GB2103806A - Improvements relating to gas detectors - Google Patents
Improvements relating to gas detectors Download PDFInfo
- Publication number
- GB2103806A GB2103806A GB08220737A GB8220737A GB2103806A GB 2103806 A GB2103806 A GB 2103806A GB 08220737 A GB08220737 A GB 08220737A GB 8220737 A GB8220737 A GB 8220737A GB 2103806 A GB2103806 A GB 2103806A
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- gas
- adsorbent material
- gas detector
- sensing means
- detector according
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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
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/20—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity
- G01N25/48—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on solution, sorption, or a chemical reaction not involving combustion or catalytic oxidation
- G01N25/4873—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on solution, sorption, or a chemical reaction not involving combustion or catalytic oxidation for a flowing, e.g. gas sample
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- Chemical & Material Sciences (AREA)
- General Health & Medical Sciences (AREA)
- Biochemistry (AREA)
- Pathology (AREA)
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Immunology (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Combustion & Propulsion (AREA)
- Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
Abstract
A gas detector for detecting a change in the composition of a gas comprises an adsorbent material (3) temperature sensing means (2) producing an electrical output signal, and monitoring means for monitoring the electrical output signal from the temperature sensing means (2) to detect changes in the temperature of the adsorbent material (2) resulting from temporary imbalances in the adsorption/desorption equilibrium between the gas and the adsorbent material (2). Preferably the temperature sensing means (2) is a pyro-electric thermometer. The detector may include a second pyro- electric thermometer (2), to compensate for changes in the ambient temperature. The detector may include a pump 8 to aid inflow of the gas or simply be provided with a diffusion screen. The adsorbent material may be treated to exclude moisture. The temperature sensing means may monitor the electrical conductivity of the adsorbent material or include a thermocouple. <IMAGE>
Description
SPECIFICATION
Improvements relating to gas detectors
This invention relates to a gas detector for monitoring a space and detecting any change in the composition of a gas filling that space. It is particularly concerned with a detector for detecting sudden changes in the composition of a gas indicating the presence of a leak of gas into the space.
It is frequently required to monitor a space to detect the presence of a flammable hydrocarbon gas because the existence of such a gas, when mixed with air, provides an explosive mixture the presence of which constitutes an extreme danger. Equally, it is desirable to be able to detect the existence of a leak from any gas storage device. For example, if gas leaks from any stored supply of poisonous gas or non-life supporting gas, then a dangerous situation is created where, if a human being enters the space where the leak has occurred they are likely to be poisoned or asphixiated.
At present flammable gas detectors usually consist of a heated device coated with catalytic material. If flammable gas is present mixed with the air surrounding the device the flammable gas is oxidised by the catalyst and this oxidation of the flammable gas causes a change in the temperature of the device, and it is this change in temperature of the device which is monitored and used to derive an indication of the presence of the flammable gas. Such devices, since they include a heated catalytic body, can represent an additional danger in the presence of a flammable gas/air mixture and also such devices can only detect the presence of flammable gases in air.
In accordance with this invention a gas detector for detecting a change in the composition of a gas comprises an adsorbent material, temperature sensing means producing an electrical output signal, and monitoring means for monitoring the electrical output signal from the temperature sensing means to detect changes in the temperature of the adsorbent material resulting from temporary imbalances in the adsorption/desorption equilibrium of the gas and the adsorbent material arising from changes in the composition of the gas around the adsorbent material.
When molecules of a gas become associated with the surface of a material by adsorption, heat is liberated and equally, when molecules of gas are liberated from the surface of an adsorbent material by desorption heat is taken from the surroundings. When the atmosphere surrounding an adsorbent material has a uniform composition an equilibrium is set up between the adsorption and desorption which means that the rate of adsorption of gas being adsorbed is equal and opposite to the rate of desorption of gas being desorbed and consequently the heat liberated on adsorption is balanced by the heat taken on desorption so that the temperature of the adsorbent material is the same as its surroundings.However, if the composition of the gas changes by, for example, the introduction of an additional component into the gas then, this additional component is adsorbed onto the surface of the adsorbent material and this changes the rate of adsorption which is then not balanced by the rate of desorption. Consequently, on the introduction of an additional component into the gas an imbalance in the adsorption/desorption equilibrium is created and this leads to an imbalance in the thermal equilibrium and hence a rise in temperature of the adsorbent material. Assuming that the composition of the gas with the additional component then remains constant a new adsorption/desorption equilibrium is then gradually established resulting in a thermal equilibrium being restored so that the adsorbent material simply cools down to the ambient temperature of its surroundings.Equally, if there is an absence of one of the components of the gas so that the composition of the gas again varies, then this also leads to an imbalance in the adsorption/desorption equilibrium and a consequent imbalance in the thermal equilibrium which leads to a fall in the temperature of the adsorbent material. Again a new adsorbent/desorption equilibrium is established. If the composition of the gas then remains constant and the adsorbent material heats up to the temperature of its surroundings.
