METHOD AND APPARATUS FOR MONITORING A SUPPLY OF GAS

Abstract: A method and apparatus for monitoring a supply of gas such as the production of methane by anaerobic degradation of organic matter utilises a monometer (14) accurately to measure predetermined volumes of the gas. Each measured volume is discharged into a chamber (18) also of a predetermined volume and containing a second gas, in order to obtain a precisely known dilution of the gas production to be monitored. The chamber (18) contains a catalytic gas sensor (36) sensitive to the gas to be monitored and which produces an output of the first gas concentration within the chamber (18), from which the production rate and volume of the first gas can be determined.

Description: METHOD AND APPARATUS FOR MONITORING A SUPPLY OF GAS This invention relates to a method and apparatus for monitoring a supply of gas. Throughout this Specification and Claims, the term "gas" is to be understood to extend to pure gas, vapours, or mixtures of gases and/or vapours, unless the context dictates otherwise. Although the invention is widely applicable, the background will be described with reference to monitoring methane production. Environmental concern has recently increased the importance of measuring the volume of methane produced by the anaerobic degradation of waste materials. In relation to landfill sites, it is important to know how much methane is produced and when. In another situation, increasing attention is being given to anaerobic degradation of sewage, because the methane produced can be used to provide power to the plant thus reducing its running costs. Although aerobic degradation may be desired to reduce the solids to the minimum, plant for such aerobic degradation may also be powered by the methane produced in the anaerobic degradation. It has thus become important to study the effect of anaerobic degradation, for example to determine, for a given material or materials, how much methane is produced, at what rate, and whether and to what extent production is affected by conditions such as temperature or the presence of other materials e.g. pollutants. The gases produced in anaerobic degradation are not all methane but contain, perhaps 20%, carbon dioxide. One conventional procedure for monitoring the production of methane is to use a pressure lock syringe to withdraw a sample periodically from a sealed vial containing the matter being tested and to analyse the sample using a gas liguid chromatograph to determine the amount of methane in the sample. The amounts produced in successive samples are recorded and added to produce a total. The procedure is time consuming, involves the use of expensive equipment, in the form of the chromatograph, and being invasive is liable to operator error. In another known procedure, the percentage of methane in the gas produced is assumed and the volume of the total gas produced is monitored by letting the pressure in the vial build up to a small predetermined degree and then releasing the pressure and logging the event. Clearly the assumption of the percentage methane, which relies on knowing the composition of the sample to be tested, can be inaccurate. In both known procedures, the pressure rises significantly above atmospheric, which changes the production conditions, as there is an increase in the gases dissolved in the culture, from which there follows an increase in acidity due to the dissolved carbon dioxide. Against this background, in accordance with one aspect of the invention there is provided a method of monitoring a supply of one or more first gas with a gas sensor, comprising the steps of: a) collecting a volume of gas containing the one or more first gas mixed with one or more gas to which the sensor is insensitive; b) diluting the collected volume in a chamber to a predetermined volume with one or more second gas to which the sensor is insensitive; and c) exposing the sensor to the diluted one or more first gas to provide a signal indicative of the concentration of the diluted one or more first gas. In a preferred form, the method is applied to monitoring a supply of, e.g. the production of, methane. A convenient sensor is a catalytic gas sensor which produces an electrical output signal dependent on the concentration of hydrocarbon gas or vapour in the range 0 to about 1%. Since the gases produced by anaerobic degradation contain, typically, about 80% methane, the gas sensor cannot be used to measure such concentrations direct. The dilution step, at a predetermined ratio of preferably about 100:1, permits use of such a gas sensor. All the methane burns on the gas sensor which gives an output signal which rises to a peak and then dies away as the combustion reduces the concentration. The integral of the output signal against time thus represents the total volume of methane in the sensor chamber, scaled by the known and constant volume of the chamber. Adding together the amounts of methane calculated in each collection provides the total amount of methane produced. Logging the time of or for each collection enables the rate of methane production to be calculated. The accuracy of the volume of gas collected in each cycle is not important unless it is also required to know the total volume of gas produced, since this is diluted to a known volume in which the concentration of methane is measured and from that the volume of methane can be calculated. Other gas sensors may be used for the same or other gasses. The value of the method lies in the dilution step which allows a sensor, which is only effective to sense relatively low gas concentrations, to be used to monitor a supply of a relatively concentrated gas . The procedure may be automated, saving time compared with the known procedure. The preferred use of a catalytic gas sensor instead of a liquid gas chromatograph represents a great saving in capital outlay and enables the concentration of methane in each sample collected to be monitored thus improving the