CA2395563C - Novel device and method for gas analysis - Google Patents

Abstract

A novel device for analysis and detection of one or more gases or gas components of a gas mixture is disclosed. At least one gas sensor is installed in a capillary tube so that the target gas molecules diffuse through the tube length before affecting the said sensor and the transient response of the device to the target gas is mainly determined by the progress of the said diffusion process. The said transient response is then recorded and used as a measured value so that the target gas detection and diagnosis ensues from its comparison with the results of the previous experiences, calibration tests or mathematical simulations. More elaborate embodiments of the invented device include plurality of capillary tubes, each furnished with its respective gas sensor(s), forming an array or a bundle gas analyzer.

Description

NOVEL DEVICE AND METHOD FOR GAS
ANALYSIS
1, Introduction and Review of the Prior Art This invention relates to the technical field of gas detection, diagnosis, and analysis;
that is to say methods of analysis and detection of one or more gases present in an atmosphere, or gas components of a gas mixture, and apparatus for use in performing such methods. This invention is of particular significance in sensors, sensor arrays and electronic nose fabrications.
Gas sensors are used for atmospheric monitoring in general, e.g. ~ coal mines, offshore installations and industrial production facilities. Gas sensors are also used to control combustion processes in engine exhaust systems, etc., for both economical and environm~tal reasons. In simple terms, a gas sensor performs as the nose of a robot or an electronic control system. The requirement for monitoring toxic gases in the environment has steadily increased in recent years as safety and health professionals have become increasingly aware of the dangers posed by these substances. Greater awareness has further prompted government regulations to address environmental monitoring and related issues. Although such monitoring serves to protect the environment as a whole, the safety of people in the workplace continues to be of most vital concern. In this regard, most toxic gases have various levels or limits, set by industry associations or regulatory agencies.
Typically, several levels are defined for each type of gas. For example, a threshold limit value sets the maximum allowable level of a gas that a Person may be exposed to for an eight-hour period, five days a week. The short-term exposure limit gives the maximum exposure that a person may be exposed to for a fifteen minute period not to exceed four occurrences per 8-how work day. The permissible exposure limit is the maximum limit a person may be exposed to the gas for any time period.
A~ere~nce to these standards requires a toxic gas detector capable of accurate detection of the toxic gases of interest. Further, as these various exposwe limits may span a large range of concentrations, the toxic gas detector must accwately measure the concentration over a wide range of concentrations. Thus Presently, the technical demands for devices which can selectively detect a certain gas or diagnose a prevailing gas or odder is rapidly increasing. Such devices, usually referred to as electronic noses, are now employed for security checks and identifications, drug defections, chemical assessments, food quality control, chemical and biochemical process control, ... etc.
A reliable and precise gas diagnosis is usually carried out by sophisticated spectroscopy or other analytical systems. However, for the said applications compactness, rigidity, simplicity, and low price are of importance. The present invention intends to provide a novel device and method for fast gas analysis.
The device is compact, simple, rigid, stable and cost effective. The device can be fabricated based on practically all the available types of gas sensors; and plwality of such devices can form reliable electronic noses.
According to the prior art, a selective gas detection, analysis and gas diagnosis have been subjects of marry investigations:
1. According to US Patent No. 4,911,892, a selective sensitivity to a pre-selected gas is achieved in a resistive semiconductor gas sensor by the means of surface decoration with noble metals such as platinum and palladium.

2. According to US Patent No. 4,347,732, selective detection of gases of certain molecular size range has been afforded by incorporation of molecular sieves onto a Zn0 based resistive gas sensor.

3. According to US Patent No. 6,284,545, a filter made of a high surface area substrate impregnated with a silver salt or copper salt, incorporated onto a gas sensor is effective in reducing the cross-sensitivity to certain gas species.

4. US Patent 6,263,723, discloses a multilayer gas sensor element having properties capable of detecting methane and carbon monoxide selectively with only 1 sensor by improving the selectivity of the semiconductor gas sensor.

5. CA Patent Application No. 2326210 discloses an electrochemical gas sensor based on a polymer solid electrolyte which selectively detects only the hydrogen concentration in the atmosphere.

6. US Patent No. 6,353,225 discloses a method for the selective detection of gases by an optical gas sensor, in which the emission spectrum of a laser diode is varied by temperature variation to match the characteristic absorption line of the target gas.

