Diffusion-type gas sample chamber

Apparatus for measuring the concentration of a gas present in a sample chamber by ambient pressure diffusion employs a nondispersive infrared gas analysis technique. The sample chamber has the form of a tube that is closed at one end, with a source of radiation and a detector mounted side by side at the other end. The inwardly-facing surfaces of the tube are specularly reflective, whereby the optical length of the sample chamber is twice its physical length. A gas filter cell located in the optical path permits the concentration of an analyte gas to be measured accurately despite the presence in the sample chamber of an interfering gas. A small ultrasonic vibrator affixed to the wall of the sample chamber prevents unwanted particles from accumulating on the semipermeable membranes through which ambient gases diffuse into and out of the sample chamber.

FIELD OF THE INVENTION 
The present invention is in the field of gas analysis, and specifically 
relates to apparatus using a nondispersive infrared gas analysis technique 
to determine the concentration of a particular type of gas present in a 
sample chamber by sensing the absorption of infrared radiation passing 
through the gas. 
THE PRIOR ART 
A comparatively new development in the field of nondispersive infrared gas 
analyzers has been the diffusion-type gas sample chamber. In a 
diffusion-type gas sample chamber, the gas to be measured enters and 
leaves the chamber by diffusion. 
One example of a diffusion-type gas sample chamber is described in the 
parent application. In that invention, the sample chamber has the form of 
a tube composed of a gastight material, having apertures covered by 
semipermeable membranes through which the gas to be measured enters and 
leaves the sample chamber by diffusion. This same approach is used in the 
present invention, with some important modifications. 
Another example of a diffusion-type gas sample chamber is described in U.S. 
Pat. No. 4,709,150 to Burough et al. In their invention, the body of the 
sample chamber is composed of a porous material through which the gas to 
be measured passes by diffusion. Burough et al. do not teach or suggest 
using the walls of the porous tube as reflective radiation-guiding 
elements. 
An example of a non-diffusion-type gas sample chamber is shown in Japanese 
Patent Publication No. 59-173734(A) of Miyazaki. In that analyzer, the 
sample cells have the form of helical tubes. The gas to be measured must 
be pressurized to force it to flow through the sample tube. 
Another example of a non-diffusion-type of gas sample chamber is shown in 
Japanese Publication No. 63-298031 by Fujimura, in which air is rammed 
into the sample chamber by motion of the sample chamber through the air. 
In the present application, the inventor will describe improvements on the 
sample chamber described in the parent application, to improve its 
performance. 
In keeping with chemical practice, the gas to be analyzed is called the 
analyte gas. In the sensor of the parent application, the concentration of 
the analyte gas is related to the absorption of radiation by a gas sample 
containing the analyte gas. A steady amount of radiation is passed through 
the gas sample chamber. Ideally, the wavelength of the radiation coincides 
with the wavelength of an absorption band of the analyte gas, so that a 
reduction in the received radiation signals the presence of the analyte 
gas, and the amount of the reduction is related to the concentration of 
the analyte gas. 
In practically all applications, the analyte gas is not the only gas 
present in the sample chamber. Typically, one might want to measure the 
concentration of carbon dioxide in air or the concentration of carbon 
monoxide in exhaust gas. 
Depending on the gases present and on the wavelength of the radiation used, 
it can happen that an absorption band of one of the other gases present 
may partially overlap the chosen absorption band of the analyte gas. When 
this situation obtains, it is impossible for the detector to distinguish 
absorption caused by the analyte gas from absorption caused by the other 
gas, which is called the interfering gas. In the following description, 
the present inventor will disclose apparatus for solving this problem, 
thereby permitting reliable measurement of the concentration of the 
analyte gas despite the presence of an interfering gas. 
A second improvement described below arose from a different type of problem 
that could occur to the sensor of the parent application. In that sensor, 
ambient gas enters and leaves the gas sample chamber through apertures in 
the wall of the chamber that are covered by a semipermeable membrane. If 
the semipermeable membranes were not used, there would be a tendency for 
particles of dust or smoke or microscope droplets of water or oil to enter 
the sample chamber and deposit themselves on the optical surfaces, thereby 
impairing the performance of the surfaces and conceivably providing 
interfering absorption bands. The purpose of the semipermeable membranes 
covering the apertures is to keep such unwanted particles and droplets out 
of the sample chamber, but without interfering with the desired diffusion 
of gases into and out of the sample chamber. 
