Note: Descriptions are shown in the official language in which they were submitted.
1333~ 19
GAS ANA~YZERS
This application is a division of Canadian
patent application Serial No. 577,403 filed
September 14, 1988.
TE(~INICAL FIELD OF THE INVENTION
m e present invention relates to novel,
improved devices for measuring the amount of one gas in
a mixture of gases. miS gas is referred to hereinafter
as the ~selected gas, n the "measured gas, n or the
~designated gas. n
One important application of the invention at
the present time is in the provision of capnometers for
monitoring the amount of carbon dioxide in the breath of
a medical patient. Why it is advantageous to do this
has been extensively discussed in the patent and open
literature and need not be repeated herein.
For the sake of convenien oe and clarity, the
principles of the present invention will be developed
primarily by reference to that application of those
principles discussed in the preceding paragraph. Thi-s,
hcwever, is not intended to limit the scope of the
invention as defined in the appended claims.
BACRGROUND OF THE rNVENTION
The novel gas analyzers disclosed herein
operate on the premise that the con oe ntration of a
designated gas in a mixture of gases can be measured by
passing a particularized beam of infrared radiation
through the mixture of gases and ascertaining the
attenuation of the energy in a narrow band absorbable by
the designated gas with a detector capable of generating
an attenuation proportional electrical output signal.
Gas analyzers which similarly employ an
infrared source and a detector for generating an
13334 19
--2--
electrical signal representing the attenuation of the
emitted radiation by a designated gas in the mixture
being analyzed have heretofore been proposed. Such
devices are commonly referred to as utilizing
non-dispersive infrared radiation.
Generation of a detector output signal of a
high enough signal-to-noise ratio to be useful requires
that the beam of attenuated infrared radiation falling
on the detector of such an instrument be modulated.
~eretofore~ this has perhaps most commonly been
accomplifihed by interposing a spinning wheel between the
infrared radiation source and the detector. m ese
wheels, commonly known as choppers, have a series of
apertures spaced equally around their peripheries.
Consequently, as the wheel rotates, the transmission of
the attenuated beam of infrared radiation to the
detector of the gas analyzer is alternately enabled and
interrupted, typically at a frequency of less than one
hundred cycles per second.
Gas analyzers of the character just described
are disclosed in U.S. patents Nos: 3~793~525 issued
February 19, 197 4 ~ to Bursch et al. for W AL-CELL
NON-DISPERSrVE GAS ANALYZER; 4~811~776 issued May 21r
1974~ to Blau, Jr. for GAS ANALYZER; 3~987~303 issued
October 19, 1976 ~ to Steft et al. for MEM CALrANALYTICAL
GAS DETEC~OR; 4 ~011~859 issued March 15 ~ 1977 ~ to
Frankenberger for METHOD FOR CONTINU0USLY MEASURING THE
C2 CONTENT IN BREA~ING GAS; 4~204~768 issued May 27
1980~ to N'Guyen for GAS ANALYSERS OF THE SELECTrVE
RADIATION ADSORPTION TYPE wrTH A CALIBRA~ION OELL;
4 ~268 ~751 issued May 19, 1981~ to Fritzlen et al. for
INFRARED BREAIH ANALYZER; and 4,371~785 issued
February 1, 1983~ to Pedersen for MET~OD AND APPARATUS
13331~9
FOR DETECTION OF FLUIDS and in A Reliable, Accurate
002Analyzer for Medical Use, Solomon, HEWLETq'PACRARD
JOURNAL, September 1981, pages 3-21.
Gas analyzers with mechanical choppers such as
those described in the just-cited patents have a number
of drawbacks. m ey are bulky, heavy, and expensive;
have moving parts, which is undesirable; and also have
complex optical designs. m ey also tend to be less
accurate than is desirable and to lack long-term
st-ability.
Also, gas analyzers employing mechanical
choppers are relatively fragile. For example, they will
typically not work properly, if at all, after they are
dropped.
Another heretofore proposed type of gas
analyzer employing absorption of infrared radiation as a
measure of gas concentration is disclosed in U.S. patent
No; 3,745,345 issued July 10, 1973, to Liston for SINGLE
PATH, DUAL SOURCE RADIANT ENERGY ANALYZER. m e infrared
radiation souroe of the Liston device includes wires
which are heated by applying pulses of electrical
current thereto, thus providing a modulated souroe of
radiation.
While this scheme at least theoretically
eliminates the need for a mechanical chopper, it
unfortunately has several serious drawbacks. The wires
Liston employs are spaced apart. Consequently, the
infrared radiation is not uni~ormly emitted over the
area embraced by those wires. As the wires heat up and
cool down, their diameters change, affecting the
stability of the radi~tion. me devices cannot be
pulsed at frequencies greater than about 20-25Hz whereas
the desirable modulation frequency of the infrared
1333 1~9
--4--
radiation, at least for carbon dioxide, ranges from
40-100Hz.
Still other patents dealing with pulsed
infrared radiation sources are Nos. 3,922,551 issued
5 Nov~nber 25, 1985, to Williams for DETECTION OF C02 IN A
;wARIC GASEO~S ENVIRONMENT; 4,084,096 issued
April 11, 1976, to Edwards for ELECTRICALLY ACTIVATED
INFRARED SaJROE; 4,163,899 issued August 7, 1979, to
Burrough for MET~OD AND APPARA~S FOR GAS ANALYSIS; and
10 4,480,190 igsued October 30, 1984, to Burrough et al.
for NON-DISPERSIVE INFRARED GAS ANAL~rZER and in a paper
entitled Introduction to the State of the Art Gas
Sensors by Liston Edwards, Inc., Costa ~sa, California.
Williams discloses an infrared radiation
15 emitter ~np~Loying cyclic variations in gas pressure to
modulate the ~nitted radiation. This scheme is complex
and bulky, requires moving parts, and demands a large
amount of pawer.
Burroughs and Burroughs et al. are also
20 concerned with a modulation sch~ne which employs
mechanical parts and requires a large pawer input. In
addition, the scheme disclosed in those patents, which
employs an incandesoent bulb as a radiation source, is
not capable of modulating the emitted radiation to an
25 extent approaching that needed for accurate gas
analysis.
Like Liston, Edwards and Liston Edwards employ
heated wires as a souroe of infrared radiation. meir
sources therefore have all of the above-discussed
30 drawbacks of Liston's.
An infrared radiation emitter som~hat similar
in appearanoe to those disclosed herein is the subject
of U.S. patent No. 3,875,413 issued April 1, 1985, to
1~334~9
Bridgham for INFRARED RADIATION SOURCE. However, the Bridgham
infrared radiation source differs from those disclosed herein in
that he employs a thin film rather than thick film source of
infrared radiation. As a result, the Bridgham infrared radiation
emitter is not capable of being modulated and it would be
difficult to produce in quantity.
SUMMARY OF THE INVENTION
There have now been invented and disclosed herein,
certain new and novel gas analyzers employing non-dispersive
infrared radiation to measure the concentration of a selected gas
in a mixture of gases. These novel gas analyzers are free of the
drawbacks of heretofore available instruments of the same general
character.
The invention provides in one aspect a device for
emitting modulated infrared energy in a stable, repeatable
manner, the device comprising a non-metallic substrate fabricated
of a material possessing a thermal conductivity which is
approximately one order of magnitude less than the thermal
conductivity of alumina and a layer of an emissive, electrically
resistive material on the substrate. The thermal properties of
the substrate and the thermal properties of the emissive,
electrically resistive material are so related as to enable the
device to efficiently emit pulses of infrared energy at a high
energy level, at a frequency in the range of 40 to 100 Hz, with
a high signal-to-noise ratio and with a sharktooth waveform when
electrical energy is applied to the emissive, electrically
resistive material.
The invention also provides a transducer head for
generating a signal indicative of the concentration of a selected
gas in a mixture of gases containing the selected gas and
confined to a path traversing the transducer head, the transducer
head comprising an infrared energy-emitting device as defined
above which is locatable on one side of the path and detector
means which is locatable on the opposite side of the path and
comprises an infrared radiation detector.
13~3~ 1 9
The invention further comprehends the combination of
a transducer head for generating a signal indicative of the
concentration of a selected gas in a mixture of gases containing
the selected gas and an airway adapter for confining the mixture
of gases to a path traversing the transducer head. The
transducer head comprises an infrared energy-emitting device as
defined above on one side of the path and detector means
comprising an infrared radiation detector on the opposite side
of the path. The airway adapter comprises an elongated casing
with first and second end sections and there is means for
confining the mixture of gases to the path is a passage extending
from end-to-end through the elongated casing. The adapter
further comprises integral mounting means for supporting the
transducer head from the airway adapter casing which are formed
on the casing between and in an in-line relationship with the
casing end sections and apertures in the casing on diametrically
opposed sides of the passage therethrough which are aligned along
the path between the infrared energy-emitting device and the
detector and so allow infrared energy to pass from the device
through the airway adapter and the mixture of gases flowing
therethrough to the detector that infrared energy of the
wavelength absorbed by the selected gas is attenuated before it
reaches the detector so that the signal emitted by the detector
reflects the concentration of the selected gas in the mixture of
gases.
The invention also pertains to apparatus for analyzing the
concentration of a selected gas in a stream of gases containing
the selected gas, the apparatus comprising source means for
emitting discrete pulses of radiant energy of a specified
intensity and of such a wavelength that the energy is
1333 1 ~9
--6--
absorbed by the selected gas but not by the other gases present
in the stream being analyzed. Detector means detects the
intensity of the radiant energy after the energy has passed
through the gases being analyzed and provides a signal indicative
of the intensity of the radiant energy detected by the detector
means. Means converts the signal to one indicative of the
concentration of the selected gas in the stream of gases being
analyzed.
In one aspect of the apparatus, the detector means
includes a detector and the apparatus further comprises means for
applying an electrical bias to the detector to improve the
signal-to-noise ratio of the detector. The bias applying means
comprises a flyback transformer having a primary winding and a
secondary winding connected to the detector means and means is
connected to the secondary winding for providing an error signal.
There is means for integrating the error signal with respect to a
reference signal to thereby generate an integrated signal and
means employs the integrated signal to control the voltage across
the primary winding of the transformer.
In another aspect of the invention the apparatus
includes zero adjust means for so periodically adjusting the
lower end of the signal provided by the detector means and
representative of the intensity of the radiant energy impinging
on the detector means that the level of the signal is at a
selected zero threshold in the absence of such impinging radiant
energy. The zero adjust means comprises resistor means for
applying a biasing voltage across the detector means, a
microcomputer for calculating from the current flowing through
the detector means the change required to adjust the lower end of
the detector means provided signal as aforesaid and means
controlled by the microcomputer for effecting the adjustment of
the lower end of the signal provided by the detector means by
altering the magnitude of the current.
