Comparing Gage, Absolute, and Sealed Pressure

PSI, or pounds per square inch, are the conventional units for defining pressure. In all pressure measuring devices, a reference pressure must be established and the PSI has to be qualified to indicate the reference datum. The suffixes ‘G’, ‘A’ and ‘S’ identify the measured pressure as follows:

  • PSIG – Gage Pressure
  • PSIA – Absolute Pressure
  • PSIS – Sealed Pressure

Pressure transducers can measure gage, absolute, or sealed pressure by design. Atmospheric pressure is the most common reference and the units are designated at PSIG. The measuring device is known as a Gage Pressure Transducer In this instance.

Gage Pressure Transducer – PSIG

It is vented to the atmosphere and measures pressure referenced to local atmospheric pressure. The transducer will show 0 PSIG if the pressure port is exposed to the atmosphere. This happens because there is no net output, as the pressure on both sides of the diaphragm is the same.

Venting is obtained by means of a small diameter hole, which is found close to the transducer's electrical termination - cable or connector. The vent hole contains a porous, stainless steel disk, which is designed to stop harmful airborne particles entering the transducer.

PSIG transducers that are over certain pressure levels may actually be sealed, depending on accuracy class. This can be achieved because the possible measurement errors from sealing will stay within the transducer’s accuracy specification.

Absolute Pressure Transducer – PSIA

Hermetically sealed at 0 PSIA, this transducer measures pressure referenced to an absolute vacuum. The transducer will show atmospheric pressure, around 14.7 PSIA when the pressure port is exposed to the atmosphere. This happens because there is atmospheric pressure on one side of the diaphragm and a vacuum on the other.

The difference is represented by the net output, which is atmospheric pressure. PSIA transducers above certain pressure levels may not have an absolute vacuum reference, depending on accuracy class. As the possible measurement errors will stay within the accuracy specification of the transducer, they may be sealed with atmospheric pressure.

Sealed Pressure Transducer – PSIS

This transducer measures pressure referenced to the prevailing atmospheric pressure hermetically sealed within the transducer. The transducer will show around 0 PSIS when the pressure port is exposed to the atmosphere.

This happens because there is the ambient atmospheric pressure on one side of the diaphragm and a fixed atmospheric pressure on the other side. If they are not the same, then the net output will be a reading other than 0 PSIS. The net output is 0 PSIS if they are the same.

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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