US3617734A - Detection system for monitoring gaseous components in air - Google Patents

Abstract

A gas detection system in which air at near atmospheric pressure is pulse fed to a delay tube, which, because of difference in diffusion of the gaseous components, separates each pulse into components according to diffusion rate. The output of the delay tube is applied to an ionization detector having a feedback stabilized amplifier system capable of compensating long term variations without affecting the short pulses, for sensing the presence of a gas or gases of interest.

Description

United States Patent Julian H. Chaudet Fairfax, Va.; Harold G. Eaton, Jr., Washington, D.C.; Victor R. l-luebner, Diamond Bar, Calif.

[72] Inventors [51] Int. Cl H0lj 37/00 [50] Field of Search... 250/435, 43.5 R, 83.6 FT, 44; 73/23; 324/33 Primary ExaminerJames W. Lawrence [21] P 644205 Assistant Examiner-A. L. Birch t t d :J 2 Attorney-Hurvitz, Rose & Greene a en e ov. [73] Assignee Melpar,lnc.

Falls Church, Va. cominuafion'in'palt of application ABSTRACT: A gas detection system in which air at near at- 324371, 1963- mospheric pressure is pulse fed to a delay tube, which, because of difference in diffusion of the gaseous components. separates each pulse into components according to diffusion [54] ggggg g gggg gs gfig gg rate. The output of the delay tube is applied to an ionization 6 Cl 2 D i detector having a feedback stabilized amplifier system capable "9 7 m of compensating long term variations without affecting the [52] U.S. Cl 250/435, short pulses, for sensing the presence of a gas or gases of in- 73/23, 250/44, 250/836, 324/33 terest.

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OUTPUT R OPERATION AMPLIFIER V 5 H T a U .ww r K WW II? c INVENTORS JuuAM HCHAUOET, vucn'on R HUEBNER HHQOLD Gr. EATON,\\P. /aha; 614/ ATTORNEYS DETECTION SYSTEM FOR MONITORING GASEOUS COMPONENTS IN AIR CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 324,071, filed Nov. 15, 1963, entitled Gas Detector.

BACKGROUND OF THE INVENTION The present invention relates generally to detectors of the presence of certain gases in air, and more particularly to detectors of hydrogen, methane, or the like gases, which may be deleterious in certain locations.

Some gases of low molecular weight can be dangerous because they are explosive in air, when present in sufficient concentration. Such gases are hydrogen, which may subsist at missile sites; methane, which may subsist in coal mines; and certain explosive gases in submarines.

Present methods for the monitoring of low molecular weight explosive gases are performed by apparatus typically comprising hot-wire thermal-type detectors. These may be found in two forms:

A. A specimen of catalytic material, such as platinum, forms one arm of a Wheatstone bridge. The explosive air, coming in contact with the platinum, is oxidized to water; and the energy released leads to a temperature increase in the platinum and a corresponding increase in resistance. This resistance increase is manifest in a bridge unbalance, the degree of the unbalance being proportional to the quantity of combustible gas present.

B. A tungsten wire element is placed in one arm ofa bridge and heated electrically. When air passing over the element is contaminated with the explosive there is a change in the amount of heat conducted away from the bridge and a corresponding change in the resistance of the element. The resulting bridge unbalance is proportional to the amount of explosive gas present.

Thermal detectors are quite unstable, since it is practically impossible to avoid slight variations in temperature even when the gas to be detected is not being passed through them. This instability means that an attendant must be present almost constantly to make compensating changes in the instrument, to insure that it will respond properly to the combustible gases. Another inherent limitation in detectors of this type is that the thermal elements, whetherthey be platinum or tungsten, are easily eroded, and, as a consequence, they must be replaced as appreciable expense after a very shortoperational time.

SUMMARY OF THE INVENTION In accordance with the present invention, explosive gases of low moiecular weight in air are detected by employing differences of diffusion times of the gases with respect to the air, when both are present in a delay column, detection being accomplished by means of an ionization detector.

In accordance with the invention, airand a dangerous gas of low molecular weight, such as hydrogen or methane, are applied to a selective column, in bursts or pulses. Due to the greater diffusion rates of the dangerous gases, in comparison with air, each pulse, in passing through a delay tube, becomes comprised of a front of the dangerous gas followed by air. This effect is probably due to the greater rate of thermal difiusion of the lighter gases in comparison with the rate of air, but in any event the phenomenon occurs.

