A portable optical gas analyzer for remote measurement of the methane concentrationin closed spaces and rooms

S. O. Mirumyants and V. S. Maksimyuk

State Institute of Applied Optics Federal Scientific Production Center, Kazan~Submitted November 22, 2001!Opticheski� Zhurnal69, 56–59~December 2002!

This paper presents an analysis of the technical means of monitoring methane in closed spaces.A version of an optical device with a zero compensation system is proposed for continuousremote monitoring in the range of volume fractions of methane 0.1–100%, with an operatingtemperature from250 to 140 °C. © 2002 Optical Society of America

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The determination of the methane concentration attime of year in closed spaces and rooms is an importantfar-from-solved problem of modern instrumentation. To ptect the lives of watchmen, repairmen, and welders at gworks and public utilities, it is necessary to constantly motor the gas content of the ambient atmosphere. Workingclosed rooms containing stagnant air is associated withpossibility of the formation of an uncontrolled concentratiof natural gas~methane! with displacement of oxygen, whichproduces an extreme hazard of explosion~for volume frac-tions of methane from 5 to 15%! or asphyxiation~for volumefractions of methane greater than 25–30%!. To prevent suchaccidents, the state of the atmosphere needs to be monitusing a single device in a wide range of concentrationsmakes it possible to constantly measure both the low exsive level and the high methane concentration with an ogen deficit.

There currently are a number of portable local but nremote individual-type devices for monitoring the methaconcentration directly at the workplace.1–3 Potable devicesare characterized by a unitized design, which combinestwo functional parts~the sensor and the indicator! in a singlespace. However, there is also practical interest in a verwith two-unit construction, in which the two functional parof the device are spatially separated by some distancallow remote measurements. Remote devices with scontained thermocatalytic sensors have been developedthey are usually stationary and large and require line voltaRemote portable monitoring devices with removable sens~the Signal-2, the Metan-9m! or with a self-contained pane~the IGS-98! have been produced commercially. Their sentive elements have second- and third-generation thermoclytic sensors,2 used in portable methanometers of tySMM-1, PGF-2, Signal-02, SMS-2, etc., produced byAnalitpribor Factory~Smolensk! and by the Prokol’ev Factory of Automatic Mining Equipment. The Metan-9M methane leak detector, developed by the Ryazan InstrumentaFactory on the basis of a semiconductor sensor, eliminthe effect of understated readings, associated with aginthe sensitive elements and characteristic of thermocatasensors. The indicated devices, although they are comand convenient to operate, operate as gas detectors annot measure the methane concentration in a wide raThermal-action thermocatalytic sensors have substa

900 J. Opt. Technol. 69 (12), December 2002 1070-9762/2002/1

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drawbacks: in their dynamic monitoring range~from 0 to 2.5vol %! and in response rate. They are subject to poisonwhen traces of aggressive gases~chlorine, sulfur, phosphor-ous, fluorine! are present in the atmosphere, require frequmetrological checking, and are characterized by short oating life.

An alternative approach is to use optical gas analyzincluding optical-fiber analyzers.4 The optical gas analyzercurrently available commercially can be divided into twmain types: individual portable types PGA-4 and MultiwaII, and stationary optoacoustic devices.

A crucial problem in developing a portable optical ganalyzer is to create a new intermediate-class device of tunit design, which must combine such qualities as being scontained and remote-acting and must be able to meaconcentrations in a wide range, with high accuracy and looperating life. Optical measurements of the transmittancethe gaseous medium serve as the physical basis for inmentally determining the volume concentration of a specgaseous component of the atmosphere from its absorpbands. The transmittance range from 15 to 85% is optimoutside it, the error becomes substantial and is causederrors in the photometry and the zero setting of the device1,5

A good way to solve the problem is to measure not the tramittance, but its derivative~differential!. A structural two-channel layout can be implemented in which the signaformed as the difference of the working and reference fluxusing a null method of comparison, with optical, electricor gaseous compensation. It assumes that the signalsthe two channels are equal when there is no absorbing gathe measurement cell. The dynamic range of gas concentions that can be measured in the best devices with zcompensation is limited by errors in setting and maintainin time the equality of the channels, as well as by the ptometric accuracy, which is usually maintained to with1–3%.

Devices with an automatic signal-equalization procehave been widely used. The compensation system makpossible to use simple methods to compensate the errorsociated with the ambient temperature variation anddustiness of the optical elements of the system. Competion at one point can be used to construct devices witscale that does not start at zero. Practice in gas-analysistrumentation shows that no one measurement system

90020900-04$18.00 © 2002 The Optical Society of America

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preferable in all cases. Each of them possesses advanand disadvantages, and the best choice depends on theating conditions. The main technical requirements are hsensitivity, fast response, accuracy, reproducibility, a gorange of measurable concentrations, a suitable temperarange, compactness of the device, and as low a price assible.

