WO1991002243A1

WO1991002243A1 - A method and a monitor for detecting combustible gases

Info

Publication number: WO1991002243A1
Application number: PCT/GB1990/001238
Authority: WO
Inventors: Gareth Jeremy Jones; Peter Julian Iredale
Original assignee: Neotronics Limited
Priority date: 1989-08-10
Filing date: 1990-08-08
Publication date: 1991-02-21
Also published as: GB8918280D0; ZA906353B; AU6165790A

Abstract

The present invention provides a monitor for detecting the presence of a combustible gas in an atmosphere which comprises: a heatable pellistor (20) for detecting combustible gases, a bridge circuit (20, 22, 24, 26), one arm of which incorporates the pellistor (20), the other arms incorporating resistors (22, 24, 26), an amplifier (32) for measuring the degree of imbalance in the bridge circuit and providing an output signal in accordance with the degree of imbalance, a regulator (28), which is preferably a switchmode regulator, for impressing a steady state potential across the bridge (20, 22, 24, 26); the regulator (28) is connected to receive the output signal from the amplifier (32) and to impress such a potential across the bridge (20, 22, 24, 26) that, if the bridge is balanced, tends to maintain the bridge in balance or, if the bridge is not balanced, tends to bring it into a balanced state, and means (44) for detecting the potential drop across the bridge (or a parameter varying therewith) and providing an output signal in accordance with the said potential drop, which signal gives a measure of the amount of a combustible gas in the atmosphere being monitored. The invention also provides a method for detecting the presence of a combustible gas in an atmosphere.

Description

A METHOD AND A MONITOR FOR DETECTING COMBUSTIBLE GASES

TECHNICAL FIELD

The present invention relates to the detection of combustible gases and in particular to circuits for controlling combustible gas monitors.

BACKGROUND ART

Combustible gases can be detected by a sensor called a "pellistor" which is usually made from a fine coil of platinum wire coated with a ceramic material (or other heat-resistant material) to form a bead. One type of pellistor, known as an "active" pellistor, has a catalyst deposited on the surface of the ceramic material for catalysing the decomposition of a combustible gas. In use, an electric current is passed through the wire to heat the bead to a temperature (usually several hundred degrees centrigade) at which the combustible gas will decompose on the catalyst surface. The current flowing through (or the voltage drop across) the wire is measured to detect a combustible gas as follows: the decomposition of a combustible gas on the bead catalyst surface gives out energy which heats the bead and the resistance wire within it causing the resistance of the wire to increase. This is detected and provides a measure of the amount of combustible gas in an atmosphere. It is known to incorporate an active pellistor in one arm of a Wheatstone bridge as shown in accompanying Figure 1 in which the active pellistor is indicated by the reference numeral 10. A constant voltage is maintained across the bridge by power supply 12 and the bridge is balanced (in the absence of a combustible gas) by resistors 14, 16 and 18. When there is a combustible gas in the atmosphere being monitored, the resistance of pellistor 10 increases as described above and the bridge becomes unbalanced which is detected by the voltmeter /*?^■• In order to maintain the bridge in balance under varying ambient conditions, the resistor 14 is usually a pellistor that is identical in construction to pellistor 10 except for the fact that it contains- no catalyst on its surface; such pellistors are known as "passive" pellistors and, in the circuit shown in Figure 1, are sometimes known as "compensators". The use of a passive pellistor as resistor 14 removes the effe.ct of changing ambient temperature and other factors that determine the operating temperature of active pellistor 10 since the passive pellistor 14 is exposed to the same ambient conditions as active pellistor 10 and so balances out the effects of changes of ambient conditions on pellistor 10, i.e. changes in ambient conditions do not affect the balance of the bridge_*

Circuits of the type shown in Figure 1 have been described, for example, in British Patent Numbers 864 293; 1 596 623 and 2 083 630.

It is _talso known to use pellistors to measure the concentration of gases by measuring the thermal conductivity of a gas. In this case, a bridge of the type shown in Figure 1 is used but both pellistors 10 and 14 are of the "passive" type. One pellistor ,is sealed in a chamber containing a reference atmosphere while the other is exposed to the gas to be measured. If the thermal conductivity of the monitored gas differs from that of the reference, the temperature of the two pellistors 10, 14 will differ causing the bridge to become unbalanced and a voltage is generated at volt meter i~).

