The reference modes are Gauge,Absolute and Differential.
Gauge Mode
Referenced to atmospheric pressure.
0 means your test pressure is the same as atmosphere.
Gauge mode is the most common reference mode.
Can be both positive and negative.Usually the easiest to measure.
Absolute Mode
Referenced to a perfect vacuum.
0 means that it is a perfect vacuum.Absolutely no molecules.
Can only be positive numbers. You cant have negative molecules in a vessel.
More difficult measurement.
Differential Mode
Referenced to another pressure. Eg: line pressure.
0 means that there is no difference in the two pressure.
Gauge mode can be thought of as a special case of differential mode.
Can be a difficult measurement to make.
Can be both positive or negative.