7. Canadian Patent No. CA 1248176 discloses a gas analysis method based on the differential temperature variation of the detection sensitivities and transient responses of a resistive gas sensor for various gases. The same method has pr~~t~ ~~~ selectivity by application on sensor arrays.

8. US Patent No. 399,122 discloses a selective sensitivity field effect transistor gas sensor, the selectivity of which arises from the selective gas absorbent material deposited on the gale electrode.

9. Chul Han Kwon et al. have described fabrication of a multi-layered resistive gas sensor which achieves considerable selective sensing via catalytic filtering technology in: Sensors and Actuators B, 65,1-3, p. 327-330 (2000).

10. Different methods for selectivity enhancement in resistive gas sensors are also the subjects of the following articles:
a. A. Cirera, A. Vila A. Dibguez, A. Cabot, A. Cornet, and J. R. Morante, Sensors and Actuators B 64,1-3, p. 65-69, (2000).
b. I. Simon, N. Biirsan, M~ Bauer, and U. Weimar, Sensors and Actuators B, 73, p. 1-26 (2001).
c. D: S. Lee, d: K. dung, J. -W. Lim, J. -S. Huh, D: D. Lee, Sensors and Actuators: B, 77, p. 22&236 (2001).
Furthermore, higher levels of selectivity in gas sensing and diagnosis is achieved by using sensor arrays rather than a single sensor element. These are exhaustively covered in more recent patent applications and related technical journals. As an example, Rajnish K. Sharma et al. have presented resistive sensor arrays which are selectively sensitive to carbon monoxide and hydrogen: Sensors and Actuators B: 72, p. 160-166 (2(101). Also, Canadian patent applications CA 2314237, CA
2215332, and CA 2264839 have presented many examples from the prior art regarding electronic noses.
2. Summary of the Invention As described in the prior art, the transient response of a gas sensor to one target gas is different from the same for another. The difference in the said transient responses has been used in the prior art (e.g. see, Canadian Patent No. CA
1248176) for selective gas detection, but not only those response are depend~t on the nature of the target gas but also simultaneously on the concentration of the said gas. In this situation, the information regarding the nature and concentration of the target gas is intricately woven and extraction of diagnostic data from the said responses is difficult.
The present invention is based on the fact that the diffusion constant of a target gas, in air or in another gas, depends strongly on its molecular structure. For example, if the open end of an air filled capillary tube (pipe) of lrnown length is inserted into a chamber containing air polluted with a target gas of low concentration level, the time at which the concentration of the target gas at the other end of the tube reaches a predetermined portion (e.g. 20%) of that in the chamber d~ends strongly on the nature of the target gas, while the said time is, as shown later in the present disclosure, almost independent from the gas concentration level. The idea is that, by recording the progess of the diffusion process in such a tube and comparing the result with the results of previous experiences, the target gas can be identified.
Another important advantage of the present invention over the other diagnostic gas sensors and methods of prior art is the fact that the diffusion theory is well established (see e.g. J. Crank, KThe Mathematics of Diffusion", Ozford University Press, Ozford, 1975) and almost accurate simulations of the diffusion process for one or more lrnown gases in a tube are possible. This advantage further facilitates comparison of the recorded data with the results of computer simulations.

