The standard atmosphere (atm) is a unit of pressure that varies constantly from

place to place and moment to moment. It is approximately equal to typical air pressures

at sea level and defined to be 101.325 kPa, 0 PSIG, 14.7 PSIA, and 1.01 Bar.

A given pressure can therefore be measured including or excluding this atmospheric pressure.

Gauge Pressure - A given pressure is sometimes measured not as an absolute pressure,

but as the excess of that above atmospheric, this is called gauge pressure.

An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI",

but is actually thirty PSI above atmospheric pressure. In technical work, this would be written

as "30 PSIG".

Sealed Gauge - Some manufacturers seal the backside of the sensor to prevent contamination

and improve the repeatability of readings. The zero point of the transducer is usually set

to air pressure at sea level, and may not be zero where you are. Changes in altitude and

barometer will affect the pressure reading, but that is an accurate measurement of actual gauge

pressure above atmospheric. Sealed Gauge references are abbreviated as SG or S,

example: PSIS, PSISG. The readings from a sealed gauge sensor are generally similar to those

of a standard gauge pressure, with allowance for your location’s atmospheric pressure above

or below pressure at sea level.

Absolute - is a pressure, measured relative to absolute zero pressure. This is the pressure that

would occur at absolute vacuum. A reading of absolute pressure from a sensor held out in

normal air will show around 101kpa, 14.7psia and 1.01bar at sea level. This is often used as a

Manifold Pressure sensor as MAP sensors are required to read both Vacuum and pressure.