WO1989011653A1

WO1989011653A1 - Combustible gas detectors

Info

Publication number: WO1989011653A1
Application number: PCT/GB1989/000515
Authority: WO
Inventors: David Ronald Piercy
Original assignee: Wessex Instruments Ltd.
Priority date: 1988-05-16
Filing date: 1989-05-12
Publication date: 1989-11-30
Also published as: GB8811587D0

Abstract

Both a pellistor sensor (15) and a semi-conductor sensor (17) are provided, and the outputs (37, 39) respectively are applied to a combining and weighting circuit (41) to produce a combined sensor output (591). The combining and weighting circuit (41) produces a combined output (591) which is based in a low combustible gas concentration range essentially on the output (39) of the semiconductor sensor (17), in a high gas concentration range essentially on the output (37) of the pellistor sensor (15), and in an intermediate gas concentration range on a weighted combination of the two sensor outputs, the weighting varying through the intermediate range. The combined output (591) of the combining and weighting circuit (41) is applied to an output function circuit (43) which applies a characteristic having a steep slope (29) up to a threshold value, and a shallow slope (31) above the threshold value.

Description

COMBUSTIBLE GAS DETECTORS

This invention relates to combustible gas detectors.

Heretofore such gas detectors have employed catalytic sensors, such for example, as the catalytic pellistor the subject of UK Patent Serial No. 892,530 and more especially the improvement UK Patent Serial No. 2,083,630. This in order to give advance warning of build-up in concentration in flammable gases and, by activation of an alarm in good time before an explosive gas mixture level is attained, alerting users to the impending danger.

Catalytic sensors incorporating detectors relying on heterogeneous oxidation of flammable gases have however a well-known major drawback, the accumulation of catalyst poisons in the course of use of the sensors renders them insensible to flammable gas in any concentration level.

And whilst poison resistant pellistors as described, for example, in the aforementioned UK Patent No. 2,083,630 are noted the problem is not eliminated, catalyst poisoning being only retarded not eliminated in its deleterious effect on the pellistor.

Pellistor sensors are normally able to monitor flammable gases such as methane in the range 10% to over 100% lower explosive level (LEL) with sufficient accuracy for use as gas concentration monitors. Below about 5% LEL, however, low output combined with the effect of drift, due to various causes, render pellistors of little use at such low gas concentrations.

Semi-conductor gas sensors, which rely on surface adsorption phenomena ^* for their ability to detect the presence of flammable gases have in the past had no ability to measure the gas concentration. More reliable semi-conductor sensors which are now available may be used in quantitative determination of flammable gas concentration.

Reference may be made to UK Patents Serial Nos. 1,280,809 (Taguche) and 1,374,575 (Bott et al) for details of such devices.

Such semi-conductor sensors may be employed in measurement of gas concentrations in the range of several tens of parts per million (ppm) to about 2000 ppm (4% LEL) .

Moreover, semi-conductor sensors are intrinsically less susceptible to poisoning by silicone compounds than are the pellistor sensors mentioned previously.

The oil, gas and similar industries have come to rely on poison resistant pellistors. The fail-to-danger characteristic of pellistor sensors is however a matter of great concern.

An attractive feature of currently available detectors is the provision of a fully transduced output which, commonly, is signalled back to a control unit by way of a current loop, typically a 4 mA to 20 mA current loop, the minimum current, 4 mA say, corresponding to zero combustible gas concentration and the upper current limit, 20 mA, corresponding to 100% LEL (5% V/V for methane).

According to one aspect of the present invention, a gas detector for combustibly potentially hazardous gas concentration environments comprises catalytic gas sensor means the- sensitivity of which is such that a useful output, being an output representative of gas concentration present, is developed thereby only in response to gas concentrations at relatively high levels; a further gas sensor means the sensitivity of which is such that an output representative of gas concentration present is developed in response to gas concentrations at relatively low and intermediate levels; and warning means normally operable in response to an excessive output developed by either of the gas sensor means to provide an early warning of build-up in concentration of combustible gas.

Thus, a warning of a build-up in gas concentration will normally still be provided by said further gas sensor means even in the circumstance that the catalytic gas sensor has been poisoned.

Preferably, said further gas concentration sensor means comprises semi-conductor sensor means.

Preferably, the outputs of the catalytic sensor means and of the further sensor means are combined to produce a combined sensor output, and the detector comprises electric circuit means having a characteristic relating^' gas concentration to an electric circuit variable and which, in response to the combined sensor output adopts a value for the said variable varying with a rate of change which is greater in respect of variations in gas concentration below a predetermined threshold level than in respect of variations in gas concentration above the threshold level.

