Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVEN~ION
This invention relates to a nondispersive
analyzer, which can be used to monitor the concentration of
one or more gases.
BACXGRO~D ~F THE INV~NT~ON
Nondispersive analyzers are analyzers which
provide discrete radiation paths from a radiation source to
one or more means for detecting the intensity of radiation.
One of these paths is through an isolated fixed quantity of
the analyte gas as well as through the sample to be
analyzed, 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 dif~erence 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 sources of radiation,
a single source is usually used to generate the beams of
radiation over both light paths. Thus, a beam splitter or
chopper is required. Analyzers of this sort may be rela-
tively large. Further, appreciable energy is consumed,
often of the order of tens of watts. This is require~ 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
1 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 radiation source.
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DESCRIPTION OF ~'HE PRIOR ART
Elliptical cylinder and ellipsoidal reflectors
are commonly use~ in optics to focus radiation. such
reflectors are frequently used in measuring devices. Thus
U.S.P. 3,226,313 (Litterst) shows a temperature measuring
device where the object whose temperature is to be measured
(a wire) at one focus o an el:Liptical re~lector and the
detector at the other focus. U.S.P. 4,810,658 (Shanks)
shows in Figure 4a a system ~here a liquid sample in
contact with a solid waveguide is placed at one focus of an
elliptical reflector and a light source at the other focus.
Partially elliptical or ellipsoidal reflectors
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 source of the
elliptical reflector. A detector is located either on the
axis of the two foci (in Brunsting) 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
ellipsoidal reflectors. In some of the designs the light
source and the sample are at one focus of an ellipsoidal
reflector 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 Oehler; U.S.P.
4,557,603; 4,657,397 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 radiation. They do not assist in the
design of a non-dispersive gas analyzer, where the
radiation must pass through at least two gas cells to give
a comparative measurement.
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BRIEF ~ESCRIPTION OF TH2 INVENTION
According to the present invention, there is
provided a nondispersive, infrared gas analyzer, c~mprising
a body having a chamber for holding a sample gas to be
analyzed, the sample gas chamber having a reflective inner
surface, at least a portion of the inner surface being
elliptical and defining a first focus and a second focus
within the sample gas chamber and being operable to reflect
radiation between the focuses, a sealed inert gas chamber
lo for holding an inert gas, a sealed analyte gas chamber for
holding an analyte gas, an infrared radiation source
disposed at the first focus in the sample gas chamber, a
reflector disposed around the second focus for reflecting
radiation emitted by the radiation source and focused by
the reflective inner surface toward each sealed gas
chamber, and a detector associated with each sealed gas
chamber for detecting radiation, emitted by the source,
which has travelled through the sample gas and the gas in
its associated sealed gas chamber and for producing a
signal representative of the concentration in the sample
gas of the gas contained in its associated sealed gas
chamber.
In various embodiments of the invention, a
reflector, or a plurality of reflectors, or a single
moveable reflector, or a plurality of moveable reflectors
are located at or around the second focus of the ellipse,
and such reflector or reflectors direct the light beam to
a plurality of analyte chambers and detectors.
This invention can be applied, for example, to
monitoring air ~uality, detecting leakage, or as the
sensing element in a flow control system.
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DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional plan view of one
embodiment of the novel analyzsr, taken on the line 1-1 of
Figure 2.
5~igure 2 is a cross-sectional side view of the
same embodiment, taken on the line 2-2 of Figure 1.
Figur~ 3 is a cut-away perspective view of a
further embodiment in which an ellipsoidal reflector is
used to further increase the number of gases which can be
measured simultaneously.
DETAILED DESC~IPTION OF PREFERRED ~MBODIMEN~
Figure~ 1 and 2 show an analyzer for analyzing
the content of several gases in an unknown sample.
15An 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,
chambers 3, 3~, 3b, 3c, and 3d (hereinafter called the
analyte chambers) each containing one of the plurality of
gases to be analyzed for, and a chamber 4 (hereinafter
called the inert gas chamber) to contain an inert gas,
which may suitably be the sample gas in which none of the
analyte gases are present. The chamber 2 comprises a side
wall ~0, top wall 14 and bottom wall 14a. The side wall 10
is an ellipse, with its foci at fl and f2, although it may
be made as a portion of an ellipse, with a non-elliptical
or several non-elliptical portions if desired. This
reduces the effectiveness of the device; but may be usable
if some additional power loss can be tolerated. Suitably,
the side wall 10 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 a
top wall 14. The bottom portion mates with a bottom wall
14a. The two walls are made of a suitable reflective
material, such as polished aluminum.
Each of walls 14 and 14a is provided with a
plurality of holes 15, to permit the sample gas to enter
and leave the device.