Yet another advantage of the present invention is the simplicity and stability of the process and the parts of the device that determine the transient response; the structiue of the tube and the said diffusion process would not alter by aging of the system, while the transient responses relied upon in the prior art are related to complex electrochemical solid / gas interactions and prone to changes caused by aging. The method is also versatile, as it can be applied to almost all types of gas sensors; but those with fast responses and compact embodiments (e.g. see, Canadian patent appUcation CA 2,267,881) are preferred. Ideally the response of the sensor employed must be much faster than the said diffusion process. In this case, the significant features of the transient response of the invented device are independent from the instabilities encountered with the sensor employed.
The instant invention uses one or more of the said tubes, each fiunished with one or more gas sensors to record the progress of the diffusion process of the target gas molecules along the said tube(s). The recorded or stored data regarding the progress of the said diffusion process are then used for the diagnosis of a target gas or analysis of a gas mixture. This is achieved by a comparison of the said data with the recordings from the previous experiences, calibration recordings, or mathematical simulation results. The basic idea described is applicable for various important technical needs. Dii~erent embodiments of the present invention are employed for fulfillment of its various aspects:
I. It is an object of this invention to diagnose a target gas by recording the progress of its diffusion process against time in an air filled capillary tube via one or more gas sensors installed appropriately in the tube.
2. It is another object of this invention to diagnose a target gas by recording the progress of its diffusion process against time in a capillary tube filled with a predetermined gas or gas mixture via one or more gas sensors appropriately installed in the tube.
3. It is another object of this invention to measure the diffusion constant of a known gas in air or in an another gas or gas mixture by recording the progess of its diffusion process against time in an air or gas filled capillary tube via one or more gas sensors installed appropriately in the tube.
4. It is another object of this invention to carry out one or more of the functions given in 1, 2, and 3, at an elevated or reduced temperature, by provision of the appropriate heating or cooling apparatus for the said tube(s), respectively.
5. It is yet another object of the present invention to carry out one or more of the functions presented in 1-4 at an elevated or reduced pressure.
6. It is another object of the present invention to perform the functions described in 1-5 for two or more target gases simultaneously, where there are more than one target gases present.
7. It is yet another object of the present invention to perform the functions described in 1-6 by using plurality of identical or different (in geometry or material) tubes furnished appropriately with plurality of identical or different gas sensors.
8. It is yet another aspect of the present invention to present methods for simple and fast extraction of the useful diagnostic and analytical data from the transient responses recorded while performing any of the functions related to the aspects described in I -7.
3. Brief Description of the Drawings The above objects of the invention will become clearer by reference to the attached drawings in which:
Fig. I:
Schematic illustrations of 4 different embodiments of the device invented.

(a). One gas sensor is installed at the closed ~d of the diffusion pipe;
target gas diffusion takes place from the open end. The diffusion starts upon the removal of the gas impermeable lid of the tube, or by insertion of the open end into a target gas contaminated camber.
(b). One gas sensor is installed at a predetermined distance (L) from an open ~d of a long tube. The target gas diffuses from the open end of the pipe, as described for (a).
(c). The tail of the pipe shown in (b) is connected to an "exhaling" device which provides a flow of pure air or a known pure gas or gas mixture, in the opposite direction of the said difl'usion process, for facilitating a rapid recovery of the device from one test for starting another. The diffusion of the target gas starts after the flow is stopped.
(d). One gas sensor is installed in the diffusion pipe, preferably at its mid point and the diffusion takes place from both ends of the pipe simultaneously, the diffusion process starts as described in case of (a) .
Table 1: Diffusion constants of three organic vapors in air, as given by CRC
Handbook, and as resulted by the analysis of the transient response of the prototype fabricated comprising a quartz glass diffusion pipe of 4 mm bore and 70 mm length and Zn0 resistive gas s~sor.
Fig. 2:
(a). Analytically predicted normalized transient responses of the prototype fabricated comprising a diffusion pipe of 4 mm bore and 70 mm length and a fast resistive gas sensor, simulated for methanol (A), ethanol (B), and butanol (C).
(b). Traces of the first time derivatives of A (A'), B (B'), and C (C'); bars 1, 2, and 3 indicate the positions of the deflection points on A, B, and C, respectively.
Fig. 3:
Normalized transient responses of the prototype fabricated comprising a quartz glass diffusion pipe of 4 mm bore and 70 mm length and a Zn0 resistive gas sensor, for methanol (A), ethanol (B), and butanol (C) and their respective first time derivatives A', B' and C'; the deflection points of A, B, and C are indicated by the bars 1, 2 and 3, respectively. The thickness of the bar 2 indicates the range of deflection times obtained when concentration of ethanol varied in the range of - 5000 PPM in air. The trace S is the transiea~t response of the gas sensor used (without difl'usion pipe) given for the purpose of comparison.
4. Detailed Description of the Invention Semiconductor gas sensors, particularly resistive semiconductor gas sensors are compact, fast and cost ei~ective. They can also be integrated with the signal processing circuits required, but the information related to the nature of the target gas and its concentration is intricately woven in their responses, and generally, the diagnostic data extraction from the recorded responses of these sensors is difficult.
A novel device is disclosed wherein the diagnostic information regarding a target gas is readily extractable from its transient response to the said target gas.
In its most basic structure the invented device consists of a resistive gas sensor and a capillary tube. However, different embodiments of the invention can be fabricated by using other types of gas sensors, including but not limited to semiconductor, capacitive, optical, polymer, electrochemical, catalytic, capacitive, electrothermal types; the priority is inclined towards the sensors of compact sizes and fast responses.
The fact that the transient responses of many common gas sensors vary from one target gas to another has been used for gas diagnosis in the prior art. In this invention, however, the transient response employed for gas diagnosis, in fact, is not a feature of the gas sensor employed but is the result of the selective retardation imposed by the diffusion process through a specific diffusion pipe. The latter process is physical, simple and stable in nature over the lifetime of the device; while the transient responses in the prior devices are determined by complicated electro-chemical processes taking place at their sensitive surfaces and can change considerably during its lifetime.
The invention will be discussed in greater details for a few non-limiting example embodiments applied for some example applications:

Example 1:
In the example embodiment, presented schematically in Fig. l a, the device is comprised a gas sensor (e.g. in the prototype fabricated a thick-film zinl:
oxide resistive gas sensor was used), and a "diffusion pipe" of known diameter and length (e.g. in the prototype fabricated a quartz glass capillary of 4 mm internal diameter and 70 mm length was used). The gas sensor located at the closed end of the pipe is connected to a digital recorder. The pipe is filled, and in equilibrium, with clean air at atmospheric pressure. An impermeable lid physically isolates the pipe from the surrounding atmosphere which is polluted with a target gas of constant concentration Co. (The existence of the said lid is not necessary in most of the embodiments and applications of the invention, as is described in the other examples below.) In the following paragraphs a very brief quantitative analysis of the device is presented in a more technical language.
Upon the removal of the lid, at time (t) = 0, target gas molecules diffuse through the air along the pipe before affecting the gas sensor. The target gas concentration along the pipe is a function of time and distance from the open end of the pipe, C(x,t). The diffusion constant of the target gas in air, D, is related to its molecular structure. The quantitative relationship between D and the molecular parameters of the diffusing gas has been thoroughly discussed in the literature; a comprehensive account of the subject is presented by RE. Treybal, in KMass Transfer Operation" 3'~ Edition, McGraw-Hill (1980). The effective target gas concentration experienced by the gas sensor, C(L, t), and hence its transient response, G(t), would depend on the nature of the target gas. The transient responses of an ideal prototype, comprising the above given tube and an ideal gas sensor, for three different target gasses are simulated bellow. It is shown that TG
can be diagnosed by a simple mathematical operation on G(t). For the sake of the simplicity of the calculations involved, it is assumed that the response of the gas sensor to the prevailing target gas is a step fimction the amplitude of which is proportional to the effective gas concentration. The amplitude normalized G(t) and C(L, t) would then be identical. In the prototype fabricated the response of the gas sensor employed was much faster than the target gas diffusion process through the pipe used, and the approximation applied proved to be valid.