accuracy. The process is non invasive so that the procedure is not so liable to operator error. The gas may be collected at atmospheric pressure so removing the alteration in the conditions which exists in the known procedure so that there is no increase in the amount of carbon dioxide dissolved in the matter being tested and thus in the acidity thereof. Although the procedure has been described in the particular context of monitoring the production of methane it will be appreciated that the catalytic gas sensor is responsive to the presence of other gaseous or vaporous hydrocarbons, e.g. butane and ethane, and in other applications the procedure may be used to monitor the supply of these. In accordance with the invention there is also provided apparatus for monitoring a supply of one or more first gas, comprising: a) means for collecting a pre-set volume of gas containing the one or more first gas and for diluting the collected gas to a predetermined volume with one or more second gas; and b) a gas sensor sensitive to the diluted one or more first gas but not to the one or more second gas, to provide a signal indicative of the concentration of the diluted one or more first gas. Preferably,the means for collecting and diluting a volume of the one or more first gas, comprises: valve means selectively operable to direct a supply of the first one or more gas to a manometer or to direct said one or more first gas from the manometer to a chamber; means to detect manometer liquid above a first relatively high level; means to detect manometer liquid below a second relatively low level; and control means to operate the valve to direct the one or more first gas to the manometer when the liquid is detected above the high level and to direct the one or more first gas from the manometer to the chamber when the manometer liquid is detected below the low level. The manometer may have a wide section at least on the side open to atmosphere so that a substantial volume of gas is collected for a small change in liquid level. Additionally or alternatively, the liquid in the manometer may have a low density, e.g. water. Either or both these features ensure that the pressure increase during collection of the gas is insignificant. In order to permit the apparatus to be used repetitively, there is preferably included an arrangement to purge the chamber. In a preferred form, the gas sensor is operative to combust hydrocarbon gases to provide said signal indicative of concentration and thus to purge the chamber of said hydrocarbon gases. So that the preferred apparatus can be used automatically in repetitive cycles, the chamber is most preferably constructed so that oxygen can permeate or diffuse thereinto to replace that used in combusting the hydrocarbon(s) . In alternative arrangements, the chamber may be vented by a solenoid valve, or purged by supply of a gas, e.g. oxygen, via a solenoid valve. In order to automate the procedure, the apparatus preferably includes computer means for integrating the signal indicative of concentration to give a signal indicative of the volume of the collected hydro- carbon(s). For the same purpose, the control means is preferably arranged to operate the valve to direct the gas to the manometer when the liquid is detected above the high level and to direct the gas from the manometer to the chamber when the liquid is detected below the low level, repetitively. The computer means is preferably arranged for adding the integrated signals to provide a signal indicative of the total volume of the hydrocarbon(s) collected. The computer means is preferably arranged for measuring the time over which the hydrocarbon(s) are collected to produce a time indicative signal; and from the volume indicative signal and the time indicative signal, producing a signal indicative of the rate of collection of the hydrocarbon(s) . The means to detect liquid in the manometer above a first relatively high level and the means to detect liquid in the manometer below a second relatively low level preferably comprise optical sensors. If the liquid used in the manometer is, say, water, the optical sensors detect the meniscus, so that when the upper optical sensor detects the presence of the meniscus, the liquid is detected to be above the first predetermined level, and when the lower optical sensor detects the presence of the meniscus, the liquid is detected to be below a second predetermined level. If the operations of the valve means are logged, the total number of operations of the valve means represents the total volume of gas collected (for example methane and carbon dioxide) and the rate at which operations of the valve means occur represents the rate of total gas production. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a diagram of apparatus embodying the invention. Referring to the drawing, a serum vial 2 contains a sample of material, of which it is desired to study the anaerobic degradation. The vial is inoculated with bacteria which is active in anaerobic conditions to degrade the sample. The vial is closed by a butyl rubber or Teflon bung which has an opening through which a tube 4 is connected to the vial to collect gas produced in the process of decomposition of the sample by the bacteria. A further hole through the bung enables a tube 6 to be connected to purge the vial of air with, for example, nitrogen or carbon dioxide. The further hole may also be used to inject with a syringe other substances into the vial in order to assess their effect on the production of methane. After the tube 6 or the syringe has been withdrawn from the further hole, it closes so as to seal the vial against the ingress of air. The tube 4 connects the vial 2 to a valve 8 which is operated by a solenoid 10. The vial is connected to one arm 12 of a manometer 14 via the valve 8 and a tube 16. The valve is operable selectively to open an exhaust port to connect the tubes 4 and 16 to a chamber 18 within which the valve is located. The solenoid 