Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THIS INV~NTION
This invention relates to a nondispersive
analyzer, which can be used ~o monitor the concentration
one or more gases.
BACKGROUND OF THB INVBNTION
Nondispersive analyzers are analyzers which
provide discrete radiation paths from a source to one or
more means for detecting the intensity of radiation. One
of these paths is through the sample to be analyzed as well
as through an isolated fixed quantity of the analyte gas,
and another of these paths is through the sample to be
analyzed only. Therefore, that part of the radiation which
is at the wavelengths absorbed by the analyte gas is
removed from one path, but not the other, resulting in a
difference in the outputs from the energy detecting means
associated with these paths. Thus, it is only when the
sample contains the analyte gas that the difference between
the intensities of the radiation along the two paths will
be reduced. Other absorbing compounds affect both
detectors equally.
In order to ensure that error is not introduced
by reason of some variation between radiation sources, a
single source is usually used to generate the beams of
radiation over both paths. Thus, a beam splitter or
chopper is required. Analyzers of this sort may be
relatively large. Further, appreciable energy is consumed,
often of the order of tens of watts. This is required to
power the source, to operate the chopper motor for the beam
splitter, and to power the measuring circuits.
This invention provides a nondispersive analyzer
which is relatively small and light in weight, and which
does not require either a beam splitter or a mechanical
chopper. It requires very little power, approximately one
watt, and that only intermittently. It can therefore be
operated for several weeks on the energy stored in a
lightweight battery. It uses an elliptical reflector to
focus light from its energy source.
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DE~CRIPTION OF THE PRIOR ART
Elliptical cylinder and ellipsoidal reflectors
are commonly used in optics to focus light. Such
reflectors are frequently used in measuring devices. Thus
U.S.P. 3,266,313 (Litterest) shows a temperature measuring
device where the object whose temperature is to be measured
(a wire) is at one focus of an ellipse and the detector is
at another focus. U.S.P. 4,810,658 (Shanks) shows in
Figure 4a a system where a liquid sample in contact with a
solid wave guide is placed at one focus of an ellipse and
a light source at the other focus.
Partially elliptical or ellipsoidal mirrors are
shown in Canadian Patents 1,126,977 (Hogg) and 1,127,867
(Brunsting) for particle counters. The sample and light
source are located at one focus of the ellipse. A detector
is located either on the axis of the two foci (in
Brunsteig) or is reflected off this axis by a mirror (in
Hogg).
C.P. 1,228,748 (Oetliker) shows a variety of
light guiding designs for various purposes using
ellipsoids. In some of the designs the light source and
the sample are at one focus of an ellipsoid and a detector
is at the other focus. In other designs, a light source is
at one focus and a specimen to be treated by light (as for
example in a chemical process) is at the second focus.
Elliptical or ellipsoidal reflectors are not
common in spectrometry. Three patents of Oehler, U.S.P.
4,557,603; 4,6S7,397 and 4,740,086 and one of Miyatake
(U.S.P. 4,808,825) disclose infrared spectrophotometers.
However, such spectrophotometers are not of the non-
dispersive type, and have only one gas cell which is
traversed by the light. They do not assist in the design
of a non-dispersive gas analyzer, where light rays must
pass through several gas cells to give a comparative
measurement.
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BRIEF DE~CRIPTION OF THE INVENTION
According to the present invention, there is
provided an analyzer which comprises a body having a cavity
having an elliptical reflecting surface means, which
defines a first focus and a second focus, for transmitting
radiation between the focuses, a chamber for holding a
sample gas to be analyzed, one of the focuses being located
within the sample chamber, a chamber for holding an inert
gas and a chamber for holding an analyte gas, the inert gas
and analyte gas chambers being positioned along a path
extending between the second focus and at least a portion
of the reflecting surface means; a radiation source dis-
posed at one of the focuses; and detector means associated
with each inert gas chamber and analyte chamber for detect-
ing radiation passing through the sample gas and and its
associated gas chamber.
The chambers are so positioned so that radiation
focused by the elliptical reflecting surface will always
pass through the sample chamber and either the analyte
chamber or the inert gas chamber as it travels to a detect-
ing means. In certain embodiments of the invention, there
is a plurality of analyte chambers and detectors, so that
the sample gas can be analyzed for several different
components (analytes). This invention can be applied, for
example, to monitoring air quality, detecting leakage, or
as the sensing element in a flow control system.