C(x, t) was obtained by solving the diffusion equation for the geometry of the diffusion pipe employed. For a pipe with an internal diameter of a few millimeters, Fick's law is applicable. However, Knudsen diffusion equation should be considered for pipes of much smaller bores (see e.g., J. Szekly et al., KGas-Solid Reaction", Academic Press, 1979). The following boundary conditions were applied:
dC(x, t) = 0 at x = L all t dx C(x,t)=Co at x=0 all t Normalized solutions, obtained for three different values of D, are presented in Fig.2a. The D values inserted were those of CH30H, CZH30H, and C4H90H vapor in air, as given in CRC Handbook of Materials Science Vo1.14 (1984), which are also presented at the first row of Table 1. An algebraic solution was not possible, solutions were calculated numerically and results are presented in Fig.2. The functions traced in Fig.2a are the transient responses of the prototype device predicted for methanol (A), ethanol (B), and butanol (C), respectively.
The G(t) functions produced, as shown in Fig.2a, have deflection points. The calculations carried out indicate that the deflection always occurred before the corresponding G(t) had gained 20% of its maximum value. The deflection time, td, is independent from Co and is related to the length of the pipe and the diffusion constant of the target gas in air (to is almost proportional to L~2/D). Hence in theory, to is a characteristic parameter of the prevailing target gas molecule. In Fig.2.b the predicted deflection times for methanol, ethanol and butanol target gases are depicted by the bars 1, 2, and 3, respectively. A', B', and C' curves in Fig. 2b, are the first time derivatives of A, B, and C, respectively, the maximum points of which coincide with the deflection times predicted.
The analytical results were verified experimentally. The sample device fabricated consisted of a 70 mm long, 4 mm bore quartz tube and a thick film zinc oxide resistive gas sensor. The normalized transient response of the gas sensor employed (without the tube), to 4000 PPM of methanol is give in Fig.3 as trace (S). The recorded transient responses of the fabricated device for methanol, ethanol and butanol are traced in Fig. 3 as A, B, and C, respectively. The respective deflection times of the traces A, B, and C are depicted as bars 1, 2, and 3. In order to show that the deflection time is independent from the target gas concentration, the ethanol concentration was varied from 500 to 5000 PPM. The width of the bar related to ethanol, bar 2 in Fig.3, indicates the variation range of the deflection times measured for various concentrations.
The minor differences between the experimental results (Fig.3) and the predictions of the simulation (Fig.2) are caused by the deviations of the response of the gas sensor used from the assumed step fimction. It was discovered that by using a gas sensor of faster response the transient response of the prototype fits more accurately to those predicted analytically. Appropriate corrections could be applied for more elaborate diagnostic tests, but even without arty correction, the simple prototype fabricated could reliably differentiate among the three alcohols mentioned, regardless of their concentration. However, in real applications diagnosis is carried out by comparison of the recorded transient response with those of the previous experiences, which would make the said corrections unnecessary.
Another technically important point is that by using the above described method of gas diagnosis, the transient response of the device is needed to be traced only up to its deflection point, i.e. when the first time derivative of the response passes through a maximum. Hence, the time required for each diagnostic test is almost equal to td;
the extension of the curves beyond this point in Fig.3 is for the purpose of comparison with the simulation results given in Fig. 2.
It was discovered both analytically and experimentally that the differential diffusion retardation caused by the diffusion pipe was more profound in longer tubes; it was shown that the deflection time is almost proportional to the square of the length of the pipe. H~ce the diagnostic power of a device of longer pipe is higher where it is a slower device and its diagnostic tests will take longer.
It was discovered both analytically and experim~tally that the performance of the device is insensitive to the diameter of the pipe employed. This is valid for pipes of down to a few t~th of a millimeter bore. Further smaller bore diameter pipe, although different in the analytical description of its characteristic behavior, in practice should indicate higher selectivity because of the comparable size of the pipe diameter and the "molecular mean free pass" of the target gases of interest which should cause even stronger diffusion discrimination among the said target gas molecules. That is to say, the diffusion time difference between methanol and S ethanol vapors would be higher in a 10 micrometer bore pipe than in a 1 mm bore one. Moreover, smaller diameter pipes would render compact devices, particularly when plurality of pipes and plurality of gas sensors are to form an array sensor for electronic nose applications.
It was discovered both analytically and experimentally that the device operation was independent from the material of the diffusion pipe employed when the pipe diameter was in the millimeter range. Various ceramics, glass, metal and polymer pipes tested rendered similar results. However for much smaller diameter pipes, and particularly in the case of very low target gas concentrations, the interaction between the pipe wall and the diffusing target gas molecule becomes considerably important. This also can add to the selective sensitivity of the sensor in special conditions.
In the above described example embodiment of the instant invention, the simple arrangement employed included only a proper attachment of a glass capillary to a non-selective, non diagnostic bare and simple resistive gas sensor. It was shown both analytically and experimentally that the combined device has very reliable gas diagnostic features.
Since the theoretical basis of the present invention was also briefly described in the Example 1, in the following examples however, the device structure and the example application will be presented only.
Eaample 2:
The same embodiment of the invention as that used in Example 1 is employed here to diagnose and measure the concentration levels of an unknown target gas in air, in a closed chamber. The device was in equilibrium with the pure air outside the chamber; the open end of the tube was inserted to the chamber when the recording of the response of the sensor was started simultaneously. The transient response and its first time derivative, i.e. G(t) and dG(t) / dt, were both traced on a CRT
screen.
The traces resembled those given in Fig.3 after amplitude normalization. It was observed that in about 30 seconds after the beginning of the recording, the derivative trace passed a maximum point and started descending. This is the deflection time of the transient response, and the measurement was complete.
Two distinct features of the recorded transient response could easily be extracted and compared with the previous experiences of the device:
a The deflection time; measured as 29.5 seconds; where the best match among the previous calibration recordings was the average deflection time obtained for ethanol vapor. The diagnosis of the target gas was correct.
b. The value of the transient response (before normalization) at the deflection time was directly related to the concentration of ethanol vapor in the chamber via the previous calibration charts drawn for ethanol vapor;
resulting 2400 PPM for the ethanol concentration. The calibration charts were almost linear which fiwther simplified the concentration calculation process.
In this experiment using the device and the method invented, the prevailing target gas was identified and its concentration in air was measured simultaneously.
All this was achieved by using a general resistive gas sensor and a capillary tube.
'The diagnostic decision regarding the nature of the gas in this example was based on a single readout from the "time axis", while the concentration information was read from the "amplitude axis"; this isolation of information regarding the nature and concentration of the target gas is of technical significance. The fimctional steps of recording, detection of the deflection point, deflection time readout, amplitude readout, and comparisons of the read figures with the calibration tables, all can be carried out online with a personal computer.
