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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: How does one make a gas (methane) leak detector?
Summary: Thermal conductivity gas detectors
Message-ID: <3670@kitty.UUCP>
Date: 18 Mar 90 16:43:04 GMT
References: <4926@aplcen.apl.jhu.edu> <386@vela.acs.oakland.edu>
Distribution: usa
Organization: Recognition Research Corp., Clarence, NY
Lines: 25

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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