DE8CRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional plan view of one
embodiment of the novel analyzer, taken on the line 1-1 of
Figure 2.
Figure 2 is a cross-sectional side view of the
same embodiment, taken on the line 2-2 of Figure 1.
Figure 3 is a cross-sectional side view similar
to Figure 2, of a different embodiment, which can be used
to analyze a sample simultaneously for three different
gases.
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DETAILED DE8CRIPTION OF PREFFRRED ENBODINæNT8
Figures 1 and 2 show an analyzer for analyzing
the content of one particular gas in an unknown sample.
For example, the sample may be ambient air, and carbon
dioxide may be the gas whose concentration is to be
measured.
An analyzer housing (not shown) is provided.
Within the housing is a chamber 2 (hereinafter called the
sample chamber) to contain the sample to be analyzed, a
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chamber 3 (hereinafter called the analyte chamber)
containing the analyte gas, and a chamber ~ (hereinafter
called the inert gas chamber) to contain an inert gas,
which may suitably be the sample gas which does not contain
the analyte gas. The chamber 2 comprises a side wall 10,
two end portions 12a and 12b, top wall 14a and bottom wall
l~b. The side wall 10 is a portion of an ellipse, with it~s
foci at fl and f2. The end portions terminate in two
openings 13a and 13b. The shape of the two end portions
12a and 12b is not critical, and they can be elliptical if
desired. Their purpose is to close the open side of the
elliptical reflector and to form two openings 13a and 13b
which support the detectors. Suitably, the side wall is a
material which efficiently reflects radiation, for example
polished aluminum, or a composite material coated with a
reflective material.
The top portion of the side wall 10 mates with
top wall l~a. The bottom portion mates with bottom wall
l~b. The top and bottom walls are made of a suitably
reflective material, such as polished aluminum.
Each of walls l~a and l~b is provided with a
plurality of holes 15, to permit the sample gas to enter
and leave the device. If the device is used as an air
tester, the sample gas will normally be the ambient air
being tested.
In the region bounded by the end portions 12a and
12b there is a divider wall 16, parallel to the top and
bottom walls. Preferably, the divider is located midway
between the top and bottom walls.
Two windows 17 and 18 extend in gas tight
relation between the top wall 14a and the bottom wall l~b
and the divider wall 16. These windows can be made from
any gas impermeable material that is substantially
transparent at the wavelengths at which the analyte
absorbs, as is known in the art. For example, sapphire or
potassium bromide are suitable materials when measuring the
carbon dioxide content of air by infrared absorption.
At the focus fl of the ellipse, within chamber 2,
is situated radiation source 2~. The source is preferably
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linear and small in cross section. If the analyzer is to
use infrared radiation, it can be a conventional gas
chromatograph thermal conductivity detector element. One
suitable such element is a Gow-Mac* # 13-470P element.
5 Suitable wiring 25 leads from the source 24 through the
bottom and/or top walls to a suitable power source and
control circuit.
At the focus f2 of the ellipse, in gas-tight seal
in the openings 13a and 13b, are placed two detectors.
10The top wall 14a, elliptical side wall 10 and
bottom wall 14b, together with windows 17 and 18, define
the sample chamber 2. The detector 27, window 17, top wall
14a and divider wall 16, together with side walls 12,
define the analyte chamber 3. The detector 26, the window
1518, bottom wall 14b and divider wall 16, together with side
walls 12, define the inert gas chamber 4. These detectors
can be any detector compatible with the wavelength of the
radiation and the wavelengths absorbed by the analyte. For
example, when the analyzer is to be used to analyze carbon
20 dioxide in air, the detectors can be thermopile detectors,
thermistors, or pyroelectric detectors. One suitable
detector for such analysis is, for example, a model 2
thermopile detector obtained from Dexter Research Centre.
The radiation source 24, and the detectors 26 and
25 27, are placed respectively as precisely as possible on
foci fl and f2 of the elliptical reflector 10.