Example 3:
The embodiment described in Example 1, was applied for selective detection and diagnosis of hydrogen in various gas mixtures. Owing to the fact that the diffusion constant of hydrog~ is much smaller than the other target gases of interest, its respective deflection point in the transient response of the device was distinct and easy to identify. Using the prototype device described above the deflection point related to hydrogen occurred at about 7 seconds. Using the same experimental set up as described for Example 2, the same prototype was able to detect hydrogen as the target gas in many gas mixtures. That is to say, the device performed as a reliable hydrogen detector at the presence of other target gases such as methane and butane.
Example 4:
The prototype used in this experiment had a structure similar to that of Examples 1-3, different in that a micro-heater had been attached to the diffusion pipe so that it could provide a constant elevated temperature along the diffusion pipe.
Experiments of Example 1 were repeated for pipe temperatures of 80,120, and 160 °C.
The deflection time obtained for a particular gas reduced as the pipe temperature increased, which is an indication of the dependence of the diffusion constant of the target gas on temperature of the diffusion medium. This dependence, best described as D(T), is also of diagnostic value. That is to say, by application of two or more devices, the pipes of which are at different temperatures, simultaneously for performing arty of the tests of Examples 1- 3, the recorded data would have resulted the diffusion constant of the target gases) in three different temperatures, making the identification process more elaborate.
Example 5:
A different prototype was fabricated by using a 25 cm long and 4mm internal diameter tube. The same resistive gas sensor as of the previous prototype was located inside the pipe at a point 70 mm away from the open end. The prototype is schematically presented in Fig.lb. The open end of the pipe was inserted to the gas chamber for any of the above described tests. As the pipe is considered infinitely long in the calculations and in practice, whether the far end of the pipe is open or closed was of no significance in the analytical predictions and experimental results.
The prototype was equally successful in performing the tests given in Examplesl-4.
A theoretical analysis of this device was carried out by solution of the diffusion equation assuming that the diffusing species observe the tube as an infinitely long one. Hence the boundary conditions of the problem differed from those given in Example 1. An exact solution of the differential equation is possible in this case (they are error functions), which makes the theoretical predictions easier.
The mathematical analysis of the transit response of the device was also possible;
it was carried out by a summation of the solutions obtained for each of the gas components individually. The analytically predicted transient response for a known mixture of two different target gases in air, both at low concentration levels, was verified experimentally. The same method of analytical prediction was applied to the case of closed end prototypes described in previous examples; it was equally successful, but the numerical calculations involved required longer calculation times. The fact that the transient response of the invented sensor is analytically predictable for mad gas mixtures of interest is unique among the compact gas sensors available, and is of great technical significance.
Eaalmple 6:
The recovery times of the prototypes illustrated in Fig. l a-b were long. That is to say, after completion of a test, it took a few minutes for the device to get ready for another measurement; because this time was necessary for the traces of the target gas to diffuse out of the pipe. Another embodiment of the present invention, shown schematically in Fig. l c, presented a much shorter recovery time. In this prototype the prototype described in Example 5 was connected to a pure air container of positive relative pressure by a flexible capillary (e.g. pvc tubing) and a controlled electric valve. The valve is closed during the test, but it opens a weak stream of pure air briefly prior to each test The flow is in opposite direction of the target gas diffusion, forcing the target gas molecules out. That is to say, the prototype had an "exhaling" mechanism which shortened the recovery time.
Moreover, the prototype described did not need the impermeable lid mentioned in Example 1, recording started as the valve abruptly stopped the flow of the pure air.
The recorder was synchronized with the valve as its closure initiated the recording.
The amount of air flown was negligible compared to our test chamber and did not alter the conca~tration of the target gas in the latter. This prototype could provide multiple read outs from the atmosphere of a chamber when the open end of its pipe was inserted into the chamber. In other words, it could provide a continuous monitoring of a chamber atmosphere, e.g. once in each minute or two, while no disconnection from the chamber was required. The same ''exhaling" concept could be applied in conjunction with all of the above described prototypes. In case of the closed end diffusion pipes (as in Example 1 ) the exhaling stream was provided through a small hole at the close end.
Example 7:
Yet another version of the prototype described in Example 6 was fabricated which was different in using a stream of a pure gas instead of pure air for "exhaling". Such a device facilitated the measurement of the diffusion coeffici~t of one gas in another. As an example diffusion constant of hydrogen in nitrogen could readily be measured with this prototype. Also, by altering the present embodiment according to the technical feature described in Example 4, these measwements could be carried out at differ~t temperatures. This prototype was modified by installing two or more sensors along the tube, so that more data readout was possible, each related to a different effective pipe length, at each test. The latter prototype afforded a more ~~ra#e measurement of diffusion constant.
Assuming a sealed chamber, its pressure could be altered. The pipe is physically in equilibrium with the chamber immediately after the "exhaling", and the diffusion coefficients mentioned could be measured in different pressures. Hence the modified prototype provided a precision instrument for the measurement of the diffusion coe~cient of a gas in another gas or gas mixture, at predetermined temperature and pressure.
Ezample 8:
The prototype described in Example 7 was used as an oxygen sensor. The stream of nitrogen or argon was used for device recovery. Oxygen could be detected and its partial pressure be measured based on the comparison of the transient response obtained with the results of the previous experiences. (The electrical conductance of the sensor decreased as the oxygen conc~tration increased; a trend opposite to all of the above given target gas examples) Ea~~rle 9:
The device described in Example 6, was applied to chambers containing air with differ~t mixtures of three target gases of interest. The transient response recorded for each target gas mixture was distinctly different from the others. This afforded the identification of the gas mixture by a comparison of the recorded trace with the previous experiences and /or the results of computer simulations of the transient response for gas mixtures.
Example 10:
Using more than one of the disclosed devices, each being different in one or more structural features simultaneously for probing of an unlrnown gas mixture would provide a vast source of information regarding the gas mixture tested which could in turn afford a more accurate diagnosis and analysis of the mixture. The said structural differences may include one or more of the: Gas sensor type used or its operating temperature, pipe effective length, pipe temperature, exhaling gas, pipe diameter or cross-sectional shape, pipe material, etc.
In this Example three devices with respective diffusion pipe lengths of 5, 10 and 20 cm, were applied simultaneously to the same experimental conditions as described in Example 9. The reliability and accuracy in determination of the gas mixtures increased.