The changes in temperature of the adsorbent material that occur as a result of changes in the composition of the gas surrounding the adsorbent material are small, typically fractions of a degree C and accordingly the temperature sensing means and the monitoring means must be capable of monitoring such small changes of temperature. The electrical conductivity of the adsorbent material may change sharply with temperature and, in this case, the temperature sensing means producing an electrical output signal may be formed by means to monitor the electrical conductivity of the adsorbent material and output an electrical signal which varies with the conductivity of the adsorbent material.
Aiternatively the temperature sensing means may include a thermocouple arranged adjacent or within the adsorbent material to monitor the temperature of the adsorbent material. In this case, the temperature sensing means again includes means to monitor the e.m.f. produced by the thermocouple.
Such a thermocouple must have a large e.m.f. change/unit temperature characteristic so that it is capable of detecting changes in temperature of the order of fractions of a degree.
Preferably, however, the temperature sensing means is a pyro-electric thermometer. Pyro-electric thermometers typically comprise a body of pyro-electrical material with conducting plates positioned on opposite polar faces of the pyro-electric material. A pyro-electric material is one which exhibits a spontaneous electrical polarisation as a result of any difference in temperature between its opposite polar faces. Thus, if there is any difference in temperature between its opposite polar faces a difference in charge is generated on the conducting plates on opposite polar faces of the pyro-electric material.
Pyro-electric thermometers are very sensitive and changes of the order of 1 0-30C can be detected.
When the apparatus includes a pyro-electric thermometer the adsorbent material is placed adjacent one of the polar faces of the pyro-electric material.
Preferably the gas detector also includes a reference temperature sensing means. This reference temperature sensing means may be an electrical circuit which generates a reference signal that varies with ambient temperature but preferably the reference temperature sensing means is a second thermocouple or pyro-electric thermometer located remote from the adsorbent material. Preferably the reference temperature sensing means is located adjacent or within an inert material with generally similar characteristics to the adsorbent material but upon which gases are not adsorbed. Most inert column packing materials are suitable as the inert material.The reference signal obtained from the reference temperature sensor is preferably subtracted from the output signal produced by the temperature sensing means and, in this way, changes in the ambient temperature are compensated, so ensuring that only differences in temperature resulting from temporary imbalances in the adsorption/desorption equilibrium between the gas and the adsorbent material influence the monitoring means.
The adsorbent material may be any general adsorbent material that is chemically inert to the gases that it is likely to encounter and the general requirements of such material are that it has a large surface area, a high electrical conductivity, a low specific heat and a high coefficient of thermal conductivity. Oxides of cobalt and iron fit these requirements well and a mixed oxide of cobalt and iron has been found to be particularly suitable for use with the gas detector when it is required to detect the presence of flammable gases.
Further, when detecting for the presence of flammable or other gases it is desirable to be able to reduce the reaction of the gas detector to changes in the concentration of water vapour that are likely to occur in the atmosphere. To do this the adsorbent material is preferably treated to make it hydrophobic so that water vapour is not adsorbed onto its surface.
The gas detector may be surrounded by a housing including a diffusion screen, typically a sintered metal disc. In this case, the rate of diffusion of the gas mixture through the diffusion screen provides an effectively constant flow of gas to the adsorbing material. in an alternative version, the adsorbent material is contained within a tube and the gas stream is passed through the tube at a constant flow rate by means of a small sampling pump operating at a constant flow rate. In these embodiments, the housing or the tube also contains a reference temperature sensing means when this is included in the device.
