In-Situ Level TROLL 400/500/700/700H Operators Manual - Manual (Page 14)

1-800-446-7488 14 www.in-situ.com
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 Inc. 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. See
www.in-situ.com/technical_notes .
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