At the same time, compensation systems also havcertain fundamental drawbacks. For example, a residualnal is present at the time of compensation, the deviceunstable if external conditions act on it in the time peribetween two successive measurements in the ‘‘monitoriregime, which manifests itself in the failure of the readingreturn to the zero mark, and it is necessary to use diffescales with high nonuniformity. The first of these drawbacis minimized by using suitable electrical circuits with thlowest level of signal discretization. The stability of the reaings of the device when destabilizing factors act on itensured by optimizing the electrical circuit and by using apropriate electronic components. This increases the interbration time interval to a normalized value. The individuversions of the nonlinear scales of the device are hardproduce, since they can be made only after preliminary cbration for each sample to eliminate an additional errorcause of inexactness of the fabrication of the scale. Thfore, the most expedient procedure is to carry out additiomonitoring of a typical calibration characteristic of the dvice at the midpoint of the scale.

Marketing studies of the needs of the market that hbeen carried out have shown that customers have a stinterest in the development of a portable, remote, and lcost optical device that operates at both positive and negaambient temperatures.

The Kolodets self-contained portable device, developat the State Institute of Applied Optics~GIPO!, has a self-contained sensor, is intended for local and remote measments of the methane concentration in the air of clorooms, and operates in a wide temperature range.

In choosing the design, preference was given to a nscheme with electric compensation in the amplifier circuwhich increases the sensitivity and the energy potentiathe system and reduces the measurement error caused belectronic part of the device. A circuit with electrical compensation thus makes it fairly simple to set the null of tdevice, including in the automatic regime. When the abso

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ing gas appears in the measurement space, the circuit imance depends directly but nonlinearly on its concentratio

The fundamental innovation in the portable optical dvice is that the entire range of dangerous methane concetions is measured simultaneously, with explosion-dangeroxygen-starvation subranges. The sensitivity of the devicsubstantially increased by using a simple and effective ocal layout in which a mirror simultaneously acts as a focing element and as a shaper of the optical absorption trfor the gaseous component of the atmosphere to be msured. To optimize the sensitivity of the device when baground illumination is present, the optical system uses a stially oriented radiator and detector, along with stoinstalled in the ray path as well as in the focal plane ofobjective.

Extremely simple and explosion-proof circuit engineeing is used when developing the device with the cheappossible components, which in combination make it possto minimize expenditures when the device is put into comercial production and provide a high quality/cost ratio.

The closest analogs or the Kolodets device in functiopurpose are the PGF2M1 thermocatalytic gas analywhich is widely used in day-to-day practice for measurimethane leaks at facilities of the gas network, and its nmodification, the Signal-2 detector, with a self-containsensor. Among optical devices, the closest analogs arePGA-4 individual-type portable gas analyzer and the optiIR sensor of the Multiwarn II personal portable device, maby Drager Safety. The devices use a unitized design, witdifferential two-frequency optical layout based on semicoductor radiative elements with a built-in microprocess~PGA-4! or based on a pulsed source having a continuousemission spectrum with a built-in microprocessor~Multi-warn II!. The main disadvantage of these devices is thatmote measurements are impossible. Table I shows the mtechnical characteristics of these devices and of the Kolodevice that has been developed.

A comparative analysis of the devices shows thatKolodets device possesses a number of advantages.

The Kolodets device uses a compensation zero sysHowever, the apparent simplicity of its design is complicatby the nonlinearity of the output response and the necesof fabricating individual scales with large nonuniformitThis makes it possible for additional error to appear becaof the inaccuracy with which the scale is fabricated. T

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TABLE I. Characteristics of portable gas analyzers.

No. Parameter Kolodets PGF2M PGA-4 Multiwar

1 Concentration, vol%measurement limit 1 0.1–5 0.37–1.2 0–5 0–10measurement limit 2 1–100 1.2–4.2 — 10–100

2 Absolute error, vol%measurement limit 1 0.05–0.5 0.15 0.25 0.1measurement limit 2 0.5–2.5 0.5 — 1.0

3 Response time, sec 2 60–120 30 —4 Remote action yes no no no5 Working temperatures, °C 250–140 220–140 230–135 220–140

901S. O. Mirumyants and V. S. Maksimyuk

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optimum technical solution involves additional monitorinof a typical calibration characteristic of the device at opoint with a volume fraction of gas of 2.5%.

Moreover, the operation of the Kolodets device cshow instability by the indicator failing to return to the zemark. To obtain the largest effect without complicating tdesign and production technology, the device uses, awith the compensation method, stabilization of certaingime parameters, electronic elements, and thermally stsystems, and this increases the intercalibration time of mtoring the sensitivity.

