US3617734A - Detection system for monitoring gaseous components in air - Google Patents
Detection system for monitoring gaseous components in air Download PDFInfo
- Publication number
- US3617734A US3617734A US644205A US3617734DA US3617734A US 3617734 A US3617734 A US 3617734A US 644205 A US644205 A US 644205A US 3617734D A US3617734D A US 3617734DA US 3617734 A US3617734 A US 3617734A
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- gas
- air
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- delay
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
- G01N30/02—Column chromatography
- G01N30/62—Detectors specially adapted therefor
Definitions
- 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.
- 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.
- gases of low molecular weight can be dangerous because they are explosive in air, when present in sufficient concentration.
- gases are hydrogen, which may subsist at missile sites; methane, which may subsist in coal mines; and certain explosive gases in submarines.
- This resistance increase is manifest in a bridge unbalance, the degree of the unbalance being proportional to the quantity of combustible gas present.
- 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.
- 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.
- 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.
- 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.
- a gas diffusion delay device is to be distinguished from a chromotographic device.
- the latter requires selective adsorption.
- the former does not.
- 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.
- 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.
- 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.
- 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 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:
- 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.
- 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 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.
- 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.
- gas feed pulsing may not be needed, but is in any event valuable.
- 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.
- FIG. 1 is a gas flow diagram according to the invention.
- FIG. 2 is a block diagram of a detector useful in the system of FIG. 1.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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"
- 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
- said amplifier system having a delay time compensating the variations with time of pressure, temperature, and humidity of the air under observation
- 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,
- 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,
- said amplifier system having a zero stabilization time longer than the times of said the pulses of gas alone
- 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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Abstract
Description
Claims (6)
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
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US64420567A | 1967-06-07 | 1967-06-07 |
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US3617734A true US3617734A (en) | 1971-11-02 |
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US644205A Expired - Lifetime US3617734A (en) | 1967-06-07 | 1967-06-07 | Detection system for monitoring gaseous components in air |
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Cited By (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3739260A (en) * | 1970-06-30 | 1973-06-12 | Balzers Patent Beteilig Ag | Method for operating a halogen detection diode and arrangement for carrying out the method |
US3942357A (en) * | 1974-05-02 | 1976-03-09 | Anthony Jenkins | Inspection apparatus |
US3998101A (en) * | 1974-08-29 | 1976-12-21 | U.S. Philips Corporation | Method and apparatus for sampling the atmosphere in non-hermetically-sealed containers |
EP0046699A2 (en) * | 1980-08-21 | 1982-03-03 | The Bendix Corporation | Ion mobility detector provided with a membrane interface |
US4818105A (en) * | 1987-09-21 | 1989-04-04 | Hewlett-Packard Company | Burner for flame photometric detector |
US4910463A (en) * | 1987-12-17 | 1990-03-20 | Sentech Corporation | Halogen monitoring apparatus |
US5198774A (en) * | 1987-12-17 | 1993-03-30 | Williams Ii William J | Gas monitoring apparatus |
US5444435A (en) * | 1990-03-19 | 1995-08-22 | Williams, Ii; William J. | Halogen monitoring apparatus |
US6606899B1 (en) * | 2000-07-07 | 2003-08-19 | Air Products And Chemicals, Inc. | Total impurity monitor for gases |
-
1967
- 1967-06-07 US US644205A patent/US3617734A/en not_active Expired - Lifetime
Cited By (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3739260A (en) * | 1970-06-30 | 1973-06-12 | Balzers Patent Beteilig Ag | Method for operating a halogen detection diode and arrangement for carrying out the method |
US3942357A (en) * | 1974-05-02 | 1976-03-09 | Anthony Jenkins | Inspection apparatus |
US3998101A (en) * | 1974-08-29 | 1976-12-21 | U.S. Philips Corporation | Method and apparatus for sampling the atmosphere in non-hermetically-sealed containers |
EP0046699A2 (en) * | 1980-08-21 | 1982-03-03 | The Bendix Corporation | Ion mobility detector provided with a membrane interface |
EP0046699A3 (en) * | 1980-08-21 | 1982-03-10 | The Bendix Corporation | Ion mobility detector provided with a membrane interface |
US4818105A (en) * | 1987-09-21 | 1989-04-04 | Hewlett-Packard Company | Burner for flame photometric detector |
US4910463A (en) * | 1987-12-17 | 1990-03-20 | Sentech Corporation | Halogen monitoring apparatus |
US5198774A (en) * | 1987-12-17 | 1993-03-30 | Williams Ii William J | Gas monitoring apparatus |
US5444435A (en) * | 1990-03-19 | 1995-08-22 | Williams, Ii; William J. | Halogen monitoring apparatus |
US6606899B1 (en) * | 2000-07-07 | 2003-08-19 | Air Products And Chemicals, Inc. | Total impurity monitor for gases |
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Owner name: AMERICAN STANDARD, INC., NEW JERSEY Free format text: RELEASE OF SECURITY INTEREST;ASSIGNOR:CHASE MANHATTAN BANK, THE (FORMERLY KNOWN AS CHEMICAL BANK);REEL/FRAME:008869/0001 Effective date: 19970801 |