Why does my Divesoft analyzer show -0.3% helium in air?
Small negative helium readings such as -0.3% can occur because analyzer measurement uncertainty is on the order of tenths of a percent.
Of course, the correct value would be zero. However, measuring is hampered by unavoidable discrepancies on the order of tenths of a percent. As such discrepancies can be positive or negative, it therefore can happen that the analyzer shows -0.3 % or 0.3 % helium in air, just as 99.7 % or 100.3 % may be displayed when measuring pure helium.
Similar situation is with measuring of oxygen. Possible error is up to 1 % (depending on manufacturer of O2 cell).
Values are displayed as calculated, there are no hidden altering of measured data to make values look better.