When these systems are used, it is possible to emdifferent versions of the null method. It should be pointout that the detector sensitivity is not used completelythese systems, since it is virtually impossible to compensthe signals corresponding to both channels to the fluctuanoise level.

A possible drawback of typical systems is the errorsociated with contamination of the optical elements duraspiration of the gaseous medium. As a result, the compsation conditions are degraded, and the zero point ofscale is displaced. This error is absent in the measuremsystem used by the Kolodets device, which is a single-besystem in the geometrical sense and a two-channel sywith respect to procedure. This involves a measurement tnique with differential readout, for which one first checand sets the zero position of the indicator of the device idefinitely clean atmosphere, and later measures the metconcentration by placing the self-contained sensor in themedium. Since additional weakening of the transmitted flcan be compensated by an electric signal measured in a catmosphere, the zero position will always remain unchangIt should be pointed out that, by using this system and tenique, it is possible to work in an open space using natuair flow without using additional flow boosters to aspirate tgas into a sample cell.

Moreover, it should be pointed out that the versionoptical measurement method used here is not selective inabsorption band in a multicomponent hydrocarbon gas mture for solving problems of differentiating their constituenIt can only be used to analyze two-component gas mixtuof hydrocarbon gases in air, for example, methane–airpropane–air. This device, as a measurement system, cacalibrated for the concentration of methane or propane.device can also be centered on an absorption band of oatmospheric gases (CO2, O2 , CO! in a multispectral versionof the optical system.

The photometric scale of the device was calibratedunits of volume concentration of methane in air in termsreference gases, using an economic method of exponedilution with evacuation of the measurement cell to a prsure of 0.5 atm and letting in pure air with a numbercompleted cycles of 5–7. Figure 1 shows the nonlinear oput photometric response of the device. It served as the bfor calibrating the measurement scale in the volume percconcentration of methane (CH4, vol %!!. The vertical axis ofthe figure shows the angle of rotation of the needle ofindicating device in the range from 0° to 90° and the n

902 J. Opt. Technol. 69 (12), December 2002

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malized voltage of the output electric signal. They reflectfunctional dependences of the readings of the device onvolume fraction of methane in the range 0–5%~curve1! andthe range 0–100%~curve2!. The graphs are used to calibrathe scale in two different designs of the electronic part ofdevice: an analog version or a digital version with built-microprocessor.

The construction of the Kolodets device consists oself-contained sensor weighing 0.3 kg and a console weing 1.2 kg, connected by a cable up to 6 m long. The elecsupply of the device is self-contained and stabilized, withrange of input voltages from 5 to 3.5 V. The device comestwo standard models, with either an analog or a digital dplay.

The console includes compact storage batteries to suthe nonselective radiator in the frequency regime andelectronic circuit of the device. The controls and the digiindicator or needle indicator, with a scale of methane ccentration and a scale to monitor the voltage of the supsource, are located on the panel. Scale illumination is pvided for working at night, as well as light and sound signling. The optical radiation source and the photodetectorlocated at twice the focal length of the mirror objective of tself-contained optical sensor. The optical base of the deis filled with a methane–air mixture by natural diffusiothrough a grid system of the optical sensor housing. Tform of the explosion protection of the device is 1ExdIISTThe device has analog and digital signal outputs in an exnal data-collection and -processing circuit.

Our studies of the technical characteristics of the devshowed that it is highly sensitive and selective in the prence of atmospheric water vapor, that it is reliable, and tthe technical decisions in the layout of the optoelectrochannel were effective.

1V. R. Kozubovski�, Optical Devices for Monitoring the Contamination oAtmospheric Air, Their Calculation and Design~Uzhgorod, 1990!.

2E. F. Karpov and B. I. Basovski�, ‘‘Thermocatalytic method of gas analy

FIG. 1. Readings of the Kolodets device vs volume fraction of methanemeasurement ranges 0–5%~1! and 0–100%~2!.

902S. O. Mirumyants and V. S. Maksimyuk

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sis in measuring methane in a mine shaft,’’ Bezopasnost’ Truda v PrNo. 3, 34~2000!.

3 ‘‘Compact gas analyzers. Current status and development trends. Aview. Devices, automation facilities, and control systems. TS-4,’’ inAna-lytical Devices and Devices for Scientific Research, vol. 2 ~Moscow,1989!.

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4S. O. Mirumyants and V. S. Maksimyuk, ‘‘Fiber-optic systefor the remote recording of explosive gases,’’ Opt. Zh.64, No. 2, 97~1997! @J. Opt. Technol.64, 157 ~1997!#.

5P. I. Bresler,Optical Absorption Gas Analyzers and Their Applicatio~Energiya, Leningrad, 1980!.

903S. O. Mirumyants and V. S. Maksimyuk