An ionization chamber, through which the gases and the air are passed, provides a current which depends on the gas which is passing. An operational amplifier is connected with the chamber, and a negative feedback path is providedfrom the output of the operational amplifier to its input. The loop, including amplifier and feedback path, has a very long time constant ofthe order of l or 2 minutes.

A potentiometer is provided in the loop, to enable adjustment to zero of the output of the loop. When air and a low concentration of gas of low molecular weight are passing, the

output of the system may be adjusted to zero by means of the potentiometer, and holds this adjustment due to the negative feedback and the fixed gain of the operational amplifier, for slow variations in temperature and/or humidity, and due to a safe concentration of hydrogen, methane or the like, or to slow variations from this concentration.

Periodically, the flow of air plus gas is interrupted for a short interval of time. On restarting the flow of air, due to differences in diffusion time, any low molecular weight gas precedes air in the delay column. When the gaseous front reaches the ionization chamber a rapid rise of output current results, if the concentration of the gas is high, but only a small rise if it is low. Thereafter, air reaches the chamber and response of the system again goes to zero.

The use of a gas diffusion delay device is to be distinguished from a chromotographic device. The latter requires selective adsorption. The former does not. It follows that a long column or tube can be used, which may be filled with filter material capable of enhancing the normal relative rates of diffusion for the gases involved, or which may be empty. The fact that delay devices, rather than chromatographic devices are employed, is a key feature of the invention, and implies that gases of light molecular weight can be distinguished from air without provision for carrier gas or effluent, or any particular chromatographic material. Great simplification of the system results, and its practicality for use in mines, or other regions, and generally as a portable equipment, is enhanced, since only the air present in the region need be pumped through the apparatus, or available. Either methane or hydrogen can readily be detected,.by means of the apparatus .of the invention, when present in appreciable quantities, and variation with time of the quantities readily become evident as variations of response of the detector. v

The detector employed is an ionization detector, originally developed by Pompeo and Otvos. (US. Pat. No. 2,641 ,llO) It utilizes the principle that different gases possess different ionization cross sections. As a consequence of the differences, a suitable detector will produce a change in current output when the gaseous composition varies. Lovelock et al., Anal.

Chem. 35:460 (1963) have found that the relative atomic ionization cross section for H, C, N, and O are 1.0, 3.69, 3.20, and 4.56, respectively. They have expressed the current, i, of an ionization detector having volumeV, as

JPV i= K Q.

Where P, R, and T, are the pressure, gas constant, and temperature, respectively, K is a proportionality constant, X is the molar fraction, and Q is the ionization cross section of the gas. It may be seen from this relation that for the detection of a single component, such as methane, the response from other components must either be so low as not to cause interference or they must be present at fixed concentration levels. Investigation has shown that carbon dioxide would have to be present to the extent of approximately 0.2 percent to cause interference, in the detection of methane. Its nominal concentration in a mine should be around 0.03 percent. Carbon monoxide must be present at a level of 5,700 p.p.m. to give a significant output from the detector; this is well above the tolerable level of 30 to 50 p.p.m. Thus, these two components in a mine atmosphere would not, except .under very exceptional conditions, lead to a significant detector response.

Oxygen and nitrogen in air, which are present at constant levels, wouldgenerate a response; but the detector in nulled out with air in it, and the air contribution to the detector output thus effectively eliminated. Water, as mentioned above, and more particularly variations in water levels in the atmosphere, would lead to a detector response which could be interpreted as a methane response; for instance, a variation in water content in the atmosphere by some 2 percent would lead to a detector response. This is equivalent to a 60 percent humidity change. The interference from water, although not great, can be eliminated by appropriate circuitry in the amplifier section of the detecting system.

The above relation also shows that the detector will respond somewhat to variations in pressure and absolute temperature. Slight variations with time in these parameters can be tolerated, but excessive changes, if not compensated, are not tolerable. Such changes are very slow, and are, in accordance with the invention, compensated by long time constant circuitry in the amplifying section of the detector.

A system of sensing methane, for example, consists essentially of the following components:

1. A Y connection for metering into the system, from tank reservoirs, of air and methane,

2. delay column packed with a molecular sieve 13X 3. a cross section ionization detector with power supply for sensing components coming from the column,

4. an associated electronic circuit for detecting the ionization within the detector,

5. a recorder which serves as a readout, and

6. pulsing devices for periodically interrupting flow of gas to the delay column.