The above described bridge circuit has two main disadvantages; firstly it uses a large amount of power because of the need to pass current through both the pellistors 10 and 14, the compensating pellistor 14 having, of course, the same power consumption as the active pellistor 10. The second disadvantage arises when the pellistor 10 is of the "active" type sincethe actiyifcy of the catalyst can change with the increasing temperature o£ the active pellistor 10; thus, when there is combustible gas in the atmosphere being detected, the temperature of the active pellistor 10 increases and so causes an alteration in the activity of the pellistor catalyst. Furthermore, the change in the resistance of the active pellistor 10 in the presence of combustible gases causes the power available to the compensating pellistor 14 to vary, thereby giving an errc- signal. This makes the output of the bridge non-linear especially when the concentration of combustible gas is above the lower explosive limit (LEL) of the gas concerned. It is known, for example from British Patent Application No.

2 185 577, to dispense with the compensating pellistor 14 thereby reducing the power requirements of the combustible gas detector, which is very important in a portable monitor using large and cumbersome batteries. In the arrangement described in the Patent Application No. 2 185 577, a single pellistor is maintained at a pre-set temperature by means of a controllable current source. This temperature is continuously measured, and corrected for the prevailing ambient temperature by means of a temperature probe dependent on ambient temperature. The response of such a circuit is more linear to combustible gases than the Wheatstone bridge arrangement since the pellistor temperature is kept constant. The circuit described in British Patent Application No. 2 185 577 sets the temperature of the pellistor by applying pulses of power across it and measures the resistance of the pellistor by passing a small current through it between these power pulses and measuring the voltage drop across the pellistor resulting from the small current. The number of power pulses is adjusted to keep the pellistor resistance, and therefore the pellistor temperature, constant. The total amount of power supplied to the pellistor is then measured. When the pellistor is exposed to a combustible gas, the decomposition of the gas on the pellistor surface gives off energy which would normally increase the temperature of the pellistor. However, since the temperature of the pellistor is held constant by the circuit, the power supplied to the pellistor by the circuit is reduced; the amount of power supplied to the pellistor is used as an output signal giving a measure of the amount of the combustible gas in the atmosphere being monitored. A similar system is used to measure the thermal conductivity of a gas. While the circuit of the type described in British Patent

Application No. 2 185 577 overcomes the disadvantages of the Wheatstone bridge arrangement, it has disadvantages of its own. The circuit, because it operates in a pulsed mode is complex. Expensive components must be used to maintain the required degree of accuracy. This is because it is necessary to measure a small current in between the high power pulses and because the accuracy of the final measurement depends upon the accuracy of the pulse power, the pulse rate, the small current and the voltage drop measurement across the pellistor.

DISCLOSURE OF THE^•INVENTION

The present invention is intended to overcome the disadvantages of this type of circuit while still using a single pellistor and so maintaining the advantage of reduced power consumption over the old Wheatstone bridge arrangement. In accordance with the present invention, there is provided a monitor for detecting the presence of a combustible gas in an atmosphere, which monitor comprises: a gas sensor for detecting combustible gases that can be heated by passing an electric current therethrough, a bridge circuit, one arm of which incorporates the sensor and the other arms incorporate linear resistors, a balance detecting means capable of measuring the degree of imbalance in the bridge circuit and providing an output signal in accordance with the said degree of imbalance, .means for impressing a steady state potential across the bridge to heat the gas sensor, which potential-impressing means is connected to receive the output signal from the balance detecting means and to impress such a potential across the bridge that, if the bridge is balanced, tends to maintain the bridge in balance or, if the bridge is not balanced, tends to bring it into a balanced state, and means for'^* detecting the potential drop across the bridge (or a parameter varying therewith) and providing an output signal in accordance with the said potential drop, which signal gives a measure of the amount of a combustible gas in the atmosphere being monitored. Thus it will be seen that the monitor of the present invention supplies sufficient power to the bridge circuit to maintain the pellistor at a constant resistance (and therefore at a constant temperature). The arms of the bridge that do not contain the pellistor contain linear resistors, i.e. resistors of substantially constant resisstance so that the current flowing through any of the resistors is linearly dependent on the voltage across it. The resistance values of the resistors should be chosen to minimise the power consumption of the bridge circuit, i.e. the resistor in series with the pellistor can be of low resistance while the other resistors can be of high resistance. The amount of a combustible gas in the atmosphere being monitored is proportional to the difference between (i) the power consumption of the bridge circuit when there is no combustible gas in the monitored atmosphere and (ii) the power consumption of the bridge circuit when the unknown amount of combustible gas is in the monitored atmosphere, in other words, the amount of combustible gas is proportional to the change of the square of the potential across the bridge upon the combustible gas being introduced into the atmosphere being monitored.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a prior art circuit and is described above; and