In a still further aspect of the apparatus there is
means for so turning the source means on and off as to cause the
source means to emit the discrete pulses of radiant energy at a
1~33~ ~
frequency in the range of 40 - 100 Hz and for precisely
controlling the frequency with which the source means is turned
on and off. The means includes a source driver for turning the
source means on by connecting it across an electrical power
source and then turning the source means off in each of
successive duty cycles. There is timing means for first
activating and then deactivating the source driver in each of the
duty cycles and there is means which keeps the source driver from
connecting the source means across the power source if the timing
means malfunctions.
Still further, an aspect of the apparatus provides a
detector zero adjust means which comprises means for detecting a
biasing current flowing to the detector means and means including
a feedback circuit for generating a zero adjust current equal in
magnitude to the detected current.
A still further aspect of the appartus provides timing
means for first turning the source means on by connecting it
across an electrical power source and then turning it off in each
of successive duty cycles and means is provided for keeping the
source means from failing by preventing the connection of the
source means across the power source if the timing means
malfunctions.
More particularly, in the novel, herein disclosed,
instruments, the infrared radiation is emitted from an also
novel, thick film source and focused by a mirror on the mixture
of the gases being analyzed. After passing through that body of
gases, the beam of infrared radiation is passed through a filter.
That filter absorbs all of the radiation except for that in a
narrow band centered on a frequency which is absorbed by the gas
of concern in the mixture being analyzed. This narrow band of
radiation, which typically extends ca. 190 angstroms to each side
of the frequency on which the radiation is centered, is allowed
to reach a detector which is capable of producing an electrical
output signal proportional in magnitude to the magnitude of the
infrared radiation impinging upon it.
13~3 1 i~
--8--
Thu8, the radiation in that band i8 attenuated
to an extent which is proportional to the con oe ntration
of the designated gas in the mixture of gases being
analyzed. me ~trength of the signal generated by the
5 detector i8 consequently inversely proportional to the
conoentration of the designated gas and can be inverted
to provide a ~ignal indicative of that conoe ntration.
In a typical instrument involving the
principles of the present invention, the analog detector
10 output signal is converted to a digital form because
digital control over the operation of the gas analyzer
can be affected much more rapidly than would be the case
if analog control were employed. The digital signal is
processed in a microcomputer and displayed to show the
j~fi~ntaneous digital concentration of the gas of
concern in the mixture being analyzed. Also, other
information can be extracted from the signal and
displayed. In medical applications of the invention,
~uch information includes minimum inspiration carbon
dioxide and respiratory rate. This capability is a
significant advantage of the invention. For example,
with respect to medical applications of the invention,
other parameters listed above are as important as
instantaneous concentration of carbon dioxide in
determining a patient's medical condition.
m e herein disclosed gas analyzers also
feat~re a novel source for the infrared radiation. m is
source includes a thick film infrared radiation emitter.
The emitter is composed of a film of an emissive,
electrically resistive material on a substrate of a
material possegsing l~w thermal conductivity such as
steatite or, less preferably, alumina.
13~3~
g
Associated with this infrared radiation
emitter is a novel power supply which applies pulses of
electrical energy to the emissive film of the infrared
radiation emitter at a frequency of 40-100 Hz. This
modulation of the infrared radiation emitter is employed
because the detectors disclosed herein operate properly
only if the infrared radiation falling on them is
modulated; and modulation of the emitter makes it
possible to provide a suitably modulated beam of
radiation without employing moving parts.
The beam of infrared radiation from the thick
film emitter is passed through the mixture of gases
being analyzed. An attenuated beam emerges from this
mixture and impinges on two juxtaposed infrared
radiation detectors preferably made of a
state-of-the-art material such as lead selenide. An
optical filter which will transmit only a first narrow
band of infrared radiation oe ntered on a frequency
absorbed by the gas of interest is placed in front of
one of these detectors as discussed above. m erefore,
the radiation impinging on that detector will be
attenuated to an extent which is dependent upon the
concentration of the designated gas in the mixture being
analyzed. A second optical filter capable of
transmitting infrared radiation in a second, similarly
narrow band centered on a frequency which is not
absorbed by the gas of interest is placed in front of
the second infrared radiation detector.
Because of their juxtaposition, the infrared
radiation reaching both detectors will, for all
practical purposes, be attenuated equally by
contamination along the optical path between the
infrared radiation emitter and the detectors. Also, it
1333~
--10--
will be egually affected by thermal drift and by any
other minor infitAhilities in the infrared radiation
emitter.
Consequently, by ratioing the signals
generated by the two detectors, as is done in the novel,
herein disclosed instruments, the effect of foreign
substan oes in the optical path between the infrared
radiation emitter and detectors and the effect of any
instabilities in the infrared radiation emitter can be
eliminated.
Compensation is also made for variations in
ambient temperature and pressure and for oxygen and
nitrogen assumed to be present in the gas mixture in
medical and other applications of the invention.
Lead selenide detectors are preferably
employed in the novel gas analyzers disclosed herein
because of that material's relatively high sensitivity
and its comparatively lcw cost. However, lead selenide
detectors are very temperature sensitive with
temperature variations affecting both the bulk
resistivity and the responsiveness or sensitivity of the
detector material. A novel heater arrangement is
therefore preferably provided to maintain the detectors
at a constant and precise temperature because the errors
that would be produced by variations in detector
temperature cannot be eliminated simply by employing the
ratioing technique discussed above. In fact, the signal
from the infrared radiation detector can be lost,
despite ratioing, if the detector temperature varies as
little as 0.1C.
mis heati~g of the detectors also has the
added advantage that it keeps un~anted condensation from
1~33-149
--11--
form~ng on the optical components of the assembly in
which the detectors are incorporated.
The novel temperature control circuitry
disclosed herein is capable of controlling the detector
temperature to within 0.01 to 0.001C. In general, that
circuitry receives an analog signal from a temperature
sensor disposed in heat transfer relationship to the
infrared radiation detectors, converts that signal to a
digital form, and utilizes the digital temperature
signal in a feedback loop to control the duty cycle of a
strip heater. That heater is incorporated in the
detector assembly and is located adjacent and in heat
transfer relationship to the infrared radiation
detectors.
Also, an electrical bias, typically on the
order of -100 volts, is applied to the detectors of the
novel gas analyzers disclosed herein. It has been found
that this appreciably increases the signal-to-noise
ratio of the lead selenide detector outputs.
Another important task of the electronic
circuitry preferably employed in the the novel gas
analyzers disclosed herein is to perform an auto zero
function. In the circuitry employed in these gas
analyzers, both the bottom and the peak of each detector
generated signal pulse are measured because a more
accurate reading of the designated gas concentration can
be obtained by ascert~ining the actual magnitude of the
pulse rather than merely its peak value. m e auto zero
circuitry shifts the waveform so that the bottom of the
wave is always returned to a constant zero threshold
before a subsequent, concentration indicative signal is
generated. This insures that the waveform does not
1333~9
drift to an extent which might introduce errors into the
concentration indicative signals.
One advantage of the novel gafi analyzers just
described is that they are ~maller and lighter than
S those which have heretofore been available. As a
consequence, the analyzer can be incorporated into
larger patient monitoring ~ystems in medi~
applications of the invention.
Also, the unit containing the infrared
radiation emitter and detectors can be associated via a
novel airway adapter directly into the system through
which a patient's breath is exhausted rather than being
disposed at a more remote location to which samples are
transmitted for analysis as is commonly done in other
gas analyzers. This is an advantage because distortion
attributable to the trAnr~l~sion of the sample to the
remote location is eliminated. Also eliminated are
problems commonly encountered with the lines through
which the fiample is routed -- water in the line,
clogging of the line with foreign material, etc.
Another significant advantage of the novel gas
analyzers disclosed herein is that they are much simpler
than those above-discussed instruments which employ a
spinning wheel or other mechanical arrangement to
modulate the emitted infrared radiation. As a result,
these novel instruments have a potentially lower initial
cost; and they are potentially easier and less expensive
to service and maintain.
Also, the power required to operate the gas
analyzers disclosed herein is on the order of one
magnitude lower than is needed to run a prior art
instrument such as one employing a spinning wheel type
of transduoe r, for example. As a result, these novel
1~33 ~ll9
-13-
instruments can be battery pcwered. mis is a distinct
advantage as there are a number of applications e.g.,
involving emergency care or otherwise requiring
transportability- where a battery powered instrument is
5 advantageous, if not required.
Yet another advantage of the novel gas
analyzers disclosed herein over those employing spinning
wheels and other moving components is that they are
substantially more rugged and shock resistant.
Furthermore, these novel gas analyzers have a
much faster response time than heretofore available gas
analyzers operating on the non-dispersive infrared
radiation absorption principle. This is important
because the ghape of the exhaled carbon dioxide waveform
is significant in the diagnosis and treatment of many
medical conditions.
Another important feature of the present
invention is a novel ainway adapter which i8 employed
to: (1) couple the gas analyzer into the system
20 handling the mixture of gases being analyzed, (2)
provide a path of precisely ~iren~ioned span for the
mixture of gases, and (3) provide an optical path
traversing the thus confined body of gases between the
infrared radiation emitter and the infrared radiation
detector. miS adapter can be inexpensively made of an
appropriate plastic. Accordingly, it can simply be
thrown away after being used. This is simpler and less
expensive than cleaning and sterilizing the prior art
counterpart of a heretofore available instrument as has
theretofore been necessary in applications involving a
human patient, for example.
In a typical application of the invention, a
transducer head containing infrared radiation emitting
133~
-14-
and detecting components is assembled to the disposable adapter
and the airway adapter of the resulting assembly is installed
in the pathway followed by the gases being analyzed.
Other important aspects and features and additional
advantages of the invention will be apparent to the reader from
the foregoing and as the ensuing detailed description and
discussion proceeds in conjunction with the accompanying
drawing.
