A chart is perhaps better able to explain the differences. This chart shows that a 0 to 50 psia sensor is different from either a psig or psis sensor by 14.7 psi. A 0 to 50 psig or psis sensor would equate to a psia sensor calibrated from 14.7 to 64.7 psia. The 50 psi span is the same, but it is offset by the 14.7 psi which takes the absolute sensor to full vacuum.