Gas detector working principle (Open path)

Most widely usedGas detector are of two types

a)  Open path gas detectors

b)  Closed path gas detectors

apart from this we also use (Electro-chemical principle GD, MOS GD, Catalytic detection GD)

Let us consider open path gas detector

The opecl (optic path eclipse developed by detronics) is an open path IR (Infra Red) gas detection system that provides continuous monitoring of combustible hydrocarbon gas concentration in the range of 0 to 5 LFL metres over a distance of 5 to 120 metres

LFL stands for Lower Explosive Limit per distance and can be calculated from mA as

[{(mA reading-4)/16}*5]=LFL meters

The system consists of two stainless steel modules a transmitter and a receiver along with mounting fixture.

Both modules are powered from an external 24 volt dc.The receiver provides the measurement signal output and an internal magnetic calibration switch.  The transmitter houses dual xenon flash lamps. Both modules are installed at approximately the same elevation and must be aligned to point directly at one another. No direct electric connection is required

(a transmitter/receive module look like is as shown above)

Theory of operation

The opecl transmitter module illuminates a direct linear path ending at opecl Rx module.As flammable hydrocarbon gases intersect the light beam between the two modules certain IR(Infra Red) wavelength are absorbed by the gas while other wavelengths are not.The amount of IR absorption is determined by the concentration of the hydrocarbon gas A pair of optical detectors and associated electronics located in the receiver module measures the absorption.

The change in the intensity of the absorbed light (active signal) is measured relative to intensity of light at a non absorbed wavelength (reference signal) The microprocessor computes the gas concentration and converts the value into 4 to 20 mA which is then communicated to external control room .The output 4-20 mA corresponds to 0-5 LFL

OPECL is capable of detecting most hydrocarbon gases and vapors including methane, ethane, propane, butane, and propylene. Gas type and other operational parameters are selected via digital communications. The factory calibrated setting is methane.

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