In experiments conducted, air was first passed at a flow rate of 200 cc./min. from the air tank through the column and detector. A current output from the detector of l.3l l amperes was nulled to give a zero output at the readout.

Methane was then metered into the air stream to give the desired concentration level of methane in air. The detector response over and above the nulled value was found to be related to the methane concentration.

With the techniques indicated above, methane at a concentration level of 0.2 percent can be detected. Levels as high as 6 percent to 7 percent of methane in air can be detected without difficulty. The system responds to methane in a matter of seconds.

The detector itself consists of two parallel plate separated by an air gap of Az-inch, insulated from each other by a Teflon spacer. Tritum titanate foil pieces are placed on each of these plates. The activity of the tritium was approximately 0.6 curies for each piece. One of the pieces of tritium foil was connected to a power supply, indicated in FIG. 2 (22% volts) while the other foil was connected to a Jarrell-Ash electrometer amplifier. The air or air plus methane coming from the filter passes between the two plates on which the tritium foils are mounted.

The fl-emanations from the tritium under the applied potential gradient are sufficient to ionize the gas that enters the detector. It is to be noted that these emanations are of such a low energy level (18 kiloelectron volts) that shielding of the radioactive source is not a problem. The detector walls themselves form a adequate shield.

The response characteristics of the detector with air flowing through it at a rate of approximately 20 cc./minute shows a rapid rise followed by a flat region. The flat region of the curve, about 22% volts, the so-called plateau region, is the region in which the cross section detector operates. A large advantage is realized in operating the detector in this region in that slight changes in applied potential do not lead to a detector output variation. Thus, with this detector regulation of applied potential is not necessary. The plateau current is constant for a given gas or a given gas mixture; it only varies when the gas composition changes, i.e., for the instant application, when methane, or hydrogen issues into the detector.

The response of the detector was found to be linear with methane concentration; it amounted to a current of 8.8Xl0 amperes for each I percent of methane concentration. Since the noise level of the electrometer is only about amperes, detection at a level of 0.2 percent was realized.

The drift rate in the electronic system over a 5-hour period with the electrometer used was 2 l0 amperes. A solid-state electrometer can be provided which has practically a negligible drift rate.

Reviewing now the general system, continuous insertion of air into a delay column is accomplished, with no carrier. The column may be an open tube or a molecular sieve column employing a silicate or diatomaceous earth, where hydrogen or methane in air is to be detected. These materials have no chromatographic effect on air, methane or hydrogen, but merely impose different relative delays for the different gaseous components.

A solenoid, periodically energized by a timer or programmer diverts air momentarily at intervals of perhaps 1 minute. The flow is then reestablished. When reestablished the gases are allowed to diffuse at rates appropriate to each. Hydrogen or methane, if present, precedes the air components to the detector and generates there a pulse of output characteristic of the hydrogen or methane, where a considerable change exists in the concentration of the latter, as compared with air plus hydrogen or methane. The time constants of the feedback loop of the detector circuitry may be 2-5 minutes. This circuitry maintains zero reading for any given relative concentration of hydrogen or methane which subsists for a considerable time, and does so despite secular changes in pressure, temperature or humidity. A sudden change, however, as occurs following interruption of gas flow, and on its reestablishment, results in a short term change, to which the circuitry cannot accommodate in a short time. Gas feed pulsing enables detection of small quantities of hydrogen or methane, alone, by comparison with air plug hydrogen or methane.

Where very rapid temporal changes in concentration or methane occur gas feed pulsing may not be needed, but is in any event valuable.

It is an object of the invention to provide a novel system for measuring the percentage of certain gases in air.

Another object of the invention is to provide a system for measuring the content in air of hydrogen or methane, the system employing a differential gas diffusion column subjected to pulses of air containing either methane or hydrogen.

It is a more general object of the invention to provide a method and apparatus for measuring the content of one gas in another, by means of a delay column and an ionization tube, and without using chromatographic devices, where the gases have different diffusion constants.