E~RIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 iS a pictorial view of a portable, hand-held
gas analyzer embodying and constructed in accord with, the
principles of the present invention;
FIG. 2 is an exploded, partially pictorial vièw of
a transducer head incorporated in the gas analyzer
of FIG. 1 to generate a collimated beam of
13334~
-15-
infrared radiation and to generate a signal indicative
of the strength of a selected band of the infrared
radiation after the beam has passed through a mixture of
gases cont~in;ng a gas which is of a concentration that
5 is to be ascertained and which is capable of selectively
absorbing infrared radiation in that narrcw band;
FIG. 3 iS a vertical section through the
transducer head of FIG. 2;
FIG. 4 is a plan view of a modulated infrared
10 radiation emitter incorporated in the transducer head of
FIG. 2;
FIG. 5 is a section through an assembly which
includes the radiation emitter and a support in which a
mirror for collimating and focusing tne emitted
15 radiation is formed;
FIG. 6 is an exploded view of a filter and
detector assembly incorporated in the transducer head of
-FIG. 3 to screen out infrared radiation which is not in
a band of interest and to then produce one electrical
20 signal indicative of the magnitude of the infrared
radiation in that band and a second output signal with
which the first signal can be so compared as to: (a)
eliminate errors attributable to foreign material in the
optical p~th bebween the infrared radiation emitter and
25 the detectors of the detector assembly, and (b)
compensate for drift in the detectors of the de~ector
assembly;
FIG. 7 iS a pictorial view of an ai~way
adapter employed in the gas analyzer of FIG. 1 to
30 confine the gas being analyzed to a path having a
precise, known, trans~erse dimension and to provide an
optical path from the infrared ~adiation emitter of the
1333449
-16-
transducer head shown in FIG. 2 through the mixture of
gases to the detectors of the transducer head;
FIG. 8 iS a longitudinal section through the
ainway adapter;
FIG. 9 iS a bloc~ diagram of the gas analyzer
of FIG. l;
FIG. 10 iS a schematic diagram of one of two
e~sentially identical systems utilized in the gas
analyzer of FIG. 1 to process the signals generated by
10 the infrared radiation detectors in the transducer head
of FIG. 2;
FIG. 11 is a schematic diagram of a clock
timing generator employed to control the operation of
the system shown in FIG. 10 and the mo~ulation of the
15 infrared radiation emitter of FIG. 5;
FIG. 12 iS a schematic diagram of a supply
source driver employed to apply electrical pulses to the
infrared radiation emitter of FIG. 5 at a frequency of
40-100~z;
FIG. 13 iS a schematic of a circuit employed
in the gas analyzer of FIG. 1 to control the temperature
of the infrared radiation detectors;
FIG. 14 iS a schematic of a circuit employed
in the gas analyzer of FIG. 1 to apply an electrical
25 bias to the infrared radiation detectors and thereby
improve the signal-to-noise ratio of those detectors;
and
FIG. 15 is a graph shawing, for two different
carbon dioxide concentrations: (1) a pulse which has a
30 typical sawtooth voltage waveform and which is
indicative of measured gas concentration obtained by
conversion of changing detector resistance to voltage,
and (2) the segments of a detector heater duty cycle in
13331~9
--17--
which the heater is respectively turned on and off and a
6egment of that cycle in which a zero adjustment is made
to canpensate for shift of the pulse away fr a
constant zero voltage threshold.
1333~ ~9
-18-
DETAlLED DESCRIPTION OF T~E INVENTION
The operation of the novel gas analyzer
disclosed hereinbelaw is controlled by a
microcomputer 18 based on an Intel 8088 chip (see
FIG. 9). A logic diagram for the routines run by the
microcomputer is attached as Appendix A. (Pages 55a - 55m)
ffl e microcomputer itself is not part of the
present invention. For this reason and because one
skilled in the relevant arts could routinely program a
general purpose computer to follow the routines shown in
Appendix A, the microcomputer will not be described in
detail herein.
Referring now to the drawing, FIG. 1 depicts a
15 portable, hand-held gas analyzer 20 embodying and
constructed in accord with the principles of the present
invention. Analyzer 20 is specifically designed to
monitor the concentration of carbon dioxide in th-e
exhAl~tions of a medical patient ~ e.g., a patient
being ventilated during a surgical procedure.
m e major components of gas analyzer 20 are a
portable, hand-held, sel~-powered unit 22 and an
assembly 24 of a transducer head 26 and an airway
adapter 28. Transduoe r head 26 is connected to the
25 hand-held unit 22 of gas analyzer 20 by a conventional
electrical cable 30.
In the the application of our invention
depicted in FIG. 1, gas analyzer 20 is employed to
measure the expired carbon dioxide level of a medical
patient. This expired carbon dioxide level can be
employed by medical personnel to control the operation
of a mechanical ventilator hooked up to the patient to
assist him in breathing. In oe rtain major surgical
* Trademark
133341~3
--1~
procedures, the ventilator completely takes over the
breathing function for the patient.
In this application of the invention, airway
a&pter 28 is employed to connect an endotracheal
tube 32 inserted into the patient's trachea to the
plumbing 34 of the mechanical ventila~or (not shown).
The airway adapter also confines the expired gases to a
flaw path 35 with a precise, transverse dimension D.
The airway a & pter algo furnishes an optical path
between an infrared radiation emitter 36 and an infrared
radiation detector unit 38, both components of
transducer head 26 (see FIG. 2).
m e infrared radiation emitted from emitter 36
traverses the mixture of gases in airway adapter 28
where it is attenuated because part of the radiation is
absorbed by the designated gas in the mixture of gases
being analzyed. The attenuated beam of infrared
radiation is then filtered to eiiminate energy of
fre~uencies lying outside a narrcw band which is
absorbed by the gas being measured. The remaining
infrared radiation in that band impinges upon a
detector 42 in detector unit 38. Detector 42 thereupon
generates an electrical signal proportional in magnitude
to the intensity of the infrared radiation impinging
upon it. This signal is transmitted over cable 30 to
the hand-held unit 22 of gas analyzer 20. That unit
contains the microcomputer 18 and electronic circuits
(see FIG. 9 and FIGS. 10-14) for controlling the
operation of transducer head 26 and for converting the
signal emitted by detector 42 to one indicative of the
concentration of carbon dioxide in the patient's
exhalations. Additional information may also be
extracted from the detector-generated signal. m is
13~344~
-20-
includes minimum inspired carbon dioxide, respiration
rate, and end tidal carbon dioxide.
Turning now to FIGS. 6 and 7, the airway
a&pter 28 of the illustrated gas analyzer 20 is a
one-piece unit typically molded from Valox polyester or
a comparable polymer. Polymers such as Valox are
preferred because they provide the ruggedness required
by a ~uitable airway adapter. Also, airway adapters can
be molded to extremely close tolerances from such
polymers. mis is necessary because the intensity of
the infrared radiation impinging upon detector 42 is
depe~Aent upon the length of the optical path between
emitter 36 and the detector, and the length of that path
is controlled by the width of the airway adapter.
Consequently, unless close tolerances are maintained,
calibration of each individual airway adapter 28 would
be required; and this might be impractical at worst and
economically prohibitive at best. Furthermore, airway
adapters of the illustrated configuration and fabricated
from polymers such as Valox*are relatively inexpensive.
Consequently, they can be disposed of after being used
rather than being sterilized and recycled, the
conventional treatment for devices of this type.
Airway adapter 28 has a generally
parallelepipedal center section 44 with a bore 46
extending from end-to-end therethrough and two hollow,
cylindrical end sections 48 and 50 having through
passages 52 and 54. End sections 48 and 50 are axially
aligned with oe nter section 44 along a common
longitudinal centerline 56. m e passages 52 and 54 in
those end sections consequently cooperate with the
bore 46 through center section 44 to form a single,
* Trademark
13~3~9
-21-
continuous, elongated passage 55 extending from
end-to-end of the ainway adapter.
Referring now to FIG. 1 as well as FIGS. 6
and 7, transducer head mounting recesses 58 and 60 are
5 formed on opposite sides of airway adapter center
section 44. These recesses furnish transduoe r head
embraceable support surfaces at the inner ends 62 and 64
of the recesses and flanges 66 and 68 at opposite ends
of the recesses. Those flanges position the assembled
10 transduoer head 26 longitudinally along the airway
adapter.
As is apparent from FIG. 6, airway adapter 28
is symmetrical with respPct to: (1) a longitu~;n~lly
extending centerplane 70, and (2) a vertically extending
15 centerplane 72. This is important from a practical
viewpoint because transduoe r head 26 can consequentially
be assembled to airway adapter 28 in the orientation
- shown in FIG. l; or it can be turned end-for-end or
upside down and still be ass~mhl~hle to the ailway
20 adapter. Consequently, in addition to its other
advantages discussed above, airway adapter 28 is user
friendly.
As is best shown in FIG. 7, apertures 74
and 76 are formed in the center section 44 of ainway
25 adapter 28 at the inner ends or transduoe r embraceable
surfaces 62 and 64 of recesses 58 and 60. mese
apertures are aligned along the optical ~ath mentioned
above and identified generally by reference
character 78. m at opkical path extends from infrared
30 radiation emitter 36 transversely across the ainway
adapter and the mixture of gases flowing therethrough to
the infrared radiation detector 42 in the detector
unit 38 of transducer head 26. Apertures 74 and 76 are
13~3 149
--22--
large cc~pared to the apertures in the most comparable
cos~ponents of heretofore proposed gas analyzers. As a
result, the novel airway adapters of the present
invention are much less apt to cause errors in the
5 concentration of the measured gas which can be
attributed to dimensional variations.
To keep the gases flawing through the airway
adapter 28 from escaping through apertures 74 and 76
without attenuating the infrared radiation traversing
10 optical path 78, the apertures are sealed by sapphire
windGws 80 and 82. Sapphire windc~s are employed
because other materials such as glass or plastic would
absorb the infrared radiation to an extent that would
significantly degrade the quallty of the signal
15 generated by detector 42. Sapphire windows are
available from a variety of co~Dmercial sources.
Typically, these windows will be on the order of 0.020
inch thick.
Referring now to FIGS. 2 and 3, we pointed out
20 above that transducer head 26 includes an infrared
radiation detector unit 38 which has a detector 42.
Also included in the detector unit are a strip heater 84
of conventional construction and a thermistor 86 which
is employed to sense the temperature of the detector
25 unit. me thermistor and strip heater are incorporated
in a system designed to keep detector unit 38 at a
constant, precise temperature. As is best shcwn in
FIG. 3, both the strip heater 84 and thermistor 86 are
juxtaposed adjacent, and in intimate heat transfer
30 relationship to, the detector 42 and the housing or
casing 88 of detector unit 38.
As shc~n in FIG. 2, housing 88 is composed of
three separate components 90, 92, and 94 which define
13331~
-23-
two separate cells 96 and 98. Cell 98 houses infrared
radiation detector unit 38, and cell 96 houses infrared
radiation emitter 36. Located between cells 96 and 98
is a rectangularly sectioned, open top recess 100 in
5 which the main body section 44 of airway adapter 28 is
fitted (see FIG. 1).
Transdu oe r housing component 90 has a
generally Lrshaped configuration provided by a
horizontal base 102 and a vertically extending side
10 wall 104. The side wall has a slot 106 therein and an
aperture 107 which acc~m~dates cable 30. Apertures 108
and 110 adjacent the upper edge of side wall 104 allow
assembly 24 to be suspended from an appropriate support
(not shown).
The second component 92 of transducer head
housing 88 has: (1) a horizontal base 112 resting on
the base 102 of housing component 90, (2) a vertical
wall 114 adjacent the vertical wall 1-04 of component 90,
and (3) a U-shaped center section 116 with top wall
20 forming flaps 118 and 120 extending laterally therefrom.
