GB2185579A

GB2185579A - Apparatus for detecting gaseous hydrocarbons

Info

Publication number: GB2185579A
Application number: GB08700712A
Authority: GB
Inventors: George Mcgregor
Original assignee: GAS MEASUREMENT INSTR Ltd
Current assignee: GAS MEASUREMENT INSTR Ltd
Priority date: 1986-01-15
Filing date: 1987-01-13
Publication date: 1987-07-22
Also published as: EP0231073A3; GB8700712D0; DE3782744D1; GB2185579B; ATE82801T1; EP0231073A2; GB8600858D0; EP0231073B1

Abstract

Apparatus (10) for detecting combustible gaseous hydrocarbons comprises a chamber (12) for receiving gaseous samples and an electric circuit (13) incorporating first and second pellistors (14, 15) housed within the chamber (12). Pellistor (14) has a catalytic coating. Circuit (13) has constant current source (20) delivering its current output to the pellistors (14, 15) which are series connected and, in parallel therewith, to a pair of high value reference resistors (21, 22) which are also series connected. A first voltage measuring device (26) is connected between the inter-pellistor junction and the inter-resistor junction and provies an LEL measurement. A second voltage measuring device (29) is connected across pellistor (15) and provides a volume gas measurement.

Description

GB 2 185 579 A 1

SPECIFICATION

Apparatus for detecting gaseous hydrocarbons 5 This invention relates to apparatus for detecting combustible gaseous hydrocarbons, such as methane, in a mixture with air.

Variousforms of apparatus forcletecting combust ible gaseous hydrocarbons in a mixture with air are 10 alreadywell known and utilise Wheatstone bridges incorporating resistance elements of pellistorform.

One such Wheatstone bridge comprises an arm formed bytwo pellistorswhich are generally similar in shape,volume and electrical resistance butone 15 pellistor is provided with a catalytic coating which is 80 effective at gas concentrations below the upper ex plosive limit (LI.E.L.) to cause combustion of the gas on the coating thereby unbalancing the bridge byan extent dependent upon the concentration of gas.

20 Both pellistors in this bridge are located in the gas stream. For gas concentrations substantially above the UELthe catalytic combustion effect of the coat ing is inhibited due to lack of oxygen and sothe bridge returnsto balance. In orderto cletectthese 25 higher levels of gas concentration, either for quantit ative measurement purposes orfor distinguishing thatthe balanced bridge condition arisesfrom higherlevels of gas ratherthan zero gas concentra tion, a second Wheatstone bridge is required,com 30 prising an arm formed by two identical pel I istors; of which one is located in air out of the gas stream and the other is located in the gas stream so thatthe thermal conductivity of the gas stream affects the rate of heat loss from only one pellistorthereby un 35 balancing the second bridge by an extent dependent 100 upon the concentration of gas. In this second bridge neither pellistor incorporates a catalytic coating.

The pellistors themselves are well known el ements and,for example, are described in UK Patent Specifications 892530,1447488 and 2044937.

It is an object of the present invention to provide an improved form of apparatus for detecting combust ible gaseous hydrocarbons.

According to the present invention there is prov ided apparatus for detecting combustible gaseous hydrocarbons comprising a chamberfor receiving samples of an atmosphere to be checked forthe pre sence of gaseous hydrocarbon content and an electr ical circuit part of which isformed by first and second pellistors; housed within the chamber, one of said pellistors incorporating a catalytic coating, the elec trical circuit comprising a constant current source delivering its current outputto the pellistors which are series connected and, in parallel therewith,to a pair of high value reference resistorswhich are series connected, firstvoltage measuring means being connected between the inter-pellistorjunction and the inter-resistorjunction to provide an LEL measurement, and second voltage measuring means being connected across the other of said pel listors to provide a volume gas measurement.

Preferably, a single measurement display device is provided having a gated inputwhereby onlythe output of one measuring means is displayed at a time, depending upon the level of the displayed measurement.

An embodiment of the present invention will now be described byway of example with reference to the accompanying schematic drawing.

As is shown in the drawing apparatus 10 cornprises an instrument case 11 within which is located a gas chamber 12 and an electrical circuit 13 part of which isformed byfirst and second pellistors 14,15 housed within the chamber 12. Chamber 12 is prov- 75 ided with an inlet port and outlet port, to the latter of which is connected a flexible tube incorporating a hand pump 16 whereby samples of an atmosphere to be checked forthe presence of gaseous hydrocarbon content are drawn through the chamber 12 so asto envelop the pellistors 14,15.

