The use of multisensor systems in monitoring hazardous atmospheres

Sensors

9 (1986)

and Actuators,

19 - 25

19

THE USE OF MULTISENSOR SYSTEMS IN MONITORING HAZARDOUS ATMOSPHERES* B BOTT

and T A JONES

Health and Safety Executwe, Sheffield S3 7HQ (UK) (Received 1986)

October

24,

1985,

Research m revised

and

Laboratory

form

January

Services

Dwmon,

3, 1986,

accepted

Broad

Lane,

January

28,

Abstract

An expernnental multlsensor system has been used to provide an early unarnblguous mdlcatlon of a fire or heating m a mme by momtormg the gaseous products evolved The system could dlstmgulsh between the gases evolved from a fire and those evolved from diesel engines or from explosives The methane and total flammable gas content were also measured SIX sensors of three different types m combmatlon with oxldlzmg layers and absorbent traps were used Sensors based on electrical conductlvlty changes m lead phthalocyanme were used to provide a measure of NO2 and NO, Sensors based on conductlvlty changes m smgle crystals of ZnO were used to indicate CO and total products of combustlon/heatmg, and catalytic sensors were used to measure the CH4 and the total flammable gas levels The system has been used on a samphng system at the surface of a mme The potential of multlsensor systems for unambiguous mdlcatlons of a hazardous sltuatlon has been demonstrated The problems associated with settmg up a more complex system that would automatically indicate the presence of a hazard on the basis of a combmatlon of sensor responses are briefly discussed Introduction

The concept of using a number of different sensors that are not necessarily inherently selective, under computer control, m order to obtain an unambiguous mdlcatlon of a hazardous sltuatlon has often been discussed The practical marufestatlons of such systems as applied to particular problems have been rare One area m whrch the possible use of such a system has been proposed IS that of unambiguous detection of heating or fire m the mmmg industry by detectmg the products evolved from heated or burning *Paper presented Actuators (Transducers 0 Crown copyright

at the Third International Conference on Sohd-State ‘85), Phlladelphla, PA, U S A , June 11 - 14,1985

1986

Elsevler

Sequola/Prmted

Sensors

and

m The Netherlands

20

materials When coal 1s heated a number of different gases are evolved at different temperatures This 1s also true of materials such as wood and PVC, which are widely used underground Many of these gases have been ldentlfled [l] Some, such as CO, are evolved from all these materials, but not necessartiy early m the heating process The products evolved earliest from coal, wood and PVC have been Identified [2] as aromatic hydrocarbons, acetic acid and hydrogen chloride respectively A sensor based on a single crystal of ZnO [ 31 has been shown 123 to be sensltlve to these products and should therefore provide a means of detecting a fire mvolvmg any of these materials at an earlier stage than a sensor that detects CO specifically A problem m detectmg fires by momtormg gaseous products 1s that of possible interference from products evolved from the use of explosives and those exhausted from diesel engines Several of the gases, including CO, from these sources are the same as those evolved from a heating, but because of the high temperatures at which reactlons occur m both shot flrmg and m diesel engines, nitrogen oxldes (NO and NOz) are evolved m significant quantities, these are not evolved m a heatmg A sensor that could measure NO, independently of the other gases evolved would therefore provide a means of dlfferentlatmg between the different processes The system of sensors described here was aimed at provldmg an unambiguous early warning of fire or heating and also at momtormg the explosive hazard The sensors Three different types of sensor were used Two of them were based on the electrical conductivity changes effected m semlconductmg materials The third was a catalytic device for measurmg the flammable gas concentration The last has no direct relevance to the particular problem of fire detection, but would be important m any general system for mme atmosphere monltonng A ZnO single crystal sensor 133 has been shown to be sensltlve to low concentrations of CO but msensltlve to high levels of CH4 and Hz0 at an operatmg temperature of around 400 “C If, therefore, the device 1s used behind an absorbent trap (charcoal or molecular sieve) the only serious mterference will be from H2 It has, however, been shown [2] that the device 1s sensltlve to a wide range of gases mcludmg aromatic hydrocarbons, acetic acid and hydrogen chloride, and many of the other products evolved when coal, wood and PVC are heated m a~ An NO, sensor based on a film of lead phthalocyanme deposited on a heatable substrate [4] has been shown to be sensitive to NO, at concentrations below 10 ppb and to retam sensltlvlty to above 10 ppm The high sensltlvlty 1s confined to the strongly electrophlhc gases NOz, Clz, F2 and OS, but the device 1s msensrtlve to most common pollutants mcludmg CH4, H,, H2S, HCl and a wide range of hydrocarbons A poison-resistant catalytic gas sensor [ 51 m which the catalyst 1s dispersed throughout a porous alumma bead m order to limit access of