Apertures 122 (only one of which is shown) are
formed in the vertically extending side walls 126
and 128 of housing component center section 116. These
apertures are aligned along the optical path 78 between
infrared radiation emitter 36 and the detector 42 in
infrared radiation detector unit 38.
The third component 94 of housing 88 has a
vertically extending front wall 130 and vertical side
walls 132 and 134. A slot or recess 136 is formed in
the front wall 130 of housing component 94. This recess
has the same dimensions as: (1) the recess 106 in
vertically extending wall 104 of housing component 90,
1333~
-24-
and (2) the gap 137 between the side walls 126 and 128
of the center section 116 of housing component 92.
Housed in cell 96 of casing 88 along with
infrared radiation emitter 36 i8 a fitting 138 with a
transversely extending passage 140 formed therethrough.
Disposed in passage 140 are: (1) a spherical
detent 142; (2) a spring 144, which biases detent 142
toward the inner end 146 of fitting 138; and (3) a
plug 148. That plug is threaded into the through
bore 140 of fitting 138 and retains the detent and
detent spring in that bore. A flange (not shcwn) at the
end of paggage 140 nearest the inner end 146 of detent
fitting 138 keeps detent 142 from falling out that end
of the passage.
With transducer head 26 assembled to airway
adapter 28 as shown in FIG. 1, for example, detent 142
is trapped in one of four complementary recesses 150 in
airway center section 44 (see FIG. 7) to secure the
transdu oe r head to the ainway adapter. These recesses
open onto the ends of the transducer head receiving
recesses 58 and 60 in the airway adapter center
section 44 and thereby facilitate the separation of the
transducer head 26 from the airway adapter.
Four detent trapping recesses are provided so
that transducer head 26 may be coupled to airway
adapter 28 in any one of the several orientations
discussed above. Two of those recesses are located at
the bottom 62 of transducer head receiving recess 60 and
at the opposite ends of that recess. The other two
detent trapping recesses are located at the bottom 64 of
the second transdu oe r head receiving recess 58 of airway
adapter 28 and at the opposite ends of that recess.
133~49
-25-
m e infrared radiation emitter 36 housed in
the just-described transducer head casing 88 is shown in
more detail in FIGS. 4 and 5. Turning now to those
figures, emitter 36 is of a unique thick film
5 construction. It includes a substrate 152 which, in one
actual embodiment of our invention, is 0.240 inch long,
0.040 inch wide, and 0.003 inch thic~. m is substrate
is formed from a material having low thermal
conductivity. Steatite (a polycrystaline material
10 con~Ai~ing magnesium oxide and silicon dioxide) is
preferred because it has a thermal conductivity which is
on the order of one magnitude less than conventional low
thermal conductivity materials such as alumina. This is
important because it significantly reduces the power
15 required to heat the emitter to its operating
temperature.
~ cwever, alumina can be employed instead of
steatite. If it is, the substrate is preferably coated
with a film of a dielectric material having low thermal
20 conductivity such as a dielectric glass.
The thickness of substrate 152 is an important
parameter in the successful operation of emitter 36. To
provide satisfactory performances, the emitter substrate
must be in the range of 0.0025 to 0.0035 inch thick.
As is apparent from FIG. 1~, for an infrared
emitter configuration to be of any value, a careful
match between the thermal properties of the emitter's
substrate and the thermal properties of the emissive
material on that substrate must be made. This
30 correlation exists in the novel, representative emitters
illustrated in FIGS. 4 and 5 and discussed above and is
required because the substrate and the emitter are
fabricated from different materials and have quite
1333~9
-26-
different thermal properties as can be seen from
conventional h~n~hook data on the substrate and emltter
materials we employ.
Specifically, and as is apparent from FIG. 15,
5 the electrical detector output in a system equipped with
an infrared emitter embodying tne principles of our
present invention has a fihark's tooth waveform. This
particular waveform is advantageous; and it or a
comparable waveform is required because, as is also
10 shcwn in FIG. 15, the result is a large change in the
detector's peak output voltage for a relatively small
change in the level of the carbon dioxide detected by
the system. Thus, our novel systems have a high, and
obviously desirable, sensitivity to variations in the
15 concentration of the carbon dioxide being monitored. In
contrast, if a less than optimum match of su~strate and
emitter is made -- as in those previously disclosed
infrared radiation emitters employing an alumina
substrate and a thick film emissive element, for example
20 -- there is only a very small change in the peak
direction output voltage for a comparable change in the
concentration of the gas being monitored.
It is concededly possible that one might be
able to obtain a useful indication of change in carbon
25 dioxide concentration from a signal which varied to a
lesser extent for a given change in carbon dioxide
concentration. However, those skilled in the arts to
which our invention relates will recognize that this
would require much more complex and correspondingly
30 expensive signal processing circuitry i~ it were
possible at all. Thus, a system with an emitter as
shcwn in FIGS. 4 and 5 and capable of generating an
output with the characteristics shcwn in FIG. 15 has an
1333 1 ~
-27-
obviou~, and significant, economic advantage at the very
least.
One might assume that increased modulation --
perhaps ccmparable to that we obtain -- could be
5 obtained by increasing the input of power to the
emitter. Such an assumption would, hcwever, be
incorrect because of the probability that the substrate
or the emissive component, or both, would fail under the
increased load.
In conjunction with the foregoing, it will be
appreciated by those to whom this specification is
addressed that the materials we employ as emissive
elements, in the thicknesses we use, will heat up and
cool very rapidly, producing sharp spikes of energy that
15 can be reproduced in the form of electrical signals by
the detectors of our novel systems. By matching the
thermal characteristics of this emissive element to the
thermal characteristics of the substrate to which it is
mated, this spikelike waveform and the appurtenant
20 sensitivity of our novel systems to small changes in the
concentration of the gas being measured can to a large
and useful extent be retained upon mating the emissive
element to the matched substrate as is evident from
FIG. 15.
Still another important advantage of matching
the thermal characteristics of the emissive element to
those of the substrate is that -- as is also apparent
from FIG. 15 -- this novel approach to infrared
radiation emitter construction can be utilized to
30 control the ratio of the energy emitted from the
emissive elements of our novel infrared radiation
emitters to the energy conducted away from the emissive
element through the substrate. m is is important
13~34~9
-28-
because a high peak level of emitted energy is needed
for ~ensitivity and discrimination between only slightly
different concentrations in the gas being measured; and
the alternative method of reaching this goal, increasing
the input of power to the emitter, can cause the emitter
to fail.
Bonded to the upper sur~ace 154 of
substrate 152 are two T'shaped electrical conductors 156
and 158. In the exemplary infrared radiation emitter 36
illustrated in FIGS. 4 and 5, the head of each conductor
is 0.020 inch long; and the gap 160 between the
conductors is 0.030 inch. The conductors will be in the
range of 18 to 22 microns thick.
Conductors 156 and 158 are preferably formed
f a platinum and gold containing cermet obtained by
printing an ink such as DuPont's 4956 on the surface 154
of substrate 152 and then firing tne substrate.
Superimposed on conductors 156 and 158 and
bonded to the upper surface 154 of the substrate with
its ends overlapping conductors 156 and 158 is a film or
layer 162 of an emissive, electrically resistive
material. me preferred material is obtained by firing
Electro-Science Labs ESL381Z Ink. mis ink has an
operating temperature in the range of 250-300 degrees
centigrade after it is fired.
The illustrated, exemplary, emissive layer 162
is 0.070 inch long, and the two ends 164 and 166 of the
emitter overlap 0.020 inch onto the conductor 156 and
the conductor 158 of emitter 36. Thus, the total
overlap constitutes 57 percent of the total area of
emissive layer 162. This is within the preferred and
operable range of 50 to 60 percent.
13~3~9
-2g-
Overlaps in the range just described are
preferred because they tend to keep the current density
at the interfaces between emissive layer 162 and
conductorg 156 and 158 from becoming too high and
5 causing emitter 36 to fail by burnthrough or fatigue
cracking of the emissive layer.
That we can thus prevent failures of
emitter 36 i8 surprising. Heretofore, it has been
believed that successful performance of a thick film
10 device with an active layer-to-conductor overlap could
not be obtained with an overlap exceeding about 15
percent.
Also contributing to the resistance to failure
fram exposure to excessive current densities is the
15 T-shaped configuration of conductors 156 and 158. This
is at least potentially superior to the more
conventional rectangular or straight sided conductors as
far as resistance to emissive layer burnthrough is
concerned.
A film 167 of essentially pure platinum metal
may be superimposed on the layer 162 of electrically
resistive material to increase the emissivity of
infrared radiation emitter 36. This layer of platinum
is on the order of 10 to 200 angstroms thick.
As is shcwn in FIG. 5, the novel infrared
radiation emitter 36 just described is supported from an
emitter mount 168 with the emissive element 162 facing
the mount as by two emitter supporting posts 170
and 172. m ese posts, which extend through
30 apertures 174 and 176 in emitter support 168, are
electrically connected to conductors (not shown) in
cable 30 and to the conductors 156 and 158 of infrared
radiation emitter 36.
1 3 ~ 9
--3~
A parabolic mirror 178 is formed in the upper
surface 180 of emitter support 168 facing the emissive
layer 162 of infrared radiation emitter 36. This mirror
collimateg the infrared radiation emitted from
5 emitter 36. It also focuses that radiation into a beam
directed along the optical path 78 between the emitter
and the radiation detector unit 38 of transduoer
head 26.
The ;~ mhly of infrared radiation emitter 36,
10 support 168, and posts 170 and 172 is mounted in a
protective can or housing 181 shawn in FIG. 2 and
diagralmnatically in FIG. 5.
While infrared radiation emitters of the
character just described can be ~mployed to particular
15 advantage in gas analyzers of the character disclosed in
the specification, this is by no means their only use.
Instead, they can be employed in virtually any
application in which a modulated beam of infrared
radiation of constant and knGwn characteristics can be
20 used to advantage.
It was pointed out above that the detector 42
on which the beam of infrared radiation impinges after
traversing the mixture of gases flGwing through the
passage in ai~way adapter 28 is incorporated in detector
25 unit 38 along with the thermistor 86 and heater 84 of
the system ~ployed to maintain detector 42 at a
preciæ, constant temperature. Also incorporated in
that unit, and ahead of the detector, in an optical
filter 182. That detector unit component filters out
30 infrared radiation of wavelengths other than one
absorbed by the gas being measured.
Also preferably included in detector unit 38
are a second detector 183 and a second optical
1333~9
--31--
filter 184. Thi8 filter is designed to pass only
infrared radiation of a wavelength that i8 not absorbed
by the gas being measured but is adjacent the band of
absorbable radiation.
