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In article <386@vela.acs.oakland.edu>, bbesler@vela.acs.oakland.edu (Brent Besler) writes:
> I am pretty sure the leak detectors are ionization type.  There is a high 
> potential across two plates sufficient to ionize hydrocarbon gases, but not
> oxygen or nitrogen.  A current proportional to the HC conc. is passed through
> the sensing element.

	The above is not true.  You may be alluding to a Flame Ionization
Detector (FID) used on a gas chromatograph.  Such a detector employs a
hydrogen _flame_ to ionize the gas in addition to a DC potential imposed
across the detection electrode (which is connected to an electrometer).

	The traditional explosive gas detector, the MSA "Explosimeter", uses
a differential thermal conductivity detector.  Some specific gas detection
methods use a catalyst whose temperature increases in the presence of the
target gas; for example, some CO detectors employ a catalyst which oxidizes
CO to CO2, giving off heat in the process.

	There are also various solid-state detectors for hydrocarhon gases,
some of which employ catalytic coatings applied directly to silicon
temperature sensors.

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