In-situ Level TROLL 400, Operator'S Manual, Page: 15 / 85

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About the Pressure/Level Sensor
A pressure transducer senses changes in pressure, measured in force per square unit of surface area, exerted by water or
other fluid on an internal media-isolated strain gauge. In-Situ offers instruments with either absolute (non-vented) or gauged
(vented) pressure sensors.
The "Absolute vs. Gauged: Comparing Absolute and Gauged Pressure Sensors" technical
note describes the difference between absolute and gauged pressure sensors and explains
the proper use of each type of sensor in different applications.
Absolute Pressure Sensor
An absolute or non-vented pressure sensor measures all pressure forces detected by the strain gauge, including atmospheric
pressure (P
ATM
). The unit of measure is PSIA (pounds per square inch absolute), measured with respect to zero pressure.
The back of an absolute pressure sensor is sealed from the atmosphere. Therefore, the front of the absolute pressure sensor
responds to both atmospheric pressure and the pressure head of water above the sensor.
Absolute Sensor
1 Sensor back
2 Sensor front
3 Water pressure, P
W
(e.g., 20 PSI)
4 Atmospheric pressure P
ATM
(e.g., 14.5 PSI)
5 Vacuum
Gauged Pressure Sensor
A gauged or vented pressure sensor eliminates the effects of atmospheric pressure because the vent tube in the cable allows
atmospheric pressure to be applied to the back of the sensor. The unit of measure is PSIG (pounds per square inch gauge),
measured with respect to atmospheric pressure.