As discussed above, this results in two
signals being generated. The one generated by
detector 42 is indicative of the concentration of the
measured gas in the mixture flGwing through ai~way
adapter 28. me second detector output signal
10 (generated by detector 183) is not attenuated by the
gas being measured. As discussed above, these two
signals can consequently be ratioed to eliminate
significant errors in the measured concentration of the
designated gas. These errors are attributable to such
15 factors as foreign substances in optical path 78 (for
example, condensation on airway adapter windGws 80
or 82) and drift or other minor, uncompensated for,
instabilities in detectors 42 and 183.
me j ust-described assembly of detectors 42
and 183 and optical filters 182 and 184 is illustrated
in FIG. 6 and identified by reference character 186.
In this assembly, detectors 42 and 183 are
mounted in spaced apart relationship on the surface 188
of a substrate 190.
Detectors 42 and 183 are preferably made from
lead selenide because of the sensitivity to infrared
radiation which that material has.
Leads 192 illustrated schematically in FIG. 6
connect detectors 42 and 183 to the signal processing
30 system of gas analyzer 20 (see FIG. 9).
Also mounted on the base or substrate 190 of
assembly 186 is a fiiter support 193. Formed in that
support are apertures 194 and 196 corresponding in
1333~9
-32-
location and configuration to detectors 42 and 183.
Apertures 194 and 196 provide an interference free path
through filter support 193 for the infrared radiation
beamed along optical path 78 from emitter 36.
Mounted on filter support 193 and spanning
apertures 194 and 196 are the above-discussed optical
filters 182 and 184. Those filters are conventional and
commercially available and will accordingly not be
described in detail herein.
In the illustrated, exemplary embodiment of
our invention (designed to measure the concentration of
carbon dioxide in a mixture of gases), filter 182 is
preferably designed to pass to detector 42 only infrared
radiation in a narrow band (typically 150 angstroms
15 wide) centered on a frequency of 4.25 microns.
Filter 184, on the other hand, is designed to transmlt
to detector 183 only infrared radiation in a similarly
narraw band centered on an adjacent frequency of 3.69
microns. m at energy is not absorbed by carbon dioxide.
Filter support 193 also supports a protective
filter frame 198. This frame has two apertures 200
and 202. Filters 18Z and 184 are fitted in these
apertures and surrounded by the end and edge portions of
the protective frame.
Finally, the assembly 186 shawn in FIG. 6
includes a top cover 204. An aperture 206 in the top
cover allows the infrared radiation beamed from infrared
radiation emitter 36 along path 78 to reach filters 182
and 184 in assembly 186 without interference from, or
30 attenuation by, that cover.
The base or substrate 190 of assembly 186,
filter support 193, and filter frame 198 are preferably
fabricated of a material having moderate thermal
1333~
--33--
conductivity such as alumina because it is necessary for
the assembly to attain and maintain a constant
temperature within a reasonable time at a 1~ power
input level. Top cover 204 is fabricated of a polymer
5 such as Kovar*because that material has the same
eX~n~ion coefficient as alumina.
Turning next to FIG. 9, the electrical signals
generated by the lead selenide infrared radiation
detectors 42 and 183 in the just-described detector
10 unit 38 are transmitted to synchronous preamplifiers 210
and 212. It is the function of these preamplifiers to
amplify the electrical signals generated by the lead
selenide infrared radiation detectors.
The output signals fr~n preamplifiers 210
15 and 212 are routed to essentially identical signal
conditioning systems 214 and 216. In these systems, the
amplified, detector-generated signals are conditioned
and then subiected to a sample-and-hold process. In
this respect, and as was discussed above and is shc~n in
20 FIG. 15, the signals generated ~y detectors 42 and 183
typically have a sawtooth waveform. me sample-and-hold
technique is employed to detect the voltage at the peak
of each wave. This is done in order to measure the
partial pressure of the designated gas as a function of
25 peak signal amplitude. me sample-and-hold circuit
output is routed through a conventional multiplexer (not
shcwn) in an analog-to-digital convertor 218 to
sequentially convert the signals one at a time. The
multiplexed signal is digitized to generate an
30 appropriate digital input for microcomputer 18.
me major functions of the microcomputer 18 of
gas analyzer 20 are: (1) to control the temperature of
the infrared radiation detectors 42 and 183, (2) to
* Trademark
1333~.~3
-34-
control the modulation of infrared radiation emitter 36,
and (3) to convert the information on the concentration
of the de~ignated gas transmitted to it from
analog-to-digital convertor 218 into a form in which
5 that information can be readily utilized by the user of
gas analyzer 20.
As pointed out above, the routines follGwed by
microcomputer 18 in carrying out these tasks appear in
~ppPnnix A.
Also routed to analog-to-digital convertor 218
is a signal generated by an off-the-shelf, commercially
available, ambient temperature transducer 222. This
transducer signal is used to ccmpensate for the effect
which the temperature of the gases flowing through
15 airway adapter 28 has on the absorption of the infrared
radiation impinging upon detector 42. Changes in the
output signal from that detector which are attributable
- to temperature changes rather than changes in the
concentration of the designated gas are thereby
20 eliminated.
The novel gas analyzer 20 disclosed herein also
includes a provision for factoring local barometric
pressure into the algorithms solved in microcomputer 18
to convert the output signal from detector 42 to a
25 display indicative of the concentration of the
designated gas in that mixture of gases flowing through
airway adapter 28. miS iS important because the
detector output signal is dependent upon barometric
pressure as well as the concentration of the designated
30 gas.
In the illustrated embodiment of the
invention, the barometric pressure factor is supplied by
way of a unit 224 consisting primarily of two binary
~ 3~3449
-35-
coded decimal switches (not shcwn). The details of
these ~witches and their input connections to
microcomputer 18 are conYentional and not part of the
present invention. m ey will, accordingly, not be
5 described herein.
Another major component of the system
illustrated in FIG. 9 is a clock timing generator 226.
The clock timing generator so turns on and of~ the
synchronous amplifiers 210 and 212 that the output
signals from the latter are in a pulsed as opposed to
continuous form. mese pulses, which represent partial
pressure, are inputted to microcomputer 18 rather than
continuous detector originated signals because
microcomputer 18 operates as a sample data system.
Clock timinq generator 226 is also connected
to a supply source driver 228. That component
alternately turns radiation emitter 36 on and off to
modulate the emission of infrared radiation from
emitter 36 at the preferred frequency of 40-100 Hz.
Yet another major component of gas analyzer 20
shown in FIG. 9 is a bias supply 230. This unit is
employed to apply the above-discussed electrical bias to
lead selenide detectors 42 and 183 and thereby improve
the signal-to-noise ratios of those detectors.
m e last of the major system components sho~n
in FIG. 9 is a heater controller 232. m e input to the
heater controller is the temperature indicative signal
generated by the thermistor 86 juxtaposed to the
substrate 190 of infrared radiation detector
30 assembly 186. miS temperature indicative signal is
processed in the hea~er controller and transmitted to
analog-to-digital convertor 218. Here, this signal is
also multiplexed and converted to a digital format which
1333~9
--36--
can be inputted to microcomputer 18. As discussed
above, the microcomputer thereupon computes that portion
of the duty cycle for which the strip heater 84 of
detector unit 38 needs to be turned on to keep
5 detectors 42 and 183 at the wanted temperature. mis
duty cycle information is transmitted back to, and
control~; the operation of, the heater controller 232.
Referring still to FIG. 9, microcomputer 220
also has four inputs 234, 236, 238, and 240 identified
10 as ZERO, SPAN, N20, and 2 These four inputs are all
user-selected and initiated.
The N2O and 2 inputs 238 and 240 are utilized
to compenE;ate for nitrogen oxides and/or oxygen in the
mixture of gases being analyzed in applications of our
15 invention such as the medical application under
discussion. mese compensations are employed in
instances in which appreciable amounts of oxygen and/or
nitrogen oxides are present in the mixture because both
nitrogen oxides and oxygen affect the infrared radiation
20 absorption of carbon dioxide even though they do not
absorb infrared radiation of the 4.25 micron wavelength
reaching detector 42. Absent correction for oxygen
and/or nitrogen oxides, therefore, the detector might
report a concentration of carbon dioxide which is
25 erroneous to a significant extent.
The ZERO input 234 is likewise employed to
introduce a compensation factor into the concentration
of the measured gas calculated by microcomputer 18 in
applications such as those discussed above in which the
30 concentration of carbon dioxide in a medical patient's
exhalations is being measured. me air present in an
enclosed space, for example a hospital ro~n, typically
contains approximately 0.03 percent carbon dioxide. me
13334~9
--37--
ZERO input instructs microcomputer 18 to ~ubtract this
amount from the calculated concentration of carbon
dioxide so that the numerical concentration displayed to
the user will more accurately reflect the patient's
5 medical condition.
Finally, SPAN input 236 is employed in
applications of gas analyzer 20 in which that instrument
is utilized to measure the concentration of a designated
gas in a mixture of gases which is contained in a closed
10 cell rather than the concentration of a designated gas
in a dynamic system such as the one illustrated in
FIG. 1. In the static situation, the SPAN input is
emplc~yed to input to microcomputer 18 the kncwn volume
of the sealed cell containing the mixture of gases being
15 analyzed.
Microcomputer 18 also has four status
indicating displays 242, 244, 246, and 248. SPAN and
ZERO displays 242 and 244 are lit when SPAN and ZERO
compensations are employed.
An INOPERATIVE display 246 is illuminated when
a mechanical malfunction, for example of heater
controller 232, occurs.
Finally, ARTIFACT display 248 is illuminated
when microcnputer 18 is unable to interpret the
25 information inputted to the microcomputer from
analog-to-digital convertor 218 in a meaningful manner.
This typically happens when there is a significant
aberration in the configuration of the waveform
reflecting the concentration of the gas being measured.
30 In this event, the illumination of ARTIFACT display 248
indicates to the user of gas analyzer 20 that there are
errors in the data being received by the microcomputer.
133~
-38-
In the medical applications of our invention
for which gas analyzer 20 is particularly designed,
microc~.,~u~er 18 also has a number of outputs. One of
these i8 ~dentified by reference character 250 and the
5 label BREAI~ DETECT. mis output is enabled each time
gas analyzer 20 detects a breath, whether or not
me~h~nically assisted, by the medical patient being
monitored.
A second output is designated by reference
10 character 252 and labelled SERIAL. This output is
employed to supply a variety of information including
the instantaneous con oe ntration of the designated gas
being measured. That information is continuously
updated during the part of each operating cycle of
15 emitter 36 in which the emitter is turned on.
A second type of information that can be
supplied at serial output 252 is end tidal carbon
dioxide. This is the peak value of the carbon dioxide -
concentration in each exhalation of the patient being
20 monitored.
In addition, microcomputer 18 can supply the
patient's respiration rate and the minimum concentration
of inspired carbon dioxide, both by analysis of the
sawtooth waveform shGwn in FIG. 15.
As shown in FIG. 9, microcomputer 18 also has
the ability to convert the INSTANTANEOUS digital output
to an analog form by way of a parallel output connected
to a digital-to-analog convertor 252A. An analog output
is useful because it can be recorded by a chart recorder
30 for example. This recorder provides the user with a
hard copy record for the patient's medical chart.