BRIEF DESCRIPTION OF THE DRAWINGS The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a gas flow diagram according to the invention; and

FIG. 2 is a block diagram of a detector useful in the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, air from the open atmosphere or from any closed or semiclosed area under observation is drawn into a detection system embodying the concepts and principles of the present invention by a pump 10. The air, which may be contaminated with hydrogen, methane, or some other potentially harmful gas, is fed through a conduit or line 11 to valving apparatus to be utilized for pulse feeding the air to a delay tube or column 16, and thence to detector 17 and out via line 18.

Operation of the valving apparatus, to be explained in detail presently, to interrupt the flow of air (including any gases contained therein) from inlet line 11 to delay tube 16 at predetermined intervals of time quite obviously results in the introduction of gaps in the gas flow to the delay tube. Accordingly, a series of pulses of gas is applied to the latter tube as the gas flow is periodically halted and resumed. As a pulse of gas proceeds along the delay tube 16, the hydrogen or methane, if present, diffuses at a differential rate with respect to the air, i.e. O, N, H 0, and the like. In the case of hydrogen, which diffuses most rapidly, the hydrogen reaches the detector 17 ahead of time, with respect to the remainder of the gas. This condition subsists for a short, but sufficient time. During this time the response of detector 17 varies radically and indicates the presence of hydrogen. Promptly thereafter the flow of air into detector 17 resumes and response becomes zero. Similar effects occur in the presence of methane.

In is most basic form, the valving apparatus between inlet line 11 and delay tube 16 may consist of a single valve operable to alternately block and permit the flow of gas. Preferably, however, the apparatus between line 11 and tube 16 is as shown in H6. 1, and includes a needle valve 13 having an inlet V, to which line 11 is connected and a pair of outlets V and V, connected to lines 19 and 14, respectively. Further ineluded are a solenoid operated valve and a sample loop 20, the later arranged to receive an input via line 19. Valve 15 is provided with a pair of inlets V and V coupled to line 14 and sample loop 20, respectively, and a pair of outlets V,;, and V connected to delay tube 16 and an exhaust line for discharge to the atmosphere, respectively.

In operation of the system of FIG. 1, most of the pumped air passes through inlet V, and outlet V of needly valve 13, line 14, inlet V and outlet V of solenoidoperated valve 15, and thence into the delay column 16 and finally through detector 17. A small percentage of the total flow ofgas (i.e., a trickle or bleed flow), however, is metered through outlet V through the sample loop via line 19, and finally through inlet V and outlet V out into the atmosphere. It should be emphasized that the flow through the earlier mentioned path to the delay tube is normally several times as great as this trickle flow. However, the continual trickle of air through sample loop 20 results in its being charged with air, even though it may only be a small quantity, at all times. it should further be noted that the terminology sample loop" is employed only to emphasize the fact that a sample or small portion of the total flow is through loop 20.

Pulsing of the gas is accomplished in this embodiment of the invention by energizing the solenoid operated valve 15 to divert the flow of gas into inlet V to outlet V and thence to the atmosphere, while at the same time the trickle flow through sample tube 20 and into inlet V is directed to outlet V and into delay tube 16. In this manner the normal flow of gas into the delay tube is reduced to a trickle, and is subsequently resumed upon actuation of valve 15 to restore the original operational conditions, thereby pulse feeding the gas to the delay tube. For reasons which are not completely apparent, this operation results in greater sensitivity and response in the overall system. Onepossible explanation is that some gas flow under pressure is maintained at all times in the delay tube to provide a base or datum point for response of the detector. However, we do not wish to limit the invention to any particular explanation of theory, and in any event emphasize the point that adequate performance of the detector is achieved by simply interruptingthe flow of gas between inlet line 11 and delay tube 16 to pulse the delay tube, without provision of a trickle flow between pulses. Accordingly, the

sample loop 20 and the path with which it is associated may be dispensed with, if desired.

The detector 17 includes an ionization chamber 30, including two separated plates 31, 32, on which is placed tritium to include ionization of gas passing between the plates. Current passing between the plates, in response to voltage source 33, is a function of the gas, in terms of composition, pressure, temperature, etc., present between the electrodes. The current flow through ionization chamber 30 is bucked out by means of an adjustable voltage supplied by potentiometer 35, for the condition in which air plus hydrogen and/or methane flows simultaneously, and under given ambient conditions,

The output of ionization chamber 30 proceeds to the input of an operational amplifier 40 and thence to an output terminal 41. The latter is connected to an output recorder R. A negative feedback amplifier 42 connects the output of the operational amplifier 40 to its input, and the time constant of the loop is about 1 minute or more.