Electrical circuit 13 primarily comprises a constant current source 20 delivering its current outputto the pellistors 14,15 which are series connected and, in parallel therewith, to a pair of reference resistors 21, 85 22,which are series connected. The nominal value of each resistor 21, 22 is several orders of magnitude greaterthan that of each pellistor 14, 15 sothatclespite variations in resistance value of the pellistors 14, 15 dueto the effects of detected gas, as will be ex- 90 plained, the current through the pellistors 14,15 essentially remains constant. Pelistors 14,15 are generally similar in shape, volume and electrical resistance to each other but pellistor 14 is provided with a catalytic coating which is effective at gas con- 95 centrations below the upper explosive limit to cause combustion of the gas on the surface of pellistor 14. Pellistor 15 does not possess this catalytic coating.

A power supply 24 is provided to energise current source 20, supply 24 normally being in the form of dry batteries, and in orderto derive measurement outputs of gaseous hydrocarbon content a high impedance differential voltage amplifier 26 is connected between the junction of pellistors 14, 15 and the junction of reference resistors 21,22,the output of 105 amplifier 26 being delivered to a meter 27 (or other form of visual display unit) via a gating device 28. A second measurement output is obtained from a high impedance voltmeter 29 (and which may also incorporate an amplifier) the output of which is delivered 110 to meter 27 via gating device 28. Gating device28 delivers only one of its inputs to the meter 27 and selection of which input is so delivered is effected by a comparator30 which compares one outputvoitage level against a reference level (Ref 1) and controlsthe 115 operation of device 28 according to a predetermined algorithms as will be explained. Conveniently the re ference level Ref 1 is compared againstthe output of voltmeter 29. Also, an autoranging facility can be provided by comparator 30.

120 In operation of the apparatus 10 power supply 24 is initially switched on so as to energise current source 20. When the atmosphere within chamber 12 is air, both peilistors 14,15 function as constant resistors, as do resistors 21,22, the latter being sized relativeto 125 the pellistors 14,15 so thatthe amplifier 26 provides a zero output signal. Voltmeter 29 detects a constant voltage which is conveniently offset within voltmeter 29 to provide a zero output signal. Since the output of voltmeter 29 is less in value than Ref 1 comparator 30 130 causes gating device 28 to direct the output of ampli- 2 GB 2 185 579 A fier26 to meter 27 which therefore displays an output of zero. When the atmosphere within chamber 12 contains a small quantity of detectable gaseous hydrocarbon, such as methane, the catalytic coating 5 provided as part of pellistor 14 but not present in pel listor 15 causes combustion of the gas on the surface of pellistor 14 resulting in heat being applied to pel listor 14thereby increasing the resistance of pellistor 14. Because the current applied to the series connec tion of pellistors 14,15 is constantthe voltage across the pellistors and thevoltage atthe junction of pel listors 14,15 changes and because resistors 21,22 are connected in parallel with the pellistors 14,15 there is a small change in voltage value atthejun 15 ction of the resistors 21,22. Thevoltage change at thejunction of resistors 21,22 is half thevoltage change acrossthe pair of pellistors 14,15when the resistors 21,22 are of equal value. Accondingly, amplifier 26 detects a changed differential inputvol tage giving riseto an outputvoltage signal capable of being displayed by meter27. In accorclancewith the known operation of pellistors 14thisvoltage signal is proportional to the concentration of com bustible gas in the gas sample so that the meter can 25 be scaled to provide a reading of gas concentration.

Also, because pellistor 15 remains unaffected bythe catalytic action of pellistor 14voltmeter 29 continues to provide a substantially zero output signal and it is the output of amplifier 26which istransmitted through gating device 28 to meter 27 because the voltage level of the voltmeter 29 output remains belowthat of Ref 1. However, pellistor 15 is subjected to the thermal conductivity of the sampled gas, which results in small changes in pellistor resistance even below the upper explosive limit (UEL) concentr ation. Accordingly, the value of Ref 1 is setto a value belowthe upper explosive limit concentration (con - veniently, about twice the lower explosive limit con centration) and consequently when the output signal from the voltmeter 29 exceeds the set level of con centration comparator 30 causes gating device 28 to direct the output of voltmeter 29 to be displayed on meter27.