In a dirty environment, where heavy concentrations of contaminant particles 
and droplets are encountered, it is possible for the semipermeable 
membranes to work so well that the membranes become clogged with the 
contaminants. This could impede the diffusion of gas through the 
semipermeable membrane. In the description below the inventor will 
disclose a solution for this problem. 
SUMMARY OF THE INVENTION 
The problem of an interfering gas in the gas sample chamber is solved in 
accordance with the present invention by inserting a gas filter cell into 
the gas sample chamber. Since the radiation emitted by the source at one 
end of the chamber is reflected from the other end of the chamber back to 
the first end, the radiation passes twice through the gas filter cell. 
The gas filter cell is filled with the interfering gas, which may even be 
pressurized to increase its concentration. In passing twice through the 
gas filter cell, the radiation generated by the source is greatly 
attenuated at wavelengths corresponding to the absorption bands of the 
interfering gas. Since interference occurs only at wavelengths where the 
absorption bands of the interfering gas overlap the absorption bands of 
the analyte gas, the great attenuation of the radiation of such 
wavelengths by the gas filter cell substantially eliminates the 
possibility of interference. 
Although gas filter cells have long been used for unidirectional filtering 
of a beam of radiation, no reference has been found to their use in a 
bidirectional mode. The bidirectional mode is especially attractive in the 
gas sample chamber of the present invention because it conserves space, 
the radiation pathlength being twice the physical length of the cell. 
In the present invention the gas filter cell is far superior to any 
conventional interference filter because the absorption spectrum of the 
gas filter cell necessarily is identical to the absorption spectrum of the 
interfering gas. 
The problem of dust particles and/or droplets clogging the semipermeable 
membranes was solved by affixing small low-powered ultrasonic vibrators to 
the body of the gas sample chamber adjacent the apertures through which 
the gases enter and leave the chamber. The vibrations are transmitted to 
the semipermeable membranes through the wall of the gas sample chamber. As 
the semipermeable membrane vibrates at a microscopic amplitude, portions 
of it collide with the unwanted particles and droplets knocking them away 
from the semipermeable membrane. 
The novel features which are believed to be characteristic of the 
invention, together with further objects and advantages thereof, will be 
better understood from the following description considered in connection 
with the accompanying drawings in which a preferred embodiment of the 
invention is illustrated by way of example. It is to be expressly 
understood, however, that the drawings are for the purpose of illustration 
and description only and are not intended as a definition of the limits of 
the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an absorption band 1 of carbon dioxide and an absorption band 
2 of carbon monoxide. In keeping with the usual practice in the art, the 
graphs of FIG. 1 show transmission through standard gas samples as a 
function of wavelength, so that the absorption bands will appear as dips 
in the graphs. FIG. 1 does not show the absorption spectrum of a mixture 
of the two gases. 
As indicated by the hatched area of FIG. 1, the two absorption bands 
overlap in the vicinity of 4.55 microns. 
If one were attempting to measure the concentration of carbon monoxide, it 
would be called the analyte gas. If the gas sample included some carbon 
dioxide, in addition to the carbon monoxide and some non-absorbing gases, 
then the absorption measured at 4.55 microns would be too large owing to 
the absorption by the carbon dioxide, leading one to think the 
concentration of carbon monoxide is greater than it really is. This type 
of error is referred to as interference, and the carbon dioxide would be 
called the interfering gas. Not only is the total absorption in this case 
greater than it should be if only the analyte gas were present, but to 
make matters worse, it is not possible to determine how much of the 
absorption is caused by the analyte gas and how much is caused by the 
interfering gas. 
Although interference is very detrimental to the objective of a gas 
analyzer, it can be used to advantage in other instruments, as described 
by the present inventor in U.S. Pat. No. 5,335,534 issued Aug. 9, 1994, 
for "Testing Method for Toxic Gas Sensors." 
The absorption spectra of all of the common gases are now well documented, 
and the possibility of interference can be determined by reference to a 
handbook, well in advance of any measurements. In most practical 
situations, interference is the rule rather than the exception. For 
example, gaseous products of combustion might be expected to include both 
carbon dioxide and carbon monoxide in proportions related to the 
completeness of the combustion. Therefore, it is highly desirable in a 
practical gas analysis instrument to be able to measure the concentration 
of the analyte gas in the presence of one or more interfering gases. How 
this is achieved in the present invention will now be described in 
relation to FIG. 2, which shows a preferred embodiment of the present 
invention. 