21

poisons to the catalytic surface provides a sensor some lo3 times more resistant to poisons than the conventional pelhstor. The device measures flammable gases at concentrations up to the lower explosive hmlts with a dlscrlmmatlon of about 0 l%, and can give a measure of flammablhty of the atmosphere almost mdependently of the nature of the flammable constltuents The system As a first step m developing a system to be used for underground morutormg, an experimental assembly was installed on the ‘tube bundle’ system [6] at a pit This 1s a widely used technique whereby the atmosphere at different points m the pit can be sampled by pumping gas along small-bore tubes to the surface. At the surface the gas 1s analysed conventionally for CO and CH4 by infrared techniques In the expernnental system used to date, s1x sensors made up of pans of those described above were employed m conJunction with oxldlzer layers and absorbent traps arranged as shown schematically m Fig 1 NOr

NOI

CO

+ H,

SAMPLED

Fig

1 Overall

system

sensor

TOTAL POC

CH4

tI

Am

arrangement

TOTAL FLAMMABLES

(POC

products

of combustion)

The gas, pnor to mtroductlon to one ZnO sensor, was passed through a trap contammg either activated charcoal or 5A molecular sieve, this removed most of the contammants and thus a measure of the CO level was obtamed. The only serious interference was from Hz, but smce the mam source of Hz m a mine atmosphere 1s heatmgs this was not, m this instance, considered to be a serious problem The second ZnO sensor was used without protection and therefore responded to many of the products evolved from a heating The first lead phthalocyanme sensor was used behmd an oxldlzer bed conslstmg of chromium tnoxlde dispersed on s&a gel, this converted NO to NO, at ambient temperatures but allowed unhmdered passage of NO2 This

22

sensor provided a measure of the total nitrogen oxldes The second PbPc sensor was used without the oxidizer to provide a measurement of NO2 only The concentration of NO m the atmosphere could be derived from the drfference m the levels mdlcated by these two sensors One catalytic sensor was used behmd a charcoal or molecular sieve absorbent and thus, to a good approxunatlon, gave a measure of the methane content, the second unprotected sensor measured the total flammables m the atmosphere Since both ZnO and PbPc are semlconductors, they have to be mamtamed at constant temperatures The sensors themselves were located m diffusion arms that were blanked-off cavltles orthogonal to the channel along which the gas flowed [7] This eliminated most of the fluctuations caused by variation m gas flow rates The effects of ambient temperature changes were elnnmated by including the substrate heater m an electromc clrcult that mamtamed it at a constant temperature [8] The conductlvlty changes caused by gas adsorption on the ZnO crystals were measured m terms of the out of balance voltage on a simple bridge arrangement This voltage was amplified by a variable gam amphfler and thus provided span control The current through the PbPc films, driven from a I volt source, was amplified using a high input nnpedance amplifier The catalytic sensors were used m a conventional Wheatstone bridge, the out of balance voltage, which IS a measure of the gas concentration, bemg processed through a variable gain amphfier The operating condltlons of the sensors were not under computer control, the temperature of operation was pre-set and both zero and span for all the sensors were under manual control However, the slgnal processmg and display were computenzed. The computer provided a faclhty for calculating the conductances from the voltage measurements, automatically prmtmg out data at predetermmed intervals, provldmg real-time analogue outputs on a chart recorder and/or VDU, retrospectively dlsplaymg data from selected trme intervals and also for collectmg all the data onto magnetic tape and disc m readily accessible form The computer used was a dedicated system based on the Essex Umverslty microcomputer board, which has mbullt mput/output lines, CMOS RAM and EPROM facllltles Thus programmes could be burnt m an EPROM so that the system was not adversely or permanently affected by power cuts or accidental switch off

Results and dlscusslon The PbPc NO, sensor was found to be totally unaffected by the pressence of products from a fire The NO2 sensor mdlcated very low levels even where the NO, sensor mdlcated concentrations of several ppm from shot fn-mg operations, this was probably due to NO2 being adsorbed on the walls of the tube bundle system Both PbPc sensors retamed stability and sensltlvlty over periods m excess of s1x months of almost contmuous operation The

23

PO& SENSOR

SENSOR

0

Fig 2 Traces from POC (products durmg onset of an underground fire

of combustion)

sensor, CO sensor and NO, sensor

stablhty of the ZnO sensor used for CO detection was such that zeru setting and recahbratlon were only reqmred at approximately four-week mtervafs Because of Its better dlscrnmnatlon at low CO concentratmns, an mdlcatlon of fire or heatmg was obtamed earher on this sensor than on the mfrared analyser The matn problem with the ZnO sensor when used for detectmg the products from a heatmg IS that of settmg the gam of the amphfler. This 1s because the sensor 1s not detecting a single gaseous component but several components m a murture m whleh the relative concentrations of the drfferent components can vary appreciably The gam can thus only be set from a knowledge of the atmosphenc varlatlons at the site sampled Data obtamed at the onset of a fire (Fig 2) show that the total products sensor gave an earlier mdlcatlon of the event than the semiconductor CO sensor Figure 2 also shows very clearly that the NO, sensor 1s unaffected by the products from combustion, but it can be used to attnbute peaks shown by the CO and POC sensors to shot firing, diesel exhaust fumes can be srmllarly identified The data show that this arrangement of sensors can be used to @ve an un~bl~ous mdlcatlon of heatmg m the presence of fumes from shot flrmg and diesel engmes The performance of the eatalytlc sensors IS enhanced by their use m conJun&on with the microprocessor The measurement obtamed zs an average of 10 readings taken over a 5 second period, this ellmmates thermal noise and allows dls~r~~natlon of about 50 ppm CW4. A crltlclsm of a system such as this IS that some of the measurements, m p~tlcul~ those of the total products, are nonquantltatlve The CO

