Why isn’t DO percentage appropriate for stabilization criteria aside from the fact that you can’t document or track your DO over time?

Why isn’t DO percentage appropriate for stabilization criteria aside from the fact that you can’t document or track your DO over time?

DO percentage, actually, as a percentage of saturation, would be acceptable as an indicative parameter of stabilization in terms of its use.  But many regulatory guidance documents don’t recognize this, they’re looking for a DO value.  And this is because measuring DO has always been a field parameter.  You’re going to get a much more accurate measurement of DO in the field than you would after taking the sample to the laboratory and measuring DO there, by that time it’s going to generally be low.  If you were watching for stabilization, while you flipped between milligrams per liter concentration for DO or looked at percent saturation, its should be consistent because the important thing here is as long as the temperature is not varying widely, you should see the same results. DO percent saturation changes with temperature.  So if you had temperature changing over time as you purged, that could be a problem.