Gauge pressure transmitters and absolute pressure transmitters are two types of pressure measurement devices used to measure pressure in different reference frames. The main difference between them lies in the reference pressure they use for their measurements:

1. Gauge Pressure Transmitter:

   • A gauge pressure transmitter measures pressure relative to atmospheric pressure or ambient pressure.
   • The reference point for gauge pressure transmitters is the local atmospheric pressure at the measurement location.
   • Gauge pressure transmitters typically provide pressure readings in units such as psi (pounds per square inch) or bar, where zero pressure corresponds to atmospheric pressure.
   • In a gauge pressure measurement, positive pressure readings indicate pressures above atmospheric pressure, while negative pressure readings indicate pressures below atmospheric pressure (vacuum).

2. Absolute Pressure Transmitter:

   • An absolute pressure transmitter measures pressure relative to a perfect vacuum, where zero pressure corresponds to absolute zero pressure.
   • The reference point for absolute pressure transmitters is a perfect vacuum, which is considered to have zero pressure.
   • Absolute pressure transmitters typically provide pressure readings in units such as psia (pounds per square inch absolute) or bar absolute.
   • In an absolute pressure measurement, positive pressure readings indicate pressures above absolute zero pressure, while negative pressure readings are not possible since absolute zero pressure is the lowest possible pressure.

In summary, the key difference between gauge and absolute pressure transmitters is the reference pressure they use for their measurements: gauge pressure transmitters measure pressure relative to atmospheric pressure, while absolute pressure transmitters measure pressure relative to a perfect vacuum. The choice between gauge and absolute pressure measurement depends on the specific requirements of the application and whether the pressure readings need to be referenced to atmospheric pressure or absolute zero pressure.