The detectors are connected by suitable wiring
28~ and 28b to a suitable instrument control and data
collection apparatus 29. For example, the apparatus 29 can
30 include a microprocessor connected and programmed to
control the timing and switching functions necessary for
operating the instrument, to store and analyze the data,
and to display the processed data as required.
A suitable power supply (not shown) is provided
35 for the radiation source 24 and the instrument control and
data collection apparatus 29. Such a suitable power supply
can be, for example, several AA size alkaline cells
connected in series and/or parallel.
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If it is desired to correct for changes in
ambient temperature, a thermistor 31 can be placed in close
proximity to the detectors, and its response used in the
known way to compensate the data collection apparatus 29 by
wiring 32.
A suitable means for confirming proper operation
of the instrument can be provided if desired. This can be
done, for example, by providing a screen with known
radiation removal characteristics, which can be moved into
the beam passing from the radiation source to one detector.
The detector reading should then vary in the known manner
from the screen characteristics.
Prior to operation, the analyte chamber 3 is
filled with the analyte gas and the inert gas chamber 4 is
filled with inert gas. For example, if it is intended to
measure the concentration of carbon dioxide in ambient air,
air with the carbon dioxide removed can be used in chamber
and chamber 3 is filled with carkon dioxide.
The gas mixture to be analyzed is allowed to
diffuse, or is otherwise introduced, into chamber 2 through
holes 15. The radiation source is energized. Since
sidewall 10 is elliptical in shape, with the source 24
located at one focus fl, the radiation emitted from source
24 is focused at f2. A portion of such radiation passes
through sample chamber 2 and analyte chamber 3 to impinge
on detector 27 at f2. A further portion passes through
sample chamber 2 and inert gas chamber ~ to impinge on
detector 26.
This focusing by the elliptical reflector permits
detector readings to be obtained with little expenditure of
energy, allowing a relatively low-powered radiation source
to be used. It has been found, for example, that the
source, the detectors and the data collection and analysis
device can be operated in one embodiment on a total power
of approximately 400 mw.
The data received from the two detectors 26 and
27 are analyzed in known fashion. If the unknown sample in
sample chamber 2 does not contain the analyte gas, the
difference between the output of detectors 26 and 27 will
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remain at a fixed known value (when corrected for
temperature variation by means of thermistor 31). If,
however, there is some of the analyte gas in the sample in
sample chamber 2, the difference between the readings of
detectors 26 and 27 will exhibit a reduced value
characteristic of the concentration of the analyte in
sample chamber 2.
Figure 3 shows an embodiment in which analysis is
carried out simultaneously for three analyte gases. In
that Figure, the same numbering is used as in Figur-s 1 and
2 where parts are the same. However, instead of the single
analyte chamber 3, three analyte chambers 3a, 3b, and 3c,
for three different analytes are provided, and these are
separated by walls 16a, 16b, and 16c. As before, chamber
4 contains a gas which is inert with respect to the
measurement to be made. If the device is to analyze air,
the inert gas can be air from which the analyte gases have
been removed. Four detectors 27a, 27b, 27c, and 2C are
provided on the focus f2 to measure the radiation passing
through the chambers 3a, 3b, 3c, and ~.
As will be noted from the description, only a
portion of wall 10 need be elliptical. Even a small
segment of an elliptical wall is helpful, as it will direct
some of the energy from the radiation source is a beam
directed to the detectors. Generally, it is preferred that
the wall ID form at least one-quarter of an ellipse, and
that this be at the end of the device proximate to the
focus fl and remote from the focus f2. However, the device
can be arranged with wall 10 as a full ellipse.
Alternately, walls 10, 14a and 14b can together form an
ellipsoid, or a portion of an ellipsoid, to further focus
the radiations. However, this is generally not preferred
as it results in more difficult construction techniques.
The analyzer has been described with respect to
a source of infrared radiation, and a detector for such
radiation. However, depending on the intended analyte, it
may be preferable to have the radiation source 24 as a
visible or ultraviolet light bulb or light pipe from an
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external bulb, and to have a suitable detector for
ultraviolet or visible light.
The invention has been shown with reference to
certain embodiments, but it will be obvious that variations
can be made by one skilled in the art without departing
from the spirit of the invention, which is as set out in
the appended claims.
