This technical note describes the difference between absolute and gauged pressure sensors and explains the proper use of each type of sensor in different applications for In-Situ, Inc. LevelTROLL and AquaTROLL instruments. Figure 1 provides a general illustration of the forces measured by a pressure sensor’s internal strain gauge.
Absolute Pressure Sensors
An absolute or non-vented pressure sensor measures all pressure forces detected by the strain gauge, including atmospheric pressure (PATM). 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. This is shown by the following equation and drawing:
PMEAS = PW + PATM
Gauged Pressure Sensors
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. The forces applied to a gauged sensor are shown in the following equation and drawing:
PMEAS = PFRONT – PBACK = (PW + PATM) – (PATM) = PW
Which Sensor Should I Use?
Barometric pressure applies a direct stress upon open wells and surface water. Locally, barometric effects can change significantly from location to location as a result of topographical and micro-meteorological changes. Therefore, it is important to compensate for the barometric pressure changes when monitoring water elevation.
Atmospheric pressure fluctuations will cause water level changes in the monitoring well if the monitoring well is open to the atmosphere. Barometric data must be collected concurrently to post correct water level for barometric pressure effects, for both absolute and gauged sensors. However, a gauged sensor will provide better accuracy because it measures true water pressure.
In general, barometric effects do not cause changes in water level in unconfined aquifers. A gauged sensor will work well in this situation. Again, absolute sensors need to be corrected with barometric data to obtain accurate water level elevations.
Accuracy of Absolute and Gauged Sensors
Accuracy is defined as the degree to which instrument readings approach the true value. The In-Situ® Level TROLL® 