Figure 2 shows a circuit in accordance with the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

As can be seen, the circuit of Figure 2 includes a bridge circuit incorporating a pellistor 20, and resistors 22, 24 and

26; the pellistor 20 is in contact with an atmosphere which is being monitored for its content of combustible gas. Power is supplied to the bridge by a regulator 28, which is connected via a line 29 to the power supply of a monitor e.g. batteries 30. The combined resistance of resistors 22 and 26 is preferably at leas t 100 t imes the combined res is tance of pel l is tor 20 and res is tor 24. The state of balance of the bridge is assessed by connecting the balance points of the bridge to the inputs of an error ampl ified 32 that provides an output along the l ine 34 in accordance with the imbalance (if any) of the bridge. Line 34 is connected to the regulator 28 to set the vol tage across the bridge. Regulator ^δ is preferably a switchmode regulator of a known type although it could be a linear regulator or an emitter follower but the latter two consume more power than a switchmode regulator and it is for that reason that the switchmode regulator is preferred. The arrangement of the error amplifier 32 and the regulator* 28 is such that the potential d if ference maintained ac ro s s the br id ge by the re gul a tor 28 i s reduced i f the resistance of -pellistor 20 increases from its equilibrium value in which he bridge is balanced and the potent ial d ifference maintained across the bridge is increased if the res is tance of pell is tor 20 falls below its equilibrium value and thus the c ircuit in Figure 2 provides a feedback loop to maintain the bridge always in balance. If there is a combus tible gas in the atmosphere being monitored, the temperature of pell is tor 20 increases and its resistance increases accordingly. The bridge thus becomes unbalanced and so an error s ignal is sent by amplifier 32 along line 34 causing the regulator 28 to reduce the potential across the bridge to a level at which the resistance of the pellistor is returned to its equil ibrium value and so the bridge becomes balanced again. In this way, the resistance and the temperature of the pel l is tor 20, even in the presence of a combustible gas, are maintained at values within a narrow range. The amount of gas in the atmosphere is generally proportional to the reduced amount of power drawn by the bridge in the presence of combustible gas as compared to the power drawn by the bridge in the absence of a combus tible gas. A s ignal proport ional to the power used by the bridge is provided along output line 46 >y output device 44, which measures the voltage V between lines 36 and the ground (or fixed potential) line 38 of the monitor; the device44 also incorporates an EPROM or a known analog shaping circuit to derive the signal proportional to the power used by the bridge from the measured voltage (the power being proportional to the square of the voltage between these two lines) and provide the output signal to line 46. The resulting signal can be sent to a display, a printer, a recording device and/or a plotter and can be used to trigger an aural and/or visual warning device.

Provided that the gain achieved in error amplifier 32 and regulator 28 is sufficiently high, the only sources of error in the circuit shown in Figure 2 are such things as the temperature coefficients of the resistors 22, 24 and 26 in the bridge and the performance of the error amplifier 32. However, these factors can easily be calculated because the circuit operates in a steady state and the design can be optimised to give the required accuracy; such design optimisation cannot easily be achieved when using a pulsed power input to the pellistor.

When the pellistor is powered by a pulsed current (as described above in connection with British Patent Application No.

2 185 577), it has to be run at a fixed resistance. However, since the initial (cold) resistance of individual pellistors varies, the fixed resistance does not correspond to a fixed temperature of operation and this can give rise to errors. In the arrangement according to GB 2,185,577, it is possible to modify the circuit for the particular pellistor used in two ways so that all pellistors operate at a single, uniform temperature: (a) by changing the resistances of the componentsin the circuits; however, this has the disadvantage that pellistor assemblies are not readily interchangeable without first modifying the circuit, and

(b) by fitting a trim resistor in parallel with the pellistor such that .the parallel combination of the pellistor and the trim resistor is equal to the lowest S

conceivable resistance of any pellistor; however, the disadvantage of this method is that the trim resistor wastes power.

The above problem can be solved by the circuit of Figure 2 by fitting a trim resistor 40 in parallel with resistor 22 and this has the advantage over the above solutions of the pulsed pellistor that the trim assembly dissipates very little power because the current flowing through resistor 22 is very small and, furthermore, the resistor 22 can be incorporated within a single pellistor assembly and so the pellistor assembly can be inserted into any desired monitor and operates at the desired temperature .