The signal conditioners 214 and 216 employed
to process the amplified pulses generated in
1333~49
--39--
preamplifiers 210 and 212 from the detector 42 and 183
output signals are significant components of the system
illustrated in FIG. 9. me two amplifier units are
essentially identical. The one of these identified by
5 reference character 214 is depicted in more detail in
FIG. 10 along with the associated preamplifier 210 and
the bias ~;upply 230 employed to increase the
signal-to-noise ratio of detector 42. Only this signal
conditioner will be described herein. It is to be
10 understood by the reader that this description is
e~ually applicable to signal conditioner 216 and its
associated preamplifier and the bias supply for
detector 183.
The bias supply is typically a minus 100
15 volts. The bias is applied to detector 42 through a
resistor R253 which is employed to convert the bias
voltage to a corresponding bias current.
As is apparent from FIG. lU, resistor Eoe53 is
physically incorporated in transducer unit 38 in series
20 with detector 42 rather than in hand-held unit 22 with
the r~nainder of the electronic circuitry. By selecting
a resistor R253 of the appropriate resistance,
variations in the lead selenide detectors can be
compensated for so that the performance characteristics
25 of each detector unit 38 will be the same. The
incorporation of resistor R253 in the detector unit
therefore all~s a transducer head 26 to be replaced in
the field without recalibration of the gas analyzer
electronics.
An electrical signal is generated by
detector 42 when the'gas being measured is present in
the mixture of gases being analyzed. This signal is
s~nmed with a bias voltage VBIAS at a sur~ming
13331~9
--40--
junction 254, and the resultant signal is applied to the
inverting input of an operational amplifier 255 which is
connected to operate as a high gain amplifier in order
to boost the signal by a factor of approximately 100. A
5 feedback circuit consisting of a resistor R256 and a
compenfiating capacitor C257 i8 connected be~een the
output of operational amplifier 255 and its inverting
input to control the amplifier gain.
Operational amplifier 255 i8 p~ered from both
10 plus 12 volt and minus 12 volt power supplies. Bypass
capacitor~; C258, C259, C260, and C261 and resistors R262
and R263 are connected between the power supplies and
the operational amplifier 255 to provide noise
filtering. This is necessary because the input signal
15 to operational amplifier 255 very small and because the
amplifier has a very high gain. Consequently, unless
noise is eliminated or minimized, the input signal can
easily be lost.-
The output signal fr~n operational
20 amplifier 255 is applied to a voltage divider networkconsisting of resistors R264 and R266 and having an
approximately 10:1 ratio and then through a dropping
resistor R268 to a series source, drain-connected, field
effect transistor 270. When turned on, transistor 270
25 applies a voltage to the inverting input of an
operational amplifier 272. ~hat amplifer is ~nployed to
drive the high voltage, transistor-based amplifier stage
or current convertor 273 described belc~w.
Field effect transistor 270 is turned on by
30 applying a control voltage to its gate. This control
voltage, designated VAz in FIG. 10, is varied as
necessary by microcomputer 18 in a manner discussed
belc~ he adj ustment compensates for the
1333~g
-41-
above-dlscussed drift of detector 42 from the threshold
level as shown in FIG. 15.
The control or auto zero voltage VAz is
applied through a capacitor C274 to the anode of a
5 diode 276 and the cathode of a diode 278. Diode 276 is
connected to the gate of field effect transistor 270,
diode 278 is connected to ground, and a biasing
resistor ~280 is connected to ground in parallel with
diode 278.
m e purpose of the circuit containing
diodes 276 and 278 and resistor R280 is to reference VA2
signal to ground. This accomplished by the repetitive
VA2 signal charging capacitor C274 through diode 278.
m e internal operation of operational
15 amplifier 272 is controlled by a feedback network. That
network consists of a serially connected capacitor C282
and resistor R284 and a second capacitor C286 connected
across the serially connected capacitor and resistor.
Capacitor C282 is the prLmary feedback control
20 component. The serially connected resistor R284 and the
parallel connected capacitor C286 constitute a network
which provides control loop compensation.
This voltage output signal from ope~ational
amplifier 272 is converted to the above-mentioned
25 current signal in above-mentioned current convertor 273.
That circuit consists of transistors 288, 290, and 292
and resistors R294, R296, R298, and R300. m is current
convertor is a conventional one. Its function is to
provide a zero adjusting current equalling the bias
30 current flowing to detector 42 through resistor R253.
With the biasing current and zero adjusting current
equal, the signal applied to the inverting teDminal of
operational amplifier 255 is accurately indicative of
1333~9
-42-
the concentration of the measured gas as detected by
detector 42.
A resistor R302 is connected between
detector 42 and a junction 303 between transistors 288
5 and 292 in the zero adjust circuit just described to
provide a fixed A.C. impedance. A pair of
capacitors C304 and C306 are provided to provide a lcw
A.C. impedance.
It was pointed out above with respect to the
10 circuitry shown in FIG. 9 that both the operation of the
preamplifier 210 just described and the supply source
driver 228 which turns infrared radiation emitter 36 on
and off are controlled by clock timing generator 226.
In particular, the preamplifier and supply source driver
15 are so regulated by the clock timing generator that the
zero adjust current just described is applied to lead
selenide detector 42 at the end of the off segment of
the on-off cycle of infrared radiation emitter 36. This
mode of operation is selected because the times when the
20 zero sample-and-peak samples are taken are adjacent to
each other.
Referring still to FIG. 10, the concentration
indicative output signal from operational amplifier 255
is also transmitted from the divider network consisting
25 of resistors R264 and R266 to the non-inverting terminal
of an operational amplifier 310. Operational
amplifier 310 has a 20:1 gain controlled by a feedback
resistor R312.
m e inverting ter~inAl of operational
30 amplifier 310 is connected through a resistor R314 to
groun~. miS resistor provides a ground reference.
Also connected to the non-inverting terminal
of operational amplifier 310 is a factory-selected gain
1 3 3 3 ~ L~, t~J/
--43--
adjust resistor R316. The responsitivity of lead
selenide varies fr~ detector to detector. Each
resistor R316 i8 matched to the detector with which it
is to be associated so that the response of each
5 detector will be the same. Again, that eliminates the
calibration of gas analyzer 20 that would otherwise be
r~uired each time a transducer head 26 is replaced.
The just-described gain adjust resistor R316
and a resistor R318 located between operational
10 amplifiers 255 and 310 together constitute an attenuator
which has a maximum attenuation of not more than 2:1.
As a result, maximum attenuation occurs when there is no
current flow at the junction 320 between resistors R316
and R318. mis just-described attenuator is provided to
lS provide gain adjustment as required by each different
detector 42.
The output signal from operational
amplifier 310 is applied to the input of a
sample-and-hold circuit 322 which is provided to sample
20 the peak or the zero signal. This circuit is
conventional and will accordingly not be described
herein. It is, hcwever, a significant feature of our
invention as it keeps the voltage across
analog-to-digital convertor 218 constant while
25 analog-to-digital conversions are being made at a level
representing the analog input as of a precisely known
time.
Circuit 322 is designed to sample the signal
transmitted to it frn operational amplifier 310
30 periodically, typically at intervals on the order of 2
milliseconds, while infrared radiation emitter 36 is
turned on. Also, the output f rc~ operational
amplifier 310 can be sampled after the infrared
13~34~9
--44--
~itter 36 has been off for a specified time. This
produoes a signal which is indicative of the zero value
of the signal generated by lead selenide detector 42.
Consequently, the just-mentioned signal is one which can
5 be ~pl~yed by microc~mputer 18 to perform the zero
adjustment diF:c~ ed above.
me analog output signal fr~n sample-and-hold
circuit 322 is routed to the multiplexer in
analog-to-digital convertor 218 where the data and
10 reference signals and zeros are sequentially ~witched to
the circuitry which performs the analog-to-digital
conversion. The multiplexer output is converted to a
digital input for microcc~mputer 18, enabling the latter
to furnish the displays and perform the control
15 functions discussed above.
We pointed out above that clock timing
generator 226 is incorporated in the electrical
circuitry of gas analyzer 20 to turn infrared radiation
emitter 36 on and off and to provide detector generated
pulses which can be sampled, either: (1) during that
part of the duty cycle in which the infrared radiation
emitter 36 is turned on, or (2) during that part of the
cycle and also during the off part of the duty cycle to
provide zero pulses for adjustment purposes.
This clock timing generator, illustrated in
more detail in FIG. 11, emits timing signal s at a rate
selected by microcc~nputer 18. me microcrputer output,
D, is applied to, and controls the operation of, two
AND gates 350 and 352. ~he D signal is applied to the
latter gate, 352, through an inverter 354 to cause the
sample-and-hold circuit 322 to select either gas
concentration data or zero inf ormation on co~nand.
1 3 3 .~
--45--
Timing clock generator 226 also includes: (1)
a conventional 14-stage ripple counter 356 which is
supplied with a 4 mE~z drive signal and a divide-by-six
prograTmoable divider chip 358 with a triple input to a
5 conventional decoder 360 connected through an
inverter 362 to ground. This inverter is provided to
provide a logic one to decoder 360.
The 4 mHz signal is reduced by ripple
counter 356 to either a 480 HZ or a 240 HZ output
10 signal. mis output signal is routed to a clock (not
shawn) in chip 358 which conse~uentially generates an
output having the six segments identified as Ql through
Q6 in FIG. 15 during each on-off or duty cycle of
infrared radiation ~nitter 36.
15Ql is the segment representing the inf rared
radiation ~nitter "ona time. During this portion of the
duty cycle, the output f rom decoder 360 enables AND
gate 350. This results in a signal which is routed to
an OR gate 363. mis causes gate 363 to output the
20 signal labelled SAM~E in FIG. 11. The SAMPI,E signal
enables supply source driver 228 which accordingly turns
- on infrared radiation ~nitter 36 for that part of the
duty cycle computed by microcomputer 18.
me SAMPI.E signal also enables synchronous
25 preamplifiers 210 and 212, allcwing the signals
generated by lead selenide detectors 42 and 183 to be
processed and transmitted to microcc1rnputer 18 in the
m~nner discussed above to furnish the displays and
provide the controls for which the microcomputer is
30 designed.
Q2 through Q5 represent that part of the duty
cycle in which the infrared radiation emitter 36 is
turned off. mis iS accomplished by removing the
1333~
--46--
enabling gignal fral~ AND gate 350 at the end of the on
segment Ql in each duty cycle of radiation emitter 36.
When this occurs, OR gate 363 shuts off. Iherefore, it
ceases to supply the SAM~E signal needed to operate
5 supply Rource driver 228 and to permit preamplifiers 210
and 212 to route zero pulses generated in those
preamplifiers fran the infrared radiation detector
output signal to the signal processing circuitry
depicted in FIG. 10.
