The Collaborative International Dictionary
Methanometer \Meth`a*nom"e*ter\, n. [Methane + -meter.] An instrument, resembling a eudiometer, to detect the presence and amount of methane, as in coal mines.
Wikipedia
A '''methanometer ''' is an instrument used to measure methane gas in the air of a mine. The Mine Safety Appliances Company Ltd. manufactured the first type - W8 Methanometer around 1950 and it was approved for use by the Ventilation Regulations of 1947. The Methanometer could be powered by an Edison battery cap lamp and it could be carried on a miner's belt with other tools. Methane is the main gas present in firedamp. It is highly explosive and had previously been detected by the blue halo effect it gave to the flame of a safety lamp.
By 1983, several instruments for measuring methane concentration in air had been developed. These relied on such methods as the differential heating of an incandescent platinum filament, or by measuring the higher absorption of infrared radiation by gas containing methane.