10 is located outside the chamber, a plunger 20 passing through a flexible membrane 22 forming one wall of the chamber 18, to operate the valve. The chamber has an impermeable cylindrical wall 24 closed at one end by the membrane 22 and at the other by a porous plug 26 the purpose of which will be explained below. The manometer 14 is constructed of a transparent material, e.g. plastics or glass. Two optical sensors 28 and 30 are located adjacent the arm 12 of the manometer to sense whether or not there is fluid at two levels. The output signals from the optical sensors are connected to a logic and solenoid driver unit 32 in a control unit 34. The optical sensors detect the interruption of light caused by the meniscus of the fluid in the manometer. When the meniscus of the fluid is sensed by the upper optical sensor 28, the logic causes the unit 32 to activate the solenoid to operate the valve 8 so as to close the exhaust port to the chamber 18. As the vial produces gases as a result of the bacterial activity on the sample, the meniscus in arm 12 of the manometer is depressed. When the meniscus level has been depressed to the level of the optical sensor 30 the logic in unit 32 causes the solenoid driver to activate the solenoid to operate the valve to connect the tubes 4 and 16 to the interior of the chamber 18. The slight pressure head in the manometer causes gas collected therein to be exhausted to the chamber 18. The pressure head, which can be quite small (typically as little as 3mm) , has no significant effect on the degradation process in the vial 2. When the valve is open, the vial is connected to the chamber 18, but as the rate of production of gas by the culture is very low compared with the rate at which the gas collected in the manometer is discharged into the chamber, any error produced thereby is insignificant. The collection of the gas in the manometer may take several hours, but the discharge into the chamber 18 may take only a second or so. It is thus possible to use a simple open/closed exhaust valve instead of a change¬ over valve which would otherwise be necessary, but a change-over valve may be used when high gas production rates are to be measured. When the meniscus reaches the level of the upper optical sensor 28, the valve is operated again to close the exhaust port to the chamber 18 and the cycle is repeated indefinitely. Inside the chamber 18 is located a catalytic gas sensor 36_ This comprises a platinum bridge network, one side of which is treated to burn hydrocarbon gases better than the other. The combustion causes the one side of the bridge thus to become hotter than the other, resulting in an increase in the resistance and consequent unbalancing of the bridge which thus produces an electrical signal indicating the concentration of a combustible hydrocarbon gas or vapour, methane in the present example, since that is the only such gas present. Provided the concentration of methane is not more than about 1%, the output of the sensor is indicative of the concentration. Since the anaerobic fermentation in the vial will usually produce a concentration of about 80% methane, the volume of the chamber 18 is about 100 times greater than the volume of the manometer swept by the meniscus, to give an appropriate dilution for the sensor. The output signal from the sensor 36 is received by a bridge circuit 38 in the control unit 34, amplified by an amplifier 40 and transmitted to an analogue to digital convertor (not shown) to produce a digital signal representative of the concentration of methane in the chamber 18, which digital signal is transmitted to a computer (not shown) for processing. The concentration of methane in the chamber varies as the volume collected in the manometer is introduced into the chamber and as it is then burned on the sensor 36. The digital concentration-representative signal varies correspondingly and is processed in the computer to produce a value representative of the integral of the concentration with respect to time. Since the volume in which this concentration exists is known and fixed, being that of the chamber 18, the value produced to represent the integral of the concentration is also representative of the total amount of methane in the chamber, and thus in the volume collected in the manometer, and may be scaled accordingly. The computer is arranged to add successive values so producing a value representative of the total volume of methane produced. The computer is also arranged to time the interval between successive cycles of operation, e.g. between successive output pulses from the gas sensor, and from that and the successive integral values to produce a value representative of the rate of production of methane. As the collected volume of gas is introduced into the chamber 18 at one end thereof by the valve 8, a corresponding amount of air is displaced from the chamber through the porous plug 26 at the other end. The plug 26 serves to prevent draughts from disturbing the mixture of air and collected gases but to allow the oxygen in the chamber to be replenished by diffusion therethrough between successive releases of collected gases into the chamber. The methane combusts relatively quickly on the sensor 36 so that no substantial amount diffuses through the porous plug 26, almost all being combusted. In alternative arrangements (not shown) the plug may be replaced with a small hole in the wall of the chamber. Such a hole would need to be large enough to permit oxygen to diffuse into the chamber but small enough to prevent draughts disturbing the combustion of the methane. The number of cycles of operation of the valve 8 represents the total volume of gas produced and this too may be recorded. Indeed, the apparatus is useful to record just that and no more in other applications. For example, by adapting the apparatus so that the gas exhausted from the valve 8 can. be delivered to a serum vial, the volume of oxygen used by an aerobic process could be monitored.