As the concentration level of the combustible gas within chamber 12 increases substantially beyond the UEL and the catalytic action of pellistor 14 is quenched due to lack of oxygen the thermal con ductivity of the gas stream through chamber 12 not iceably affects the heat loss from both pellistors 14, 15. This gives rise to a change in resistance of both pellistors 14,15 but because they are provided with a constant current and voltmeter 29 only detects the voltage across pel listor 15 the increased resistance of pellistor 15 is detected as an increased outputvol tage signal from voltmeter 29 which is displayed by meter 27. As is already known this voltage signal is proportional to the concentration of gas in the gas sample.

It will now be appreciated that the apparatus 10 is capable of measuring both volume gas and L.E.L.

with a reduced number of active and matched com ponentsthan hitherto, namelywith onlytwo pel listors 14,15.

As is shown in phantom, currentsource 20 may be autocontrolled to compensate for changes in 130 ambient conditions by provision of a low-value current sensing resistor 32 having similar characteristics to pellistor 15. The voltage developed across resistor 32 is measured by meter 33 to provide a 70 control signal to current source 20. Ambient conditions giving rise to an increase in resistance of resistor 32 cause the control signal level to increase leading to a reduction in value of the current level delivered by source 20.

The current source 20 may conveniently operate on a d.c. basis butforthe purpose of conserving energy in the batteries of power supply 24 it is preferred that current source 20 operates in a pulsed mode with an on-tooff time ratio (i.e. mark-space ratio) of 80 constant RMS value. Of course,the period of the pulsed modewaveform required to be substantially less than the thermal time constant of pellistors 14, 15.

With regard to hand pump 16 itwill be appreciated 85 thatthis pump could be motorised, conveniently electrically, and located within instrument case 11. Pump 16could alternatively be connected to the inlet port of chamber 12 either in its hand-operated or motorised form. Also, where the apparatus 10 is 90 static pump 16 maybe dispensed with and the ports of chamber 12 enlarged to permit diffusion of the atmosphere to be checked through the chamber 12.

Claims

1. Apparatus for detecting combustible gaseous hydrocarbons comprising a chamber 0 2) for receiving samples of an atmosphere to be checked forthe presence of gaseous hydrocarbon content and an el- 100 ectrical circuit (13) part of which is formed byfirst (14) and second (15) pellistors housed within the chamber (12), one of said pellistors (14) incorporating a catalytic coating, characterised in that the electrical circuit (13) comprises a constant current source 105 (20) delivering its current outputto the pellistors (14,15) which are series connected and, in parallel therewith, to a pair of high value reference resistors (21,22) which are series connected, firstvoltage measuring means (26) being connected between the 110 inter-pellistorjunction and the inter-resistorjunction to provide an LEL measurement, and second voltage measuring means (29) being connected acrossthe otherof said pellistors (15) to provide a volume gas measurement.

2. Apparatus as claimed in claim 1,characterised in that a single measurement display device (27) is provided having a gated input (28) whereby onlythe output of one measuring means (26,29) is displayed at a time, depending upon the level of the displayed 120 measurement.

3. Apparatus as claimed in claim 2, characterised in thatthe gated input (28) is controlled bya comparator (30) which compares the voltage output of the second voltage measuring means (29) against a 125 reference level (Ref 1) and when the compared voltage level is less than the reference level the gated. input (28) directs the voltage output of the firstvoltage measuring means (26) to the display device (27) and when the compared voltage level is greaterthan the reference level the gated input (28) directs the 3 GB 2 185 579 A 3 voltage output of the second measuring means (29) to the display device (27).

4. Apparatus as claimed in any preceding claim, characterised in that constant current source (20) is 5 autocontrolled to compensate for changes in ambient conditions by provision of a currentsensing resistor (32) and meter (33).

5. Apparatus as claimed in any preceding claim, characterised in that constant current source (20) op- 10 erates in a pulsed mode with a mark-to-space ratio of constant RMS value, the pulsed mode waveform period being substantially less than the thermal time constant of the pellistors (14,15).

Printed for Her Majesty's Stationery Office by Croydon Printing Company (11 K) Ltd,6187, D8991685. Published by The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies maybe obtained.

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