When utilizing a PSIG transducer, as ambient pressure changes, the back, or reference side of the sensing element is allowed to breathe by venting it to atmosphere. With changes in elevation, ambient pressure, also known as atmospheric pressure, will change. If not allowed to vent, as is the case with psis or psia transducers in which the vent is sealed, that same atmospheric change manifests itself in a slight offset in the diaphragm as the transducer reacts to that ambient change. Imagine a slight stress upward of the pictured green sensing element as a lower atmospheric pressure would tend to pull the reference side of the diaphragm towards the lower pressure. The transducer would interpret this as a force being applied to the diaphragm in an upwards direction, misconstruing that as an increase in pressure (or decrease in vacuum) from the pressure source. Since atmospheric pressure changes are quite small, this issue is normally only a problem with very low pressure ranges, typically under 50 or 100 psi. Above that range, small changes in ambient are barely noticed by the transducer.  For example, a 1 psi change in ambient is only a 1% error in a 100 psis “sealed” transducer, but would equate to a 33% error in a 3 psis transducer. Always use psig for real low pressure ranges, unless installed where ambient pressure conditions are not expected to change.