As shown in FIG. 2, the gas sample chamber of the present invention 
includes a tube 12 having a closed end 14 and having an open end. In the 
preferred embodiment, the tube 12 is composed of a metal, and has a square 
cross section. In other embodiments, the cross section is circular. 
The surface of the inner wall 16 of the tube 12 and the inwardly-facing 
surface 18 of the closed end 14 are specularly-reflective. 
In accordance with the present invention, the metal tube 12 is gastight and 
therefore filtering apertures, of which the filtering aperture 20 is 
typical, are provided at spaced locations along the tube 12 to permit 
molecules of the ambient gas to enter and to leave the space within the 
tube. Each of the filtering apertures 20 is covered by a sheet of a 
semipermeable membrane 22. 
In the preferred embodiment, the analyte gas is carbon dioxide, and the 
semipermeable membrane is composed of silicone rubber and is approximately 
25 to 50 microns thick. Because of its fragility, in the preferred 
embodiment the semipermeable membrane 22 is supported by a mesh 24 that 
spans the aperture 20. At this point in time, the exact number, location, 
and disposition of the filtering apertures does not appear to be crucial, 
although some as-yet-undiscovered arrangement may be optimal. 
The open end of the tube 12 is closed by a cap 32 in which are mounted a 
source 26 of radiation, a detector 28, and a narrow passband filter 30. 
The passband of the filter 30 is located at a wavelength at which the 
analyte gas strongly absorbs radiation and at which any other gases that 
might be present do not absorb. The plastic cap 32 serves to mount the 
source 26 and the detector 28 and the filter 30 in the open end of the 
tube 12 with the source 26 and the detector 28 facing the surface 18. 
Some of the radiation emitted by the source 26 is simply reflected from the 
surface 18 directly back to the detector 28. In FIG. 2, this component of 
the radiation is defined by the bundle 42 of rays. It is clear from FIG. 2 
that if this were the only mode of propagation, then only an extremely 
small fraction of the emitted radiation would reach the detector 28. The 
solid angle of the detector at a distance equal to twice the length of the 
tube 12 is extremely small. 
An important advantage of using the tube 12 is that it permits other modes 
of propagation from the source to the detector to occur. The amount of 
radiation contributed by the various modes of transmission is additive 
since the successive modes are characterized by progressively steeper 
rays. Compared with a simple plane mirror such as the surface 18, the 
addition of the tube 12 greatly increases the amount of radiation that 
reaches the detector 28. One might consider the bundle 42 of rays to 
represent the simplest or fundamental mode, and the ray 40 to represent 
one of the higher order modes of propagation. 
In addition to making it possible to utilize the higher order modes of 
propagation, the addition of the tube 12 produces a secondary benefit, 
namely, that the radiation travels a greater distance through the space 
within the tube as the order of the mode of propagation increases. That 
is, for the higher modes, the rays are steeper resulting in a greater 
distance of travel back and forth across the tube, notwithstanding that 
the distance traveled in the longitudinal direction remains constant and 
simply equals twice the length of the tube. 
In accordance with the present invention, a gas filter cell 50 is mounted 
within the tube 12 at any location between the detector 28 and the surface 
18. In the preferred embodiment of FIG. 2, the gas filter cell is mounted 
within the open end of the tube 12, immediately in front of the source 26 
and the detector 28, and the diameter of the gas filter cell is 
sufficiently large that substantially all of the radiation generated by 
the source 26 passes through the gas filter cell, proceeds the length of 
the tube 12, is reflected from the surface 18, proceeds the length of the 
tube 12, passes through the gas filter cell, and falls on the sensitive 
portion of the detector 28. Thus, the radiation passes through the gas 
filter cell twice-once from right to left in FIG. 2, and thereafter from 
left to right. A unique advantage results when a gas filter cell is 
mounted within the tube 12; namely, the radiation passes twice through the 
gas filter cell. As a result, the optical thickness of the gas filter cell 
is twice its actual mechanical thickness. 
In the preferred embodiment, the gas sample cell includes a front window 52 
and a rear window 54, both hermetically sealed to a cylindrical wall 56 
that may be composed of the same material as the windows or of a different 
material. The material of the windows 52 and 54 is highly transmissive of 
radiation in the wavelength interval corresponding to the absorption band 
of the analyte gas. The volume of space between the windows 52 and 54 is 
filled with the interfering gas. This volume of interfering gas 
substantially absorbs the spectral components of the radiation that lie 
within the absorption band of the interfering gas, thereby eliminating the 
interference by definition. That the small volume of interfering gas can 
have such a strong effect is not surprising when it is remembered that in 
many applications the interfering gas is present in the sample chamber in 
relatively small concentrations, whereas in the gas sample cell the 
concentration of interfering gas is much greater. In those rare instances 
where even greater absorption by the gas filter cell is desired, the gas 
in the gas filter cell can be pressurized and/or the thickness of the gas 
filter cell can be increased. In case more than one interfering gas is 
present in the ambient gas, the gas filter cell can be filled with a 
mixture of the interfering gases. Alternatively, several gas sample cells 
may be used in sequence, one for each interfering gas. 