The monitoring means may include an alarm indicator and be arranged to trigger an alarm upon a change in the concentration of the gas surrounding the adsorbent material. Also, the monitoring means may include an integrator to integrate the electrical signal representing the change in temperature caused by the temporary imbalance in the adsorption/desorption equilibrium and thereby produce an integrated output signal which gives a measure of the change in the composition of the gas present in the monitored space.
Two examples of gas detectors in accordance with this invention will now be described with reference to the accompanying drawings; in which: Figure 1 is a longitudinal section through a detector head used in the first example;
Figure 2 is a longitudinal section through a detector head used in the second example; and
Figure 3 is a block diagram of a signal processing circuit used in both examples.
The first example of detector head includes a housing 1 containing two cylindrical passages both of which contain a tubular pyro-electric temperature sensor 2. The temperature sensors 2 are made from lead zirconate titanate ceramic composition having silver electrodes applied to its internal and
external faces. The inside of one of the temperature sensors 2 is packed with an adsorbent material 3 formed of a mixed oxide of iron and cobalt and the inside of the other of the temperature sensors 2 is
packed with an inert material 4 such as glass or cotton wool. An inlet manifold 5 including a dust filter
6 is connected to one side of the housing 1 and an outlet manifold 7 is connected to the other side of the housing 1.A pump 8 and a flow control valve 8' is attached to the outlet manifold 7 and, in
operation the pump 8 draws a gas mixture through the dust filter 6, the adsorbent material 3 and the inert material 4. Preamplifiers formed by differential operational amplifiers 9 have their inputs connected to the electrodes on opposite faces of the temperature sensors 2 and provide an amplified differential signal of any difference in voltage on opposite faces of the temperature sensors. The output signals from the detector head are analysed by the circuit shown in Figure 3 and described subsequently.
The second example of detector head includes a housing 10 including two chambers 11 and 12.
A temperature sensor 13 is located in the base of each chamber 11 and 12, and the temperature sensors 1 3 are formed by plates of lead zirconate titanate ceramic having silver electrodes applied to its opposite faces. An adsorbent material 14, typically a mixed oxide of iron and cobalt is applied on top of the temperature sensor 1 3 in the chamber 11 and an inert packing material 1 5 is applied on top of the sensor 1 3 in the chamber 12. A sintered metal diffusion screen 1 6 is arranged in front of both chambers 11 and 1 2 and preamplifiers 9 formed by differential operational amplifiers are contained in the housing 10.The inputs of the amplifiers are connected to opposite faces of the temperature sensors 13 and amplify any difference in voltage that occurs on the opposite faces. The output signals from the detector head are analysed by the signal processing circuit shown in Figure 3.
During adsorption of a gas on a solid surface, the rate of adsorption can be expressed in terms of the gas pressure P, and the fraction of surface covered x:
dx
---=kaP(1-x)-kdx (1)
dt where ka and kd are the adsorption and desorption rate coefficients, respectively. the fraction of surface covered at any time, t, can then be found from the integrated form of equation (1), viz:- bP
xa= (1-exp(-t(1+bP)/Kj) (2)
1 +bP where b=ka/kd and is known as the adsorption coefficient.
Similarly, the rate of desorption of a gas can be expressed as:
dx - ----=kdx (3)
dt from which the fraction of surface covered at any time during desorption can be found from the integrated form of equation (3) viz:- bP exp (-kdt) (4)
1 +bP The heat released during adsorption causes a temperature increase which is related to the fraction of surface covered and the cooling parameters of the system which, for simplicity, are assumed to obey Newton's law of cooling.Thus, the rate of increase in temperature during adsorption can be described as: d(AT) dx =To ---- knAT (5) dt dt where T=the temperature of the adsorbent material, To=ambient temperature, a=a proportionality constant, kn=a constant and AT=T-To. Equation (5) covers the situation where a drop in temperature is caused during a desorption process.
The temperature difference at any time during adsorption can then be found from the integrated form of equation (5) by substituting for dx/dt the derivative of equation (2); thus:-
where ATa is used to denote the temperature change during adsorption.
Similarly, the temperature difference at any time during desorption can be found by substituting in equation (5) the derivative of equation (4) for dx/dt and integrating; thus:-
The observed output is directly proportional to these temperature differences and if these temperature differences are plotted against time the areas of the observed peaks are directly proportional to the amount of gas adsorbed and/or desorbed.