Preferably, the said characteristic has a first substantially linear portion having a certain gradient and a second substantially linear portion having a certain lesser gradient.

Alternatively said characteristic may have a varying slope, either discontinuous or continuously varying, in order to facilitate the signalling by analogue means of concentration levels in a specified region, for example the region near zero.

Advantageously, the electric circuit variable is electric current.

According to a second aspect of the invention a gas detector for combustibly potentially hazardous environments comprises catalytic gas sensor means the sensitivity of which is such that substantially no useful output, being an output representative of gas concentration present, is developed thereby in response to gas concentrations at relatively low levels; further gas sensor means the sensitivity of which is such that an output representative of gas concentration present is developed in response to gas concentrations at said relatively low levels; and combined output means operable in response to an excessive output developed by either of said gas sensor means corresponding to predetermined levels of gas concentrations to provide an early warning of build-up in concentration of combustible gas, as well as providing warnings at higher concentrations.

A third aspect of the invention is concerned with detecting a sensor fault in a gas detector employing two different sensors.

According to the third aspect of the invention, a gas detector for combustibly potentially hazardous gas concentration environments comprises first and second gas sensor means which have first and second operating ranges respectively of gas concentration, the operating range being taken to be the range over which the output of a detector is a usable measure of the gas concentration, the first and second sensor means being chosen such that the operating ranges overlap, in an overlap region, monitoring means monitoring the relationship between the outputs of the first and second sensors in the overlap region, and fault indicator means responsive to the monitoring means to indicate a fault in one of the sensor means when the relationship between the outputs of the first and second sensor means in the overlap region departs substantially from an established relationship.

Preferably the established relationship is determined automatically from a series of measurements of actual values for the outputs of the sensor means and is then stored.

When, for example, the first sensor is a catalytic gas sensor means which is susceptible to poisoning, this poisoning may be detected in a detector in accordance with the second aspect of the invention, by the monitoring means which ascertains the fact that the output of the gas sensor means in the overlap region is no longer what would normally be expected from the corresponding output of the second sensor, which is conveniently a semi-conductor sensor means.

It can be arranged that in this circumstance the detector is able to output a first alarm signal, which is normally associated with for example a gas concentration of 20% LEL, but is inhibited from outputting a second alarm signal which normally would indicate a higher gas concentration, such as 60% LEL. Instead a warning signal is provided, such as a flashing light, to indicate a fault.

In a preferred arrangement the first alarm signal is flashed so as to indicate firstly that the gas level may be higher than a first amount, and secondly to indicate that the sensor needs attention.

According to a fourth aspect of the invention a gas detector for combustibly potentially hazardous gas concentration environments comprises first and second gas sensor means having different first and second operating ranges respectively of gas concentration, the operating range being taken to be the range over which the output of a detector is a usable measure of the gas concentration, the first and second outputs respectively of the first and second sensor means are applied to a combining and weighting circuit means which is so configured as to provide a combined output signal which over a first, low gas concentration range is based substantially on the output of the first sensor means, over a second, intermediate gas concentration range is based on proportions of the first and second outputs which vary progressively through the second range, and over a third, high gas concentration range is based substantially on the second output.

The foregoing and other features of a gas detector for combustibly potentially hazardous gas environments and in accordance with the invention is hereinafter described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the general construction of the detector,

Figures 2, 3 and 3a are representations of alternative arrangements for the catalytic and semi-conductor sensor means,

Figure 4a is a schematic electric circuit diagram,

Figure 4b is a block schematic electric circuit diagram of a part of the circuit of Figure 4a,

Figures 5 and 6 represent current loop characteristics of current detectors and the detector of the invention, respectively, and

Figures 7 and 8 represent possible significance weightings for a range of methane concentration for the semi-conductor and the pellistor sensor respectively.

The gas detector has a flameproof housing 11 being of preferably stainless steel. The housing 11 has a forward dog-house portion 13 within which are disposed the pellistor sensor means 15 and the semi-conductor hydrocarbon sensor means 17, the latter sensor means commonly comprising a hot-wire/thermal conductivity semi-conductor sensor. Such sensors are supplied by, for example, New Cosmos UK Ltd. The dog-house portion 13 is in the form of a cylindrical extension of the main housing body 19 and is provided with a closure member, being a stainless steel sinter disc 21.