Finally, Q6 is that segment of the duty cycle
of infrared radiation emitter 36 in which the zero
adjust function discussed above is performed. In that
segment of the duty cycle, AND gate 352 is enabled by
decoder 360. ~his enables OR gate 363, thereby routing
15 the zero adjust input signal VAz discussed above to
signal conditioner 214 to cc~npensate for drift of lead
selenide detector 42.
Referring still to the drawing, FIG. 12
illustrates, in detail, the supply source driver 228
20 which is activated during that segment of the infrared
radiation emitter duty cycle designated Ql in FIG. 15 to
turn on infrared radiation emitter 36. This signal is
applied through a capacitor C364 and resistor R366 to
the gate of a field effect transistor 368. This
25 transistor is also connected across a reference voltage
source through a resistor R370 which isolates the
reference voltage (VREF) frcm ground when field effect
transistor 368 is turned on. merefore, during the Ql~
~nitter on segment of the infrared radiation emitter
30 duty cycle, field effect transistor 368 is turned off.
Field effect transistor 368 insures that the
circuit connecting infrared radiation emitter 36 to its
sources of operating voltage is, or r~nains, interrupted
1333~9
-47-
if clock timing generator 226 fails. Absent the
protection circuit, the infrared radiation emitter would
remain continuously on and would fail almost
immediately, requiring that transdu oe r head 26 be
5 replaced.
The turning on of field effect transistor 368
removes the reference voltage signal from the
non-inverting terminal of an operational amplifier 372.
Removal of this control voltage turns on amplifier 372
10 which has its output connected to its inverting input so
that it functions as a unity follcwer. A unity follower
is needed at this point in the supply souroe driver to
buffer the reference voltage.
The output from operational amplifier 372 is
15 connected through a resistor R374, which sets the
amplifier gain, to the non-inverting tenrin~l of a
~econd operational amplifier 376 in supply source
driver 228. m at operational amplifier is one component
of a differentially connected amplifier-and-follower
20 combination which also includes field effect
transistor 378. mis amplifier-and-follower combination
is employed to provide gain and a 150 milliampere output
current drive capability.
The sour oe of field effect transistor 378 is
25 connected to one end of infrared radiation emitter 36,
and the opposite end of the emitter is connected to a
minus 12 volt pGwer source having a capacitor C380
connected in parallel therewith to provide local pG~er
supply filtering. Consequently, when field effect
30 transistor 378 is partially or fully turned on, infrared
radiation emitter 36 is connected across the plus 12 and
minus 12 volt power supplies; and current flGws through
1~3~fl4~
-48-
it, cAI~s~ng infrared radiation of the wanted, controlled
character to be emitted.
m e magnitude of the power applied to infrared
radiation emitter 36 i8 crucial because variations will
5 affect the magnitude of: (1) the radiation emitted in
the 4.25 micron-centered band impinging upon
detector 42, and (2) the energy emitted in the 3.79
micron-centered band impinging upon detector 183.
Therefore, variations in the power supplied to
10 emitter 36 would cause those detectors to report changes
in the concentration of the designated gas which are
attributable to variations in that power rather than to
changes in the concentration of the measured gas as both
detectors are involved in the generation of the ratioed,
15 error compensated, gas concentration signal.
Unwanted irregularities of the character just
described are eliminated by employing a differential
resistor network to provide a precise, constant pcwer
source for emitter 36. This network includes: (1) a
20 resistor R382 connected between the inverting tenminal
of operational amplifier 376 and infrared radiation
emitter 36, (2) a resistor R384 connected between the
non-inverting input of operational amplifier 376 and
ground through the above-discussed capacitor C380, l3) a
25 resistor R386 connected between operational
amplifier 376 and the sour oe of field effect
transistor 378, and (4) a resistor R388 connected
between the inverting input of operational amplifier 376
and ground. Resistors R388 and R370 are matched as are
30 resistors R382 and R384. Therefore, the voltage applied
across infrared radiption emitter 36 is precisely
related to the reference voltage; and the infrared
radiation detector is turned on and off in an
133~9
--49--
essentially instantaneous fashion; i.e., the voltage
applied across emitter 36 follcws an essentially ~uare
waveform.
In the system depicted in FIG. 13, a constant
5 voltage source is employed to operate emitter 36 rather
than a constant power source. A constant voltage source
i8 inherently more stable, and stability is a
prerequisite to accuracy in gas analyzers operating on
the principles disclosed herein.
Furthermore, a bipolar power supply, typically
plus and minus 12 volts, is preferably employed rather
than the more common single-sided pawer supply. This
makes it possible to pGwer gas analyzer 20 with a
battery, thereby making that instrument self-contained.
We also pointed out above that lead selenide
is extr~ely temperature sensitive and that, as a
consequence, sucoessful operation of gas analyzer 20
requires that infrared radiation detectors 42 and 183 be
precisely maintained at a constant temperature. The
20 system by which this control function is accc~mplished
includes the above-discussed strip heater 84 and
thermistor 86, and it is identified in FIG. 13 by
referenoe character 390.
mermistor 86 is located in one leg of a
25 bridge which also includes resistors R392, R394,
and R396. Reference voltage VREF is applied to one
referenoe terminal 398 of the bridge, and the second
referenoe terminal 400 is connected to ground. The two
output teIminals 402 and 404 of the bridge are
30 respectively connected to the non-inverting and
inverting inputs of an operational amplifier 406 with
terminal 402 also being connected through biasing
resistor R394 to ground. merefore, operational
1333~4~
--50--
amplifier 406 i8 turned on when the detector unit
temperature sensed bsr thermistor 86 departs fran the
reference temperature because: the resistance of
thermistor 86 changes, the j ust-discussed bridge becomes
5 unbalanced, and current thereby flows in the bridge
circuit.
Connected across the output and the inverting
input of operational amplifier 406 is a law pass filter
consisting of a resi5tor R408 and a capacitor C410.
10 This filter eliminates those frequencies of the signals
applied to operational amplifier 406 which, because of
the characteristics of a thermistor, are not
representative of changes in the thermistor-sensed
temperature.
Also connected to operational amplifier 406
are capacitors C412 and C414. mese capacitors provide
local p~er supply filtering.
Because feedback resistor R408 and bridge
resistors R392, R394, and R396 cooperate with
20 thermistor 86 to perform a temperature-to-voltage
conversion, the output frc~n operational amplifier 406 is
a detector temperature-proportional voltage. This
voltage varies from zero volts at a detector t~mperature
of 35C to plus 5 volts at a detector temperature of
25 45C.
This operational amplifier output signal is
routed to the multiplexer in analog-to-digital
convertor 218 to allc~ for sequential signal conversion.
ffle multiplexer output signal is converted to a
30 digitized signal and routed to microcomputer 18.
Ihe digitized information is ccmpared in
microcomputer 18 to the operating temperature selected
for detector unit 38 through an algorit~n encoded in the
133~9
-51-
microcomputer. ffl e result i8 a computed duty cycle with
the time segment Ql (see ~IG. 15) representing that part
of the duty cycle in which strip heater 84 is re~uired
to be turned on to keep the temperature of detector
5 unit 38 at tbe selected level.
This calculated information is employed to
generate a high state output which is applied to the
gate of a power field effect transistor 416 through an
inverter 418 which buffers the digital signal. This
10 turns transistor 416 on, connecting heater 84 across a
plus 5 volt operating source to ground.
At the end of the time segment Ql in each duty
cycle, the microc~mputer output applied to inverter 418
reverts to a 1GW state. This turns transistor 416 off
15 and interrupts the operation of heater 84.
It is important, in conjunction with the
foregoing, that heater 84 keep the entire detector unit,
rather than just detectors 42 and 183 per se, at the
selected temperature (which is preferably on the order
20 of 40C). miS keeps condensation from forming on the
exposed surfaces of filters 182 and 184 and interferring
with the transmission of the attenuated beam of infrared
radiation to the detectors. Also, the keeping of the
entire detector unit at a constant temperature
25 eliminates inaccuracies that would potentially be
present if the optical filters 182 and 184 instead
expanded and contracted as they would tend to do if they
were not kept at a constant temperature.
We pointed out above that another major
30 component of the system illustrated in FIG. 9 is the
circuit 230 employed to apply a negative bias to lead
selenide infrared radiation detectors 42 and 183 and
thereby improve the signal-to-noise ratios of those
1 3 3 3 !1 4 9
-52-
detectors. This circuit, shcwn in detail in FIG. 14,
includes a flyback transformer T420, a power field
effect tran~istor 422, a secondary diode 424, a storage
capacitor C426, and a voltage control circuit 427 which
5 includes operational amplifiers 428 and 430 and a series
pass connected field effect transistor 432.
As shcwn in FIG. 14, one terminal of
transformer primary winding 433 is connected to ground
through diode 424, and the other end of that winding is
10 connected to the drain of field effect transistor 422.
m e control voltage generated by circuit 427 is applied
to the center tap of winding 433.
m e gate of field effect transistor 422 is
supplied with a 125 kHz signal from clock timing
15 generator 226.
When the clock signal goes high, field effect
transistor 422 is turned on; and current builds up in
transformer primary winding 433. m en, when the clock
signal goes low, turning off field effect transistor
20 switch 422, the energy in the magnetic field of
transformer T420 causes the voltage in primary
winding 433 to fly back to approximately 100 volts.
Tbis creates a current in winding 433 which is stored in
capacitor C426.
The negative voltage generated when field
effect transistor 422 turns off is sensed by a
resistor R434 which is connected to the inverting
terminal of operational amplifier 428. This resistor is
also connected through a capacitor C436 to the output
30 terminal of operational amplifier 428 to form an
integrating circuit. mis circuit integrates the error
between the current flowing through resistor R434 and a
reference current developed from reference voltage VREF-
133~1~9
--53--
The reference voltage i8 supplied by resistor R438, andit is also applied to the inverting tenrinAl of
operational amplifier 428.
A second and equal reference current is
5 developed frc~ reference voltage VREF by resistor R440
and applied to the non-inverting termin~l of
amplifier 428.
m e operational amplifier circuit also
includes resistors R442 and R444, which provide local
10 isolation, and capacitors C446 and C448 which provide
local filtering.
As suggested above, it is the function of the
operational amplifier circuit just described to
integrate the error between the reference current and
15 the current derived from the reference voltage.
The integrated error current is inverted by
unity gain operational amplifier 430. The error signal
is applied to the inverting ter~in~l of that amplifier
through a resistor R450, which provides amplifier input
20 resistance; and the non-inverting tenm;n~l of that
amplifier is connected to ground. A resistor R452
connected between the inverting input and the output of
amplifier 430 controls the amplifier gain.
The output of operational amplifier 430 is
25 applied to the gate of field effect transistor 432.