Claims (11)

1. A device for gas analysis or detection and diagnosis of at least one target gas or one gas component of a gas mixture comprising one or more gas sensors and a hollow tube (pipe) of finite length, characterized in that the target gas affects the said gas sensors) only after diffusing through the length or a part of the length of the said pipe; and in that the temporal variation of the response of the device to the target gas is at least partly determined by the said diffusion process.

2. A device according to Claim 1, characterized in that the said pipe is made of glass, ceramics, polymer, metal or composite materials having any cross-sectional geometry with an area in the range of 10~-6 to 10~4 mm~2, and a length in the range of 1-1000 mm.

3. A device according to Claims 1 and 2, characterized in that the said pipe is full of pure air, a pure gas, or a known gas mixture, prior to each gas detection, or analysis run.

4. A device according to Claim 3, characterized in that the said pipe is connected to a selective flow of pure air, a pure gas, or a known gas mixture.

5. A device according to Claim 4, characterized in that its detection run starts immediately after stoppage of the said flow.

6. A device according to the Claims 1 and 2, characterized in that the temperature of the said pipe could be adjusted by a controlled heating or cooling system.

7. A device according to the Claims 1 and 2, in which the gas sensors employed are of electroceramic, semiconductor, optical, thermoelectric, capacitive, catalytic, acoustical, or electrochemical types.

8. A gas analyzer head comprising a plurality of the devices defined according to the Claims 1 and 2.

9. A hydrogen sensor made according to Claims 1 and 2.

10. An oxygen sensor made according to Claims 1 and 2.

11. A carbon monoxide sensor made according to Claims land 2.