Claims: CLAIMS 1. A method of monitoring a supply of one or more first gas "with a gas sensor, comprising the steps of: a) collecting a volume of gas containing the one or more first gas mixed with one or more gas to which the sensor is insensitive; b) diluting the collected volume in a chamber to a predetermined volume with one or more second gas to which the sensor is insensitive; and c) exposing the sensor to the diluted one or more first gas to provide a signal indicative of the concentration of the diluted one or more first gas. 2. A method as claimed in Claim 1, in which steps a) to c) are performed in repetitive cycles, and wherein the chamber is purged of the sensed gas in each cycle before the start of the next cycle. 3. A method as claimed in Claim 2, including the step of: d) adding the signals produced in step c) to provide a signal indicative of the total volume of the first gas collected. 4. A method as claimed in any preceding Claim, wherein a volume of a mixture of hydrocarbon(s) with one or more gas to which the sensor is insensitive is collected in step a) and the volume of the mixture is diluted in step b) . 5. A method as claimed in Claim 4, wherein the sensor is operative to combust the hydrocarbon(s) so purging the chamber thereof. 6. A method as claimed in Claim 5, including the step of: d) integrating the signal indicative of concentration to give a signal indicative of the volume of the collected hydrocarbon(s) . 7. A method as claimed in any preceding Claim, including the steps of: f) measuring the time over which the first gases and/or vapours are collected to produce a time indicative signal; and g) from the volume indicative signal and the time indicative signal, producing a signal indicative of the rate of collection of the first gas. 8. Apparatus for monitoring a supply of one or more first gas, comprising: a) means for collecting a pre-set volume of gas containing the one or more first gas and for diluting the collected gas to a predetermined volume with one or more second gas; b) a gas sensor sensitive to the diluted one or more first gas but not to the one or more second gas, to provide a signal indicative of the concentration of the diluted one or more first gas. 9. Apparatus as claimed in Claim 8, wherein the means for collecting and diluting a volume of the one or more first gas, comprises: valve means selectively operable to direct a supply of the first one or more gas to a manometer or to direct said one or more first gas from the manometer to a chamber; means to detect manometer liquid above a first relatively high level; means to detect manometer liquid below a second relatively low level; and control means to operate the valve to direct the one or more first gas to the manometer when the manometer liquid is detected above the high level and to direct the one or more first gas from the manometer to the chamber when the manometer liquid is detected below the low level. 10. Apparatus as claimed in Claim 9, including an arrangement to purge the chamber and arranged repetitively to collect and dilute the one or more first gas. 11. Apparatus as claimed in any of Claims 8 to 10, wherein the gas sensor is operative to combust hydrocarbon gas to provide said signal indicative of concentration and thus to purge the chamber of said hydrocarbon gas. 12. Apparatus as claimed in Claim 11 when appendent to Claim 9 or 10, wherein the chamber is constructed so that oxygen can permeate or diffuse thereinto, to replace that used in combusting the hydrocarbon(s) . 13. Apparatus as claimed in Claim 11, including computer means for integrating the signal indicative of concentration to give a signal indicative of the volume of the collected hydrocarbon(s) . 14. Apparatus as claimed in Claim 13, wherein the computer means is arranged for adding the integrated signals to provide a signal indicative of the total volume of the hydrocarbon(s) collected. 15. Apparatus as claimed in Claim 13 or Claim 14, wherein the computer means is arranged for measuring the time over which the hydrocarbon(s) are collected to produce a time indicative signal; and from the volume indicative signal and the time indicative signal, producing a signal indicative of the rate of collection of the hydrocarbon(s) .

Inventors: BARTINGTON JOHN KEITH, REYNOLDS PATRICK JOHN, BARTINGTON, JOHN, KEITH, REYNOLDS, PATRICK, JOHN
Applicants: WIVENHOE TECHN LTD, WIVENHOE TECHNOLOGY LIMITED
Application year: 1992
Main classification: C02F11
IPC codes: C02F3, G01N1, G01N7, G01N33