The use of the gas filter cell 50 extends the performance of the gas sample 
chamber beyond that described in the parent application by enabling 
concentration measurements to be made on the analyte gas accurately and 
reliably, even in the presence of one or more interfering gases. 
The purpose of the semipermeable membrane 22 is to prevent airborne 
particles larger than a predetermined size from entering the space within 
the tube 12, while at the same time not interfering appreciably with the 
free diffusion of the ambient gas into and out of the space within the 
tube 12. The unwanted particles include minute droplets of moisture or oil 
and also include fine particulate matter such as particles of dust or 
smoke. If these unwanted airborne particles were to enter the space within 
the tube 12, they would deposit themselves onto the specularly reflective 
surfaces thereby reducing the reflectivity and destroying its specular 
nature. The unwanted particles would also deposit onto the source 26 and 
onto the narrow passband filter 30 reducing the transmission of radiation 
and possibly causing chemical changes to take place. All of these problems 
are eliminated through the use of the semipermeable membrane which, in the 
preferred embodiment prevents airborne particles larger than 0.3 microns 
from entering the space within the tube 12. 
The purpose of the semipermeable membrane 22 may be partly frustrated if 
the environment in which the gas sample chamber is used contains 
extraordinary amounts of dust or smoke particles, or of minute droplets of 
water or oil. During the useful life of the gas sample chamber such 
particles and droplets may accumulate on the semipermeable membrane to 
such an extent as to interfere with the desired diffusion. 
In accordance with the present invention, one or more ultrasonic vibrators, 
of which the vibrator 60 is typical, are affixed to the tube 12. The 
vibrators may operate on an electromagnetic principle or they may employ 
piezoelectric elements. The vibrators are located adjacent the apertures 
20 to minimize the power required to operate them. The vibration is 
transmitted through the wall of the tube 12 to the semipermeable membrane 
22, causing the semipermeable membrane to vibrate. The amplitude of the 
vibration may be extremely small--on the order of 0.001 micron--and 
vibrational frequencies on the order of 100 kHz are used to avoid any 
conceivable interference with the data processing portion of the gas 
sensor. 
A comprehensive theory of how the vibration of the semipermeable membrane 
prevents the unwanted particles from lodging on it has not yet been 
devised. At present the process is viewed as analogous to the condensation 
of water vapor onto a cold surface; the vibration being analogous to an 
increase in the temperature of the surface. 
The semipermeable membrane cannot prevent molecules (as opposed to 
droplets) of water from diffusing into the space within the tube 12, and 
if the components within the space are at a sufficiently low temperature, 
there is a possibility that the water vapor may condense onto the cold 
surfaces. To prevent that from happening, heater wires 34 are employed in 
the preferred embodiment to generate heat by ohmic heating when an 
electric current is passed through them. To minimize the escape of this 
heat, the metal tube 12, which is an excellent conductor, is provided with 
an insulative sheath 38. Likewise, the cap 32 is provided with an 
insulative casing 36. Because of the proximity of the wires 34 to the 
source 26 and the filter 30, these components are also protected from 
moisture condensing upon them. 
Thus, there has been described an improved diffusion-type gas sample 
chamber which differs from previous sample chambers in two important ways. 
First, one or more gas filter cells are located in the path of the 
radiation within the gas sample chamber to permit the concentration of an 
analyte gas to be measured accurately despite the presence of one or more 
interfering gases. Second, one or more small ultrasonic vibrators are 
affixed to the wall of the gas sample chamber to cause high frequency 
vibrations of small amplitude of the semipermeable membranes through which 
gases diffuse into and out of the chamber for the purpose of preventing 
unwanted particles from lodging on the semipermeable membrane and plugging 
it. 
The foregoing detailed description is illustrative of one embodiment of the 
invention, and it is to be understood that additional embodiments thereof 
will be obvious to those skilled in the art. The embodiments described 
herein together with those additional embodiments are considered to be 
within the scope of the invention.