Within the main body 19 of the housing is located the electric circuit means 23 associated with the sensor means 15, 17. At the rear of the housing 11 there is a conduit 25 through which pass power connections to and output connections 27 from and to the electric circuit means 23. Both the dog-house portion 13 and the conduit 25 may be charged with potting resin encapsulating, on the one hand, the sensor means 15, 17 and on the other the connections 27 to and from the circuit means.

Whilst the semi-conductor sensor means is represented in both Figures 2 and 3 as comprising a single unit 17 the pellistor sensor means 15 may comprise a sensitive pellet and a compensatory pellet encapsulated as a single unit (Figure 3) or as two separate units (Figure 2). The semi-conductor sensor means and the pellistor sensor means may both be encapsulated as two separate units (Figure 3a).

Referring to Figure 4a the circuit means there envisaged is based, as is common, on the provision of a 4 mA to 20 mA current loop as hereinbefore contemplated.

Circuit means could be provided the current characteristic of which is as shown in Figure 5. Given that 4 mA corresponds to zero combustible gas concentration and 20 mA to 100% LEL (5% v/v) a concentration of 2000 ppm would be signalled as 4.64 mA. The proximity of the latter level to the zero gas level (4 mA) may be such as to pose difficulties for setting the detector to provide a pre-alarm of possibly impending danger. Such a setting might be within the 'noise' level of the system.

To avoid this possibility the circuit means may be generally as depicted in Figure 4a and with reference to Figure 6

where it is shown that the current characteristic has a breakpoint at , say, 12 mA, being 50% of its annunciation range. The characteristic has a first relatively steeply inclined linear portion 29 followed at the knee of the characteristic by a linear portion 31 of gentler gradient .

Below the knee point of the characteristic the current loop would signal gas concentration at 16 mA/ 10% LEL; above it at about 0.84 mA/ 10% LEL. With such an arrangement 2000 ppm would be signalled as 10 .4 mA, a current level sufficiently far removed from the zero gas level (4 mA) as to enable the pre- alar settings to be readily achieved without fear of false alarms being signalled . Assuming the alarm settings were appropriately established the sensor and current loop may be easily powered and interpreted using standard control modules ; standard scale markings or read-outs would necessarily need to be changed . The circuit, means for achieving the foregoing functional performance of the detector may, as shown in Fig . 4a_, comprise signal recovering and scaling circuit portions 33 , 35 , respectively receiving the outputs of the pellistor and semi¬ conductor sensors 15 , 17 ; linearizing circuit portions 37 , 39 respectively connected to the outputs of the circuit portions 33 , 35 ; a combining and weighting circuit portion 41 having inputs respectively connected to the outputs of the linearizing current portions 37 , 39 ; an output function portion 43 receiving as a control input the output of the signal recovering circuit portion 35 associated with the semi¬ conductor sensor 17 ; and a detector combined output 4 to 20 mA drive circuit portion 45.

Additionally the signal recovering and scaling circuit portions 33 , 35 are respectively connected to auxiliary output 4 to 20 mA drive circuit portions 47, 49 respectively, being separate pellistor and semi-conductor sensor outputs .

The circuit portions 1 and 43 are essentially as shown in Fig . 4b_. A circuit portion 70 receives for control purposes the raw semi-conductor sensor output signal from the signal recovery and scaling circuitry 35. The circuit 70 has concentration range decision circuit means represented, at 71, and significance weighting drive circuit means, represented at 72. The significance weighting drive circuit means 72 is configured to develop in response to the output of the decision circuit means 71 two outputs 51, 53 which are in accordance with the algorithm hereinafter discussed with reference to Figures 7 and 8.

Two variable adjustable gain circuits 55 and 57 respectively receive the outputs of the linearizing circuits 37 and 39. The gains of the circuits 55 and 57 are appropriately continuously varied, the one 55 in accordance with an increasing output signal from one of the said two outputs of the significance weighting drive circuit means 72 and the other 57 in accordance with the decreasing output signal from the other of the said two outputs.

The outputs of the circuits 55, 57 are combined in a summation circuit 59 the output 59- of which is, in turn, applied to aa 1 variable slope control function circuit 61. The output 59 represents a combined sensor output, being an output based on the outputs of both sensors 15, 17.

The output of the latter circuit is, of course, applied as an input to the 4-20 mA drive circuit 45.

The circuit means may be entirely analogue circuit means and may, for compactness, employ surface mount technology. Alternatively the foregoing functions may advantageously be performed by a suitable constructed microprocessor circuit.