24

sensors and NO, sensors can be calibrated accurately and retam their mtegnty for extended periods of time The CO sensor will tend to mdlcate a level higher than the actual concentration because of interference from H2 In practice there will be small fluctuations m the background level of these gases in the atmosphere and, because of the high sensltlvlty of the sensors, these fluctuations will show up clearly m the readmgs obtained on both these sensors Hence, m order to use the system as an indicator of a hazardous sltuatlon, the decision as to whether a situation 1s dangerous or not must depend on whether the changes measured are slgnlflcant This IS particularly true of the total product measurements because of the wlderangmg sensltlvlty of this sensor It IS therefore Important to define a slgnlflcant change for each sensor and to obtain knowledge of the trends m pollutant concentration that are mdlcatlve of danger This has to be done for the particular environment being monitored, since general crlterla may not apply a change of 2 ppm CO m a mme atmosphere where the level 1s usually 5 ? 1 ppm must be significant, whereas a change of 2 ppm where the level 1s usually around 12 ppm 1s obviously less slgnlflcant Slmllarly, a change m methane concentration from 0 3 to 0 5% may be more slgnrflcant than a change from 1.1 to 1 3%, although the absolute change m concentration 1s the same and the latter has statutory slgnlflcance In the measurement of total products from heatmg the same crltena ~~11 apply, but they are much more dlfflcult to quantify and must rely to an even greater extent on a knowledge of the particular environment m which they are operating Slmllar conslderatlons will apply m different sltuatlons and different apphcations The use of systems such as this must therefore be preceded by an mvestlgatlon of the environment m which they will be used m order to define slgmflcant changes and trends m that environment The mformatlon so obtamed could then be preprogrammed mto the computer and the declslon as to whether the changes and trends observed by the sensors are slgnlflcant and are an mdlcatlon of danger can be left to the machine For total environmental momtormg, other sensors can be added to, or mdeed replace, those used in this work An obvious addltlon would be an O2 deficiency measurmg device, which would add slgnlhcantly to the mformatlon avtiable and would allow CO/O2 deficiency ratios to be mdlcated Temperature and humid&y sensors could be added very easily. Before such a system can be extended to cover the more general case of fire detection m other mdustnal sltuatlons or to other apphcatlons, the philosophy of momtormg m terms of the necessary accuracy of measurement must be carefully considered m relation to the possible use of reliable but possibly less accurate or less selective sensors Such sensors could &Ive accurate mdlcatlons of trends and changes and should be considered for some apphcatlons What 1s requrred m many apphcatlons 1s a clear mdlcatlon of a potent4ly dangerous situation or occurrence, this does not necessarily require highly accurate and selective sensors but does require good longterm stability and retention of sensltlvlty However, this approach does not preclude the development of high quality sensors, the better the sensors the

25

better the data available uses them

and thus the higher the mtegrlty

of any system that

References 1 N W Hurst and T A Jones, A rewew of products evolved from heated coal, wood and PVC, Fzre Mater, 9 (1) (1985) 1 -8 2 N W Hurst, T A Jones, B Mann, R L van Ewyck and P Walden, Analysis of gases evolved from heated coal, wood and PVC conveyer belt and their effect on zmc oxide single crystal semxonductor gas sensor, Fzre Mater, 9 (1) (1985) 9 - 22 3 B Bott, T A Jones and B Mann, The detectlon and measurement of CO usmg ZnO single crystals,Sensors and Actuators, 5 (1984) 65 - 73 4 B Bott and T A Jones, A hlghIy sensltlve NO2 sensor based on electrical conductlvlty changes m phthalocyanrne films, Sensors and Actuators, 5 (1984) 43 - 53 5 S J Gentry and P T Walsh, Poison-resistant catalytic flammable gas sensing elements, Sensors and Actuators, 5 (1984) 239 - 251 6 The Tube Bundle Technzque for The Contznuous Monztorzng of Mzne Azr, Sclentlfx Control Natlonal Coal Board, Report No 622 413,1977 7 J G Fn-th, A Jones and T A Jones, The prmclples of the detectlon of flammable atmospheres by catalytic devices, Combust Flame, 21 (1973) 303 - 311 8 B Bott and T A Jones, A mlcrocomputer controlled system for characterlsmg semlconductor gas sensors, Lab Pratt , 32 (1983) 80 - 85