With both the circuit just described and the circuit of the type described in British Patent Application No. 2,185,577 it is neces&ary to compensate for changes in ambient temperature. This is generally done by means of a thermistor 42 or similar device which performs a function similar to that of the passive bead in the Wheatstone bridge arrangement but with a lower power requirement. The circuit shown in Figure 2 has the additional advantage over the single bead circuit discussed above in connection with

British Patent Application No. 2 185 577 that the latter circuit requires four connections to the pellistor assembly (two for the pe.lliδtor and two for the thermistor) whereas the bridge circuit only uses three connections since the pellistor 20 and the thermistor 22 are both connected to ground line 38. This has the great advantage of making assembly easier.

Although the description of the circuit of Figure 2 has concentrated on the use of an active pellistor 20, the description is equally applicable to the use of a passive pellistor to measure thermal conductivity.

Instead of being a pellistor (i.e. a device containing a length off resistance wire encapsulated in an inert material), the sensor' for combustible gas could be an etched silicon chip that operates in the same way as a pellistor.

Instead of the output device measuring the voltage drop V between lines 36 and 38, it is possible instead to arrange it to measure the vol tage drop between the l ine 38 and either of the balance points 47, 48 of the bridge since the potential between the line 38 and either of the the balance points will vary with the voltage drop V across the whole bridge.

Claims

1. A monitor for detecting the presence of a combustible gas in an atmosphere, which monitor comprises : a gas sensor for detec t ing combus t ible gases that can be heated by pass ing an electric current therethrough, a br idge c ircui t , one arm o f wh ich incorporates the sens or and the other arms incorporate linear resistors , a balance detecting means capable of measuring the degree of imbalance in the bridge circuit and providing an output signal in accordance with the said degree of imbalance, means for impressing a steady state potential across the bridge to heat the gas sensor, which potential-impressing means is connected to receive the output signal from the balance detecting means and to impress such a potential across the bridge that, if the bridge is balanced, tends to maintain the bridge in balance or, if the bridge is not balanced, tends to bring it into a balanced state, and means for detecting the potential drop across the bridge (or a parameter varying therewith) and providing an output signal in accordance with the said potential drop, which signal gives a measure of the amount of a combustible gas in the atmosphere being monitored.

2. A monitor as claimed in claim 1, wherein the gas sensor is a pellistor.

3. A monitor as claimed in claim 2, wherein the pellistor has a surface coating of a catalyst capable of catalysing the decomposition of the combustible gas.

4. A monitor as claimed in claim 2, wherein the pellistor has a surface that is not capable of catalys ing the decompos it ion of the combustible gas.

5. A monitor as claimed in claim 1, wherein the resistance of the linear resistor that is connected in series with the gas sensor has a resistance substantially lower than the resistance of the gas sensor and the resistor in parallel with the gas sensor has a resistance substantially higher than the gas sensor.

6. A monitor as claimed in claim 1, wherein the arm connected in parallel with the gas sensor contains a first resistor of fixed resistance and a second resistor of variable resistance connected in parallel with the said first resistor.

7. A monitor as claimed in claim 6, wherein the gas sensor and the said first and second resistors are held on a single carrier that can be removed from the monitor.

8. A monitor as claimed in claim 1, wherein a thermistor is connected in parallel with the gas sensor.

9. A monitor as claimed in claim 1, wherein the said means for impressing the steay state potential is a switchmode regulator.

10. A monitor as claimed in claim 1, wherein the means for detecting the potential drop across the bridge provides an output in accordance with the power consumed by the bridge circuit.

11. A monitor as claimed in claim 1, wherein the gas sensor is an etched silicon chip.

12. A method of detecting the presence of a combustible gas in an atmosphere, which comprises: impressing a steady state potential across a sensor for detecting combustible gases whereby the sensor is heated to an operating temperature, detecting the resistance of the sensor, adjusting the potential across the gas sensor to maintain the resistance of the sensor substantially constant, and providing an output signal in accordance with the said potential across the sensor, which signal gives a measure of the amount of a combustible gas in the atmosphere being monitored.

13. A method as claimed in claim 12, wherein the gas sensor is a pellistor.

14. A method as claimed in claim 13, wherein the pellistor has a surface coating of a catalyst capable of catalysing the decomposition of the combustible gas.

15. A method as claimed in claim 12, wherein the pellistor has a surface that is not capable of catalysing the decomposition of the combustible gas.