It is to be remarked that the circuit means, whatever the character should in a practical implementation:

1. Supply the correct drive conditions for each detector.

2. Fully transduce the sensor outputs such that gas concentration is signalled to a control unit (not shown) over a 4 to 20 mA loop. 3. Apply weighting to each sensor output as follows:

(a) 0-1500 ppm (0.15% v/v) - maximum significance to semi¬ conductor (b) 0.5% v/v - 5% v/v (100% LEL) - maximum significance to pellistor

(c) 0.15% V/V - 0.5% V/V - a variance weighting might be employed.

Weighting characteristics would needs be proved in development of the detector, being most probably of the order 10:1 weighting varying progressively to 1:50 weighting (semi¬ conductor sensor : pellistor) as represented in Figs. 7 and 8.

4.Modification of the nominally linear output of the combined sensors to the characteristic of Fig. 6 for reasons touched on at 3 above. ₀ 5. he provision of-separate outputs from the separate sensors in addition to the combnined output for use in service if required and/or as an aid to servicing the detector.

Dealing with significance weighting, represented in Figs. 7 and 8, of the sensor outputs more explicitly, the independent ₅ outputs from the semi-conductor and pellistor detectors must be combined before the information regarding gas concentration can be placed on the 4 - 20 mA current loop.

At low methane concentrations, of the order of 1000 ppm (2% LEL) the semi-conductor is in the middle of its measuring o range, whereas the pellistor detector must be regarded as being almost within the domain of its "noise" .

Of the two outputs at this level therefore, that of the semi-conductor is of greater significance than that of the pellistor. 5 At methane concentrations of about 2% (40% LEL) the semi¬ conductor is fully saturated, and can give almost no information concerning the gas concentration, other than that it is "probably over 20% LEL" . The pellistor of course is now well above its noise level, and will be operating within its 0 "linear" range. The output from the pellistor is therefore of far greater significance than that of the semi-conductor.

The electronic circuit combining these two outputs must not simply average the two signals, since they would thereby be accorded equal significance throughout the measuring range, and 5 this would clearly be not acceptable. The circuit must therefore "weight" the significance of each of the outputs depending on where in the gas concentration range the detectors are presently operating. The precise values of the weighting coefficients and the concentration "breakpoints" have to be confirmed in the sensor development phase. The figures given in this proposal are estimates for the purposes of illustrating the principle only.

The use of the proposed sensor would necessarily be different from conventional sensors .

The first alarm level or "pre-alarm" would be set at about 1000 ppm (2% LEL) . This level should alert the operator to the fact that a second alarm (or first genuine alarm) may be expected shortly.

The second alarm would be set within the normal range of concentrations, say 20% LEL, and would be interpreted in the usual way, sounding audible alarms and if necessary starting shutdown actions, etc.

A pre-alarm on its own, continuously present, might indicate a slightly raised gas level, or an implication of pellistor poisoning, possibly together with the presence of a higher gas level. In this case, local checks with portable equipment may be necesary to confirm the safety of the plant. Standard operating rules for any site protected by these sensors would have to reflect the new information available.

The embodiment hereinbefore described with reference to Figure 4a and 4b has three outputs, from elements 45, 47 and 49, in the form of a 'processed' output from element 45, and individual auxiliary outputs from elements 47 and 49. In a modification, not illustrated, the auxiliary outputs are sequenced or time-multiplexed onto a single output line. In a further modification the 'processed' output from element 45 is also included in the multiplexed output line. Such output arrangements can be achieved by conventional electronics or with suitable software control, as will be appreciated by those skilled in the art.

In yet another modification to the illustrated gas detector arrangement the relationship between the outputs of the sensors 15 and 17 in the overlapping portions of their operating ranges is monitored by suitable monitoring means, and any substantial departure of the output relationship from an established relationship is taken to indicate possible poisoning of the pellistor. Thus, for example, the ratio of the outputs of the linearising elements 37, 39 of Figure 4a can be compared within a specified region of overlap by a suitable comparison means, and the departure of that ratio from a predetermined range can be determined and indicated. This involves the use of suitable electronics or software.

The semi-conductor sensor 17 may be considered to be unaffected by levels of catalyst-poison that would disable the catalytic detector, pellistor sensor 15. If a fault condition is signalled by the comparison means due to poisoning of the pellistor 15, the system will still be able to output a first alarm signal, but is inhibited from initiating a second level of alarm signal.

This arrangement will take into account the fact that the two outputs from the sensors are no longer harmonised, and can be arranged to 'flash' the signalled level between a nominal level, say 0% gas, and the level as reported by the active sensor (probably the semi-conductor sensor, but not always so). The rate of flashing is preferably substantially 2Hz. The effect would be to cause the first alarm level light ^" to flash on and off, thereby alerting the user to the fact that (i ) the gas level may be higher than the first level of alarm, and (ii) the sensor needs attention.

