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--Hi, John! Thanks for the reply. I am not really trying to detect different gases, i was looking at some sensors for o2 of a type i had not seen before. the spec sheet mentions grahams law, which says (for our purposes and these cells) that the output is proportional to the square root of the ratio of the molecular weight of nitrogen(28) divided by the mean molecular weight of the carrier gas(es) for the mix being analysed. Using a helium only carrier should make the reading about Square root of (28/4) times the signal ie it reads about 2.65 times higher than it should be. I mixed some heliox to test some teledyne cells, and they don't seem to suffer from this problem , although they seem to work in the same way.the reason for my interest is really using He in closed circuit systems where this kind of error would be of great significance! Mass spectrometers: as i understand it, they ARE quantitative so could be used for analysis. I am not sure how big a room you now need for one, but i think it used to be quite big when i was at school! The ionised gas should not be a problem as i think you just use a sample which you would discard. I think that they are a bit out of my price range! We hope to have a gas chromatograph (i think ! ) soonish. I'll post some info when i know. We are lucky to have a 'tame' instrument technician as a customer. Julian Bottomer
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