Air weighs 14.7 psi at sea level which is defined as 1 atmosphere. Therefore, a psia unit will read 14.7 psia at sea level. At full vacuum it will read 0 psia. Both psis and psig units will read 0 psig at 1 atmosphere. Expanding on this example, using a 0 to 100 psia sensor with a 4 – 20 milli amp output, at 0 psia the transducer output will be 4 milli amps. At 100 psia, which is the same as 85.3 psig or psis, the transducer output will be 20 milli amps. A 100 psig or psis transducer is measuring the same full scale output pressure as a 114.7 psia transducer. There is an offset, or difference of 14.7 psi between psia and either psig or psis. In other words, 100 psig or psis is equal to 114.7 psia.