Thus , if gas does become present, the sensor is able to give a warning that it has partly failed in its function, and is able to draw attention to this fact with a flashing light. Thus the ' fail unsafe ¹ situation associated with conventional apparatus is avoided.

Claims

C L A I M S

1. A gas detector for combustibly potentially hazardous gas concentration environments comprising catalytic gas sensor means (15) the sensitivity of which is such that a useful output, being an output representative of gas concentration present, is developed thereby only in response to gas concentrations at relatively high levels characterised by a further gas sensor means (17) the sensitivity of which is such that an output representative of gas concentration present is developed in response to gas concentrations at relatively low and intermediate levels; and warning means normally operable in response to an excessive output developed by either of the gas sensor means to provide an early warning of build-up in concentration of combustible gas.

2. A gas detector for combustibly potentially hazardous gas concentration environments comprising catalytic gas sensor means (15) the sensitivity of which is such that substantially no useful output, being an output representative of gas concentration present, is developed thereby in response to gas concentrations at relatively low levels characterised by further gas sensor means (17) the sensitivity of which is such that an output representative of gas concentration present is developed in response to gas concentrations at said relatively low levels; and combined output means (41, 43, 45) operable in response to an excessive output developed by either of said gas sensor means corresponding to predetermined levels of gas concentrations to provide an early warning of build-up in concentration of combustible gas, as well as providing warnings at higher concentrations.

3. A gas detector as claimed in claim 1 or claim 2 in which the further sensor means comprise semi-conductor sensor means (17).

4. A gas detector as claimed in any one of claims 1 to 3 in which the outputs of the catalytic sensor means and of the further sensor means are combined to produce a combined sensor output (59 ), and comprising electric circuit means (61) having a characteristic relating gas concentration to an electric circuit variable and which, in response to the combined sensor output (59 ) adopts a value for the said variable varying with a rate of change which is greater in respect of variations in gas concentration below a predetermined threshold level than in respect of variations in gas concentration above the threshold level.

5. A gas detector as claimed in claim 4 in which the said characteristic has a first substantially linear portion (29) below said threshold value having a certain gradient and a second substantially linear portion (31) above said threshold value having a certain lesser gradient.

6. A gas detector as claimed in claim 4 or claim 5 in which the electric circuit variable is electric current.

7. A gas detector for combustibly potentially hazardous gas concentration environments characterised by first and second gas sensor means (15, 17) which have first and second operating ranges respectively of gas concentration, the operating range being taken to be the range over which the output of a detector is a usable measure of the gas concentration, the first and second sensor means being chosen such that the operating ranges overlap, in an overlap region, monitoring means monitoring the relationship between the outputs of the first and second sensors in the overlap region, and fault indicator means responsive to the monitoring means to indicate a fault in one of the sensor means when the relationship between the outputs of the first and second sensor means in the overlap region departs substantially from an established relationship.

8. A gas detector as claimed in claim 7 in which the outputs of the first and second gas sensor means are combined (at 59) to produce a combined sensor output, and comprising electric circuit means (61) having a characteristic relating gas concentration to an electric circuit variable and which, in response to the combined sensor output (59 ), adopts a value for the said variable varying with a rate of change which is greater in respect of variations in gas concentration below a predetermined threshold level than in respect of variations in gas concentration above the threshold level.

9. A gas detector for combustibly potentially hazardous gas concentration environments characterised by first and second gas sensor means (17, 15) having different first and second operating ranges respectively of gas concentration, the operating range being taken to be the range over which the output of a detector is a usable measure of the gas concentration, the first and second outputs (39, 37) respectively of the first and second sensor means are applied to a combining and weighting circuit means (41) which is so configured as to provide a combined output signal (59 ) which over a first, low gas concentration range is based substantially on the output (39) of the first sensor means (17), over a second,^' intermediate gas concentration range is based on proportions of the first and second outputs (39, 37)_^ which vary progressively through the second range, and over a third, high gas concentration range is based substantially on the second output (37).

10. A gas detector as claimed in claim 9 comprising electric circuit means (61) having a characteristic relating gas concentration to an electric circuit variable and which, in response 1 to the combined output signal (59 ), adopts a value for the said variable varying with a rate of change which is greater in respect of variations in gas concentration below a predetermined threshold level than in respect of variations in gas concentration above the threshold level.