The feedback amplifier includes a feedback resistance 50 10" ohms) connected between its input and output and a relatively low resistance 51 to ground (47K) at the input side of the resistance 50. A time constant circuit 52, is interposed between the output of amplifier 42 and the input of amplifier 40. The circuit 52 includes two series resistances 53, 54 of value 10 and 10' ohms, and a shunt capacitor 55 connected from their junction to ground, and having a value of 2p" The presence of heavy feedback and a long time constant in the loop containing operational amplifier 40 stabilizes its output as temperature, humidity, or atmospheric pressure slowly varies, but does not prevent rapid changes in response. The relatively rapid changes in output of ionization chamber 30, which occur in response to hydrogen or methane alone, when these exist ahead of a column of air during pulsing, due to differential diffusion in the delay tube 16, are not appreciably reduced, due to the long time constant of the loop and are therefore indicated on recorder R.

While we have described and illustrated one specific embodiment of our invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be restored to without departing from the true spirit and scope of the invention as defined in the appended claims.

What we claim is:

1. A gas detector for detecting presence of a predetermined gas in air at near atmospheric pressure, comprising a differential diffusion delay column responsive differentially in respect to time to air and to said gas,

an ionization detector responsive to the diffused gas as it arrives at said detector from said delay column,

an amplifier system responsive to the output of said ionization detector,

said amplifier system having a delay time compensating the variations with time of pressure, temperature, and humidity of the air under observation,

means for adjusting said amplifier system to have a predetermined level response due to said air and said gas when combined, and

means for pulse feeding said air and gas to the input of said delay unit. 2. The combination according to claim 1 wherein said predetermined gas is methane.

3. The combination according to claim 1 wherein said predetermined gas is hydrogen.

4. An explosive gas detector, said explosive gas being intermixed with air at near atmospheric pressure, comprising a diffusive delay column having an input and an output for differentially delaying said gas and said air in passing through said delay column from said input to said output,

means applying said intermixed air and gas to said input in pulses,

an ionization detector connected to said output,

said ionization detector providing an output of amplitude determined by the character of the gas applied thereto, including its chemical composition, its temperature, its pressure and its humidity,

an amplifier system connected to said ionization detector,

said amplifier system having a zero stabilization time longer than the times of said the pulses of gas alone, and

means for setting said amplifier to have said stabilization while said gas and said air are concurrently and in steady flow passing through said column.

5. The combination according to claim 4 wherein said column is packed with molecular sieve.

6. The combination according to claim 4 wherein said ionization detector includes a radioactive source for ionizing the gas passing therethrough, and wherein the flow rate of the gas and air passing through said ionization detector is at least approximately 20 cc. per minute.

Claims (6)

1. A gas detector for detecting presence of a predetermined gas in air at near atmospheric pressure, comprising a differential diffusion delay column responsive differentially in respect to time to air and to said gas, an ionization detector responsive to the diffused gas as it arrives at said detector from said delay column, an amplifier system responsive to the output of said ionization detector, said amplifier system having a delay time compensating the variations with time of pressure, temperature, and humidity of the air under observation, means for adjusting sAid amplifier system to have a predetermined level response due to said air and said gas when combined, and means for pulse feeding said air and gas to the input of said delay unit.

2. The combination according to claim 1 wherein said predetermined gas is methane.

3. The combination according to claim 1 wherein said predetermined gas is hydrogen.

4. An explosive gas detector, said explosive gas being intermixed with air at near atmospheric pressure, comprising a diffusive delay column having an input and an output for differentially delaying said gas and said air in passing through said delay column from said input to said output, means applying said intermixed air and gas to said input in pulses, an ionization detector connected to said output, said ionization detector providing an output of amplitude determined by the character of the gas applied thereto, including its chemical composition, its temperature, its pressure and its humidity, an amplifier system connected to said ionization detector, said amplifier system having a zero stabilization time longer than the times of said the pulses of gas alone, and means for setting said amplifier to have said stabilization while said gas and said air are concurrently and in steady flow passing through said column.

5. The combination according to claim 4 wherein said column is packed with molecular sieve.

6. The combination according to claim 4 wherein said ionization detector includes a radioactive source for ionizing the gas passing therethrough, and wherein the flow rate of the gas and air passing through said ionization detector is at least approximately 20 cc. per minute.

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