Platinum Metals Rev., 1971, 15, (3), 91
Detection of Methane and of Oxygen Deficiency
Catalytic Transducers for use in Mines
The Safety in Mines Research Establishment showed two new detectors at the 1971 Physics Exhibition in London. A reliable device for measuring the methane present in coal mine atmospheres has been developed and also a system for the detection of oxygen deficiency, from which a compact personal alarm has been evolved.
Hitherto methane detection has been based on the SMRE catalytic transducer comprising a minute coil of platinum wire embedded in a bead of alumina whose outer surface is coated with a palladium-thoria catalyst. Methane present is oxidised catalytically on the palladium, thereby changing the electrical resistance of the transducer to give a measure of methane concentration. Unfortunately at high methane concentrations the amount of oxygen available is reduced and an ambiguously low signal is obtained. This problem has now been overcome by combining the signal from the palladium-thoria catalyst with that from a pure platinum catalyst at a suitable temperature. The latter element gives a signal which slowly increases with rising methane concentration until at a critical concentration the signal increases sharply. Further rises in methane concentration then cause a slow decrease in signal owing to lack of oxygen. The instrument uses a Wheatstone bridge circuit with catalytic “pellistor” elements resistant to the catalyst poisons found underground forming two arms of the bridge. It can measure both high and low concentrations of methane and other gases combustible in air.
A similar transducer measures partial pressure of oxygen by diffusing air from the atmosphere under test into a chamber into which methanol is evaporated. The latter is oxidised catalytically on a platinum element and the heat liberated is measured by a thermocouple. Excess of methanol vapour ensures that the temperature of the bead depends only on the partial pressure of oxygen present. This instrument measures this pressure in the range 5 to 80 kN/m2 to about 3 per cent accuracy, it is stable and it is resistant to catalyst poisons.
The resistance of this detector to gases which poison the catalyst by adsorbing on it and impeding the reaction makes it possible to apply its principle to measuring catalystpoisoning gases. By choosing suitable catalysts with suitable surface areas toxic gases such as chlorine and hydrogen sulphide can be detected in concentrations near their lower toxic value levels.