Dewey works by completely evaporating a small amount of water (~35 uL) into a volume of dry nitrogen.  The Picarro then sucks vapour out through one pipe while it is replaced by dry nitrogen through the other, using an open T on the feed side.  Note that you need to have little concentration dependence for this to work: it’s good with the newer 2130 but perhaps not so good with the older 1102.  This has a few advantages over IsoLyser, one of which is that it is easier to run standards of different composition over the day with a single kit.  It is also difficult to get a constant concentration out of IsoLyser which tends to be fussier to get going.  We have learned to wrap our evaporation bottle to get more constant values.  Is this because of varying kinetic fractionation with the dry nitrogen fill gas?  We typically get about 15 minutes of a constant vapour composition (on the Picarro 2130) using it – the higher draw rate and greater concentration dependence of the 1102 will give less.

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