These areas are equal to the integrals of equations 6 and 7 which both lead to the expression: knA
P (8)
Toab-k,Ab where A is the area. Under conditions where Toab knAb, the partial pressure of sample gas in a gas stream is therefore directly proportional to the area of the observed peaks. Under constant flow conditions, the partial pressure of a sample gas in a gas mixture is directly proportional to the percentage of that sample gas in the gas mixture.
These equations make it clear that if the output signals from the detector head are processed it is possible to derive a signal indicative of the partial pressure of any gas that is introduced into a gas mixture and thus derive information with regard to the change that has taken place to give information with regard to a change in the composition of a gas. The signal processing circuit to perform this processing of the output signal from the detector heads comprises a differential operational amplifier 17 which subtracts the output signals from the first pre-amplifier 9 from the output signal from the second pre-amplifier 9. Thus, the output from the amplifier 1 7 represents the temperature difference sensed due solely to the change in the adsorption/desorption characteristics of the adsorbent material.
The output of the amplifier 1 7 passes through a threshold circuit 1 8 and upper and lower threshold stores 19 and 20. The output from the amplifier 1 7 is also fed to an operational amplifier 21 arranged to differentiate the signal and the output of this amplifier 21 is fed to a second differentiating operational amplifier 22 to provide the second differential. Outputs from the amplifiers 17, 21 and 22 and also from the stores 1 9 and 20 are all fed to a signal processor unit 23 which, in practice is likely to be formed by a microprocessor which carries out the calculations to satisfy the equations described above and outputs a signal representing the function dependent upon the difference in temperature, to a sample and hold circuit 24 and associated display 25.This output function is also fed to an integrating operational amplifier 26 where it is integrated with respect to time and the output from the integrating operational amplifier 26 is fed to a further sample and hold circuit 27 with its associated display 28.
Thus, when the detector head in accordance with the first aspect of this invention is used, gas is drawn through the adsorbent material 3 and the inert packing 4 by the pump 8. Equally, in the second example of detector head in accordance with this invention the surrounding atmosphere diffuses through the diffusion screen 1 6 into contact with the adsorbent material 14 and the inert packing 1 5.
Any changes in the ambient temperature or any changes in the temperature of the gas passing into the detector heads causes a signal to be output by the preamplifiers 9 but if there is an equal change in the temperature sensors 2 and 1 3 associated with the adsorbent material 3 and 14 as with the inert packing material 4 and 1 5 then these temperature difference signals are cancelled out in the differential operational amplifier 1 7. However, if the composition of the gas entering the diffusion head changes, for example by the introduction of an additional component into the gas mixture then the adsorbtion/desorption equilibrium in the adsorbent material 3 and 14 leads to a change in the temperature of the adsorbent material and leads to the output from the first pre-amplifier 9 being greater than that from the second pre-amplifier 9. Thus, a positive output is obtained from the differential operational amplifier 1 7. This is differentiated by the operational amplifiers 21 and 22 to derive the first and second differential and the stored values of the output signal from the operational amplifier 17 obtained during calibration of the device are fed from the stores 19 and 20 into the single processing unit 23. In this, the calculations are carried out produce an output signal shown as f(y) which gives an indication of the presence of a change in the composition of the gas surrounding the detector head. This signal is also integrated in the integrator 26 to provide an indication of the percentage concentration of the additional component that has been introduced into the gas mixture.
Claims (12)
1. A gas detector for detecting a change in the composition of a gas comprising an adsorbent material, temperature sensing means producing an electrical output signal, and monitoring means for monitoring the electrical output signal from the temperature sensing means to detect changes in the temperature of the adsorbent material resulting from temporary imbalances in the adsorption/desorption equilibrium between the gas and the adsorbent material arising from changes in the composition of the gas around the adsorbent material.
2. A gas detector according to claim 1, in which the temperature sensing means is a pyro-electric thermometer, the adsorbent material being placed adjacent one of the polar faces of the pyro-electric material.
3. A gas detector according to claim 1 or 2, which also includes a reference temperature sensing means, the reference signal from which is subtracted from the output signal produced by the temperature sensing means to compensate for changes in the ambient temperature.
4. A gas detector according to claim 3, when dependent upon claim 2, in which the reference temperature sensing means is a second pyro-electric thermometer located remote from the adsorbent material.