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At the focus fl o~ the ellipse, within chamber 2,
is situated a radiation source 2~. The source i5
preferably linear and small in cross section, as can be a
conventional gas chromatograph thermal conductivity
detector element. One suitable such element is a Gow-Mac*
# 13-470P element.
A multi-faceted reflective column 31 is centered
at focus f2. Column 31 is approximately hexagonal in
cross section. It is preferrecl to deviate from a true
hexagon in that a slight curvature of the reflective faces
31a, 31b, 31c, 31a, 31e, 31~, i5 needed to focus the
radiation reflected ~rom these faces toward the
corresponding detectors 26, 27, 27~, 27b, 27c, and 27d.
The column is of a suitable material and is polished, so
that the faces corresponding to the sides of the hexagon
reflect radiation. For example, the column can be polished
aluminum.
When the source 24 is energized, radiation is
focused toward the center of column 31. Each of the faces
of column 31 reflects a beam of radiation toward a specific
location on wall 10. For example, two rays o~ radiation
are shown emanating from the source 2~. These rays are
focused toward f2 by the elliptical reflector 10. They are
then reflected by face 31d toward hole 40 which penetrates
wall 10. Behind hole 40 are oriented one behind the other
window 50 and detector 27b, which define between them, in
combination with a cylindrical side wall 60, a chamber 3b.
Similarlyl other holes 41, 42, 43, 44, and 45 are
shown, which receive reflections from faces 31e, 31f, 31a,
31b, and 310 respectively. In such a structure, a
multiplicity of detectors and their corresponding chambers
can be provided.
With a device having six chambers, it is possible
to simultaneously measure the concentrations of five
analytes in the sample in chamber 2. The sixth chamber
contains an inert gas, for example, the sample gas which
does not contain the analyte gases.
The detectors are connected by suitable wiring 2~
to a suitable instrument control and data collection
*Trademark
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apparatus 29. For example, the apparatus 2g can 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
for the source 24 and the instrument control and data
collection apparatus 2g. A suitable power supply is, for
example, several AA size alkaline cells connected in series
and/or parallel to supply the different voltages required.
If it is desired to correct for changes in
ambient temperature/ a thermistor 31 can be installed, and
its response used in the known way to compensate the data
collection apparatus 2g 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 removing a known and reproducible
part of the radiation from one beam by introducing a screen
into one beam, as is known in the art.
Prior to operation, the analyte chambers 3, 3a,
3b, 3c, and 3d are filled with the analyte gases and gas
chamber 4 is filled with inert gas~ For example, air with
the carbon dioxide removed can be used in chamber 4 where
the analyte is carbon dioxide, and it is intended to
measure the concentration of carbon dioxide in ambient air.
The gas mixture to be analyzed is allowed to
diffuse, or is otherwise introduced, into chamber 2 through
holes 15. The source is energized. Sinca sidewall 10 is
elliptical in shape, with the source 24 located at one
focus fl, the radiation emitted from source 2~ is focused
at f2. Such radiation is then reflected by reflectors 31a,
31b, 31c, 31d, 31e, and 31~ through analyte chambers 3, 3a,
3b, 3c, 3d, and inert gas chamber 4.
This focusing by the elliptical reflector onto
reflecting surfaces of column 31 permits detector readings
to be obtained with little expenditure of energy, allowing
a relatively low-powered source to be used. It has been
found, for example, that the source, the detectors and the
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data collection and analysis device can be operated in one
embodiment on a total power of approximately 400 mw.
The data received from detectors 26, 27, 27a,
27b, ~7c, and 27d are analyzed in known fashion. If the
unknown sample in sample chambe~r 2 does not contain the
analyte gas, the difference between the output of detector
26 and the other detectors wil:L remain at a fixed known
value (when corrected for temperature variation by means of
thermistor 31). If, however, there is some amount of one
of the analyte gases in the sample in sample chamber 2, the
difference between the output of detector 26 and the
detector associated with that analyte ~as will exhibit a
reduced value characteristic of the concentration of the
analyte in sample chamber 2.
In Figure 3 the reflector is an ellipsoid (rather
than an elliptical cylinder) with f3 and f4 being the foci
of the ellipsoid. Thus, the entire wall 50 is formed as an
ellipsoid. A source 51, which is as close as possible
being a point source, is placed at focus f3. A
multifaceted reflector 52 is placed at focus ~, and holes
are made in the ellipsoid at the points on the wall 50 to
which the facets of reflector 52 direct the beams of
radiation. Behind each of these holes is placed an analyte
or inert gas chamber and a detector. Two holes 53 and 54
are shown, and one associated chamber/detector assembly 55
is shown schematically.
In the example given, only two of many possible
radiation paths are shown, and the sample gas could flow,
for example, through porous supports 56 and 57.
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 departiny
from the spirit of the invention, which is as set out in
the appended claims.
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