Consequently, as the error signal increases in
magnitude, field effect transistor 432 iS turned on to
an extent determined by the magnitude of the error
current. This controls the voltage across the primary
30 winding 433 of transformer T420 and, consequentially,
the voltage across the secondary winding 454 of that
transformer, maintaining the latter at the wanted,
typically -100 volt level.
133~9
-54-
A diode 456 and a resistor R458 are connected
in series in the biasing voltage supplying secondary
winding 454 to provide isolation and recti~ication,
respectively.
In the foregoing circuit, the biasing is
typically modulated at a frequency of 125 kHz in order
to reduoe size of tbe magnetic and filtering components.
Gas analyzers embodying the principles of the
present invention do not have to be of the particular
10 character discussed above. For example, additional
detectors may be incorporated to measure the
con oe ntration of other gases present in the mixture
being analyzed, and hand-held unit 22 and the routines
appearing in Appendix A may be modified to provide
15 appropriate displays of the additional information
provided by those detectors. (Appendix A - Pages 55a - 55m)
Another modification that can be made within
the principles discussed above is to modulate infrared
radiation emitting source 36 with A.C. as opposed to the
20 disclosed D.C. electrical voltage. This has the
potential advantage of reducing the current density at
the interfaces between the radiation emitting layer 162
and the conductors 156 and 158 of an infrared radiation
emitter such as that identified by reference
25 character 36 in FIG. 4. That would further reduce the
potential for burnthrough attributable to current
densities which are too high and would also allow higher
current densities to be employed in applications where
that is deemed desirable.
Options for eliminating the effect of foreign
substances in the optical path between the infrared
radiation emitter and infrared radiation detector and
the effect of minor instabilities in the latter also
1333~
exist. For example, two emitters, each filtered to pass
only infrared radiation in a specific band, could be
employed; and the second, reference emitter could be a
diode with a sp~cific spPctral output rather than an
5 emitter of the type disclosed here.
A second option would be to substitute for one
or both of the disclosed lead selenide detectors a PIN
diode or a thermopile detector.
Furthermore, it will be obvious to those
10 skilled in the arts to which this invention relates that
gas analyzers as disclosed herein can be incorporated in
equipment designed to monitor a variety of functions
rather than being constructed as a stand-alone unit.
In addition, instruments employing the
15 principles of the present invention can be employed to
measure gases other than carbon dioxide simply by using
a different filter to change the wavelength of the
detected infrared radiation. And, the mixture being
analyzed may be of an industrial origin, as an example,
20 rather than the gases breathed out by a medical patient.
From the foregoing, it will be apparent to the
reader that our invention may be embodied in many
specific forms in addition to those disclosed above
without departing from the spirit or essential
25 characteristics of the invention. m e embodiments of
the invention disclosed herein are therefore to be
considered in all respects as illustrative and not
restrictive. m e scope of the invention is instead
indicated by the appended claims, and all changes which
30 come within the meaning and range of the equivalency of
the claims are therefQre intended to be embraced
therein.
13~3~9
55a
APPEN Dl X A
GAS ANALYZERS
(Infrared C02 Analyzer
Software Flow Chart)
1333~
55b
( MAIN ) ( INT ) 80X/SEC
t
CALL INIT 1 2 ¦ SET INTR FLAG
t
CALL WARM 1 3 ¦ INCR CYC CNT MOD 20 ¦ 250 MS
INCR PERIOD MOD 4 1 SEC
- ON CYC CNT ROLLOYER
CALL KEY ¦ 4
ENA INTERRUPT
,YES RETURN
NO / IS \ NO
<~?F~>
YESl'
CALL SPAN 15 ¦ CALL ZERO ¦ 6
NO<~YES 80X/SEC
CALL DATA ¦ 7
CALL TEMP ¦ 8
¦ RESET INTR FLAG ¦
/ \ NOTE: THERE ARE
4X /SEC YES / IS ~ 10 D~TA/ REF AND
< CYC CNT > 10 Z-RO MEASURE--
1 ! \ 71 9 / ~lEN-S PER 1/~ SEC
¦ CALL Cl ~MPU~E ¦ g NO )ISP A~ UpDAT-
COUN- R PROVI DES
¦ CALL CONVERT ¦ 10 1 IN ~0 (12 5MS) REQ
CALL DISPLAY ¦ 11
1333~
55c
C INIT )
¦ RESET DATA FLAG ¦
¦ RESET KEY FLAG ¦
¦ CYCLE CNT = 19 ¦ ¦ TURN ItEATER OFF ¦
ZERO DISPLAY ¦ ¦ ERROR = 0
TURN ON D.P. ¦ ¦ RESET CAL FLAG
TURN LFDS OFF ¦ ¦ RESET CALONE FLAG ¦
RESET 02 FLAG ¦ ¦ RESET CALKEY FLAG ¦
¦ RESET N20 FLAG ¦ ¦ RETURN
SET DZ FLAG
OUTPUT DZ
¦ START WARM TIMER ¦
¦ START LED TIMER ¦
R ESET ZER O FLAG
PERI OD = 3
ENA INTERRPUT
RESET LED FLAG ¦
1333~49
55d
WAR M
~\YES
NO ¦ RESTART LED TIMER ¦
¦ TOGGLE LED FLAG
¦ TOGGLE STATUS LED ¦
~ I
/ INTR \YES
\FLgG ~
N~/ ¦RESET INTR FLAG
/~ ¦ CALL TEMP ¦ 8
~WARM~NO
YES /\
/ IS \ NO
YES
SET KEY FLAG
¦ TURN ON STATUS LED ¦
¦START BOUNCE TIMER ¦
RETURN ¦ ~
1333~9
55e
( KEY )
~Y ~
YES
~ ~KEY FLAG~
<~NO \~
YES ¦CALL PROCESS ¦ 12
SET KEY FLAG
~ ¦ START BOUNCE TIMER
¦ RESEr KEY FLAG ¦
RETURN
1333 1~9
55f
( SPAN )
SET CALON E FLAG I N O
RESET CALKEY FLAG ¦ \~'
TURN SPAN LED ON
SEr LED FLAG I ¦ RESTART LED TIMER ¦
START LED TIMER I ¦ TOGGLE SPAN LED ¦
¦ START 2 MINUTE TIMER ¦ ¦ TOGGLE SPAN FLA~ ¦
SAVE OLD SP AN VAL
SPAN VAL = 1 ¦ ¦ RESTORE OLD SPAN VAL ¦
~YES L READ BARO PRES
\ OUT /
N~/ ¦ CORR 38T FOR BARO
~ CONV 38T TO 38 COUNT ¦
/ MINUTE \ YES
bu T ~ . COU N T
N~
¦ TURN OFF CAL LEDS ¦
<~>YES ¦ RESET CALONE FLAG ¦
NO
RESET CAL FLAG
RETURN ¦ ¦ RESET ZERO FLAG
1333~49
559
ZER O
¦ SET CALONE FLAG I ~
N O/CA ON E\
¦ RESET CALKEY FLAG ¦
¦ TURN ZERO LED ON ¦ I YES
¦ SEr LED FLAG
¦ START LED TIMER
~ ¦RESTART LED TIMER ¦
¦ SrART MINUTE TIMER ¦ ~
~ ¦ TOGGLE ZERO LED ¦
¦ SAVE OLD ZERO VAL.
~ ¦ TOGGLE LED FLAG
¦ ZERO VAL = 0
~' ~
RESET CAL FLAG
TIMER ~YES
\Y~ ¦ RESET CALONE ELAG
NO
¦ TURN OFF ZERO LED
TIMER ~ ¦ RESTORE OLD ZERO VAL¦
~ I
NO
¦ ZER O VAL = D ATA ¦
YES ¦ RESET CALONE FLAG ¦
NO ¦ SET ZERO FLAG
¦ RETURN I¦ TURN ON ZERO LED ¦
13334~9
55h
( DATA )
~CNT YES ¦ DArAWORD = ~ ¦
-0? ~
¦ REFWORD = ~ ¦
NO
¦REA~ DATA COUNT¦
¦ READ REF COUNT ¦
NO ~ YES
READ ~ DZ FLAG ~ DATA
TIME ~ TIME
¦ SET DZ FLAG ¦ ¦ RESET DZ FLAG
¦ OUTPUT FLAG l l OUTPUT FLAG
DATAWORD = DATA DATAWORD = DATA
WORD-DATA COUNT WORD+DATA COUNT
REFWORD = REF REFWORD = REF
WORD-REF COUNT WORD+REFCOUNT
¦ RETURN
133~ ~9
55i
( TEMP )
t
~ CYC CNT INCREMENTS - 12.5MS
/ IS \YES ¦ MEAS. SENSOR TEMP ¦
~TIME = ¢)~
¦ OLD ERR = ERROR ¦
NO
ERROR=MEAS TEMP
--SET VALUE
ERR=ERROR
OLD ERR
DUTY TIME =
F(ERROR, A ERR)
I
D~U TY ~YES
NO
REFWORD = REF
OLD ERR
¦ RETURN ¦ l
1333~9
55j
( COMPIJTE )
[~ATA = 10230 CONVERT DATA TO
- DATAWORD POSITIVE INCREASING VALUE
RATIO = 10230 DErERMlNE REFERENCE
--. RE~WORD CHANNEL RATIO FACTOR
I
DATA = DATA CORRECT DATA ~3Y REFERENCE
X RATIO CHANNEL RATIO
DATA = ~ATA COMPLETE AVERAGE OF
--. 10 10 DATA POINTS
RETURN
1333~3
55k
~ CONVERT )
COUNT -- DATA ADJUST DATA COUNT
--ZERO FOR ZERO OFFSET
COUNT = COUNT ADJUST DATA COUNT
X SPAN BY SPAN CAL FACTOR
TORRVAL = F(COUNT) CONVERT COUNTS TO
FROM LOOKUP TABLÉ TORR FROM TABLE
¦ READ BARO. PRESS. ¦
CORRECT TORRVAL PRESSURE COMPENSATION
NO <~YES GAS
COMPEN SATI ON
~>YES
<~,YES I IAPPLY 02-N20 CORR I
I APPLY 02-N20 CORR I ¦ APPLY 02--N20 CORR ¦
¦ RETURN ¦
13~3~4~
551
( DISPLAY )
CONVERT TORRVAL TO
B CD TEN S, U N I TS, TEN TH
OUTPUT TENS AND
UNITS TO PORT 2
OUTPUT TENTHS
TO PORT 1 D7--D4
¦ RE~URN ¦
1333~
55m
( PROCESS )
</~ YES
~ 1
NO YE~IS~>NO
¦ SET CAL FLAG ¦
¦ SET CALKEY FLAG ¦
<~,YES
NO ¦ TOGGLE N20 FLAG ¦
¦ TOGGLE N20 LED
YES
N ¦ TOGGLE 02 FLAG
TOGGLE 02 LED
¦ RETURN ¦