5. A gas detector according to claim 4, in which the reference temperature sensing means is located adjacent or within an inert material with generally similar characteristics to the adsorbent material but upon which gases are not adsorbed.
6. A gas detector according to any one of the preceding claims, in which the adsorbent material is a mixed oxide of cobalt and iron, and in which the gas detector is required to detect the presence of flammable gases.
7. A gas detector according to any one of the preceding claims, in which the adsorbent material is treated to make it hydrophobic so that water vapour is not adsorbed onto its surface to reduce the reaction of the gas detector to changes in the concentration of water vapour.
8. A gas detector according to any one of the preceding claims, which is surrounded by a housing including a diffusion screen so that the rate of diffusion of the gas mixture through the diffusion screen provides an effectively constant flow of gas to the adsorbing material.
9. A gas detector according to any one of claims 1 to 7, in which the gas detector includes a pump and the adsorbent material is contained within a tube, the gas stream being passed through the tube at a constant flow rate by means of the pump operating at a constant flow rate.
10. A gas detector according to any one of the preceding claims, in which the monitoring means include an alarm indicator which is arranged to trigger an alarm upon a change in the concentration of the gas surrounding the adsorbent material.
11. A gas detector according to any one of the preceding claims, in which the monitoring means includes an integrator to integrate the electrical signal representing the change in temperature caused by the temporary imbalance in the adsorption/desorption equilibrium thereby to produce an integrated output signal which gives a measure of the change in the composition of the gas present in gas surrounding the adsorbent material.
12. A gas detector according to claim 1 constructed substantially as described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08220737A GB2103806A (en) | 1981-08-05 | 1982-07-16 | Improvements relating to gas detectors |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8123944 | 1981-08-05 | ||
| GB08220737A GB2103806A (en) | 1981-08-05 | 1982-07-16 | Improvements relating to gas detectors |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2103806A true GB2103806A (en) | 1983-02-23 |
Family
ID=26280366
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08220737A Withdrawn GB2103806A (en) | 1981-08-05 | 1982-07-16 | Improvements relating to gas detectors |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2103806A (en) |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2191585A (en) * | 1986-06-09 | 1987-12-16 | American Sterilizer Co | Sensor indicating and controlling substance concentration |
| US4817414A (en) * | 1986-07-03 | 1989-04-04 | Dragerwerk Aktiengesellschaft | Measuring arrangement for detecting the level of combustible gases mixed with air |
| US5959190A (en) * | 1996-11-06 | 1999-09-28 | Deutsche Forschungsanstalt Fuer Luft-Und Raumfahrt E.V. | Sensor for measuring the composition of mixtures of hydrogen and oxygen gas |
| US6474138B1 (en) * | 2000-11-28 | 2002-11-05 | Honeywell International Inc. | Adsorption based carbon monoxide sensor and method |
| US6550310B1 (en) | 2000-11-28 | 2003-04-22 | Honeywell International Inc. | Catalytic adsorption and oxidation based carbon monoxide sensor and detection method |
| WO2017052462A1 (en) * | 2015-09-25 | 2017-03-30 | Provtagaren Ab | Temperature differential fluid sensor |
-
1982
- 1982-07-16 GB GB08220737A patent/GB2103806A/en not_active Withdrawn
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2191585A (en) * | 1986-06-09 | 1987-12-16 | American Sterilizer Co | Sensor indicating and controlling substance concentration |
| US4817414A (en) * | 1986-07-03 | 1989-04-04 | Dragerwerk Aktiengesellschaft | Measuring arrangement for detecting the level of combustible gases mixed with air |
| US5959190A (en) * | 1996-11-06 | 1999-09-28 | Deutsche Forschungsanstalt Fuer Luft-Und Raumfahrt E.V. | Sensor for measuring the composition of mixtures of hydrogen and oxygen gas |
| US6474138B1 (en) * | 2000-11-28 | 2002-11-05 | Honeywell International Inc. | Adsorption based carbon monoxide sensor and method |
| US6550310B1 (en) | 2000-11-28 | 2003-04-22 | Honeywell International Inc. | Catalytic adsorption and oxidation based carbon monoxide sensor and detection method |
| WO2017052462A1 (en) * | 2015-09-25 | 2017-03-30 | Provtagaren Ab | Temperature differential fluid sensor |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |