There are two ways to measure percentage of dissolved oxygen: saturation and volume per volume. The air in the atmosphere has a volume per volume percentage of 20.9% and a saturation percentage of 100%.

Some DO meters use a saturation percentage, whereas FieldKit uses a volume per volume percentage.

For example, if your FieldKit shows a low value of about 10 %DO (volume per volume), then meter measuring in saturation percentage will show 47.8 %DO (saturation). Or, if you use an aerator and get a saturation level of about 16 %DO (volume per volume) with the FieldKit, then the other meter will show 76.6 %DO (saturation).

The formula to convert one into the other is DO (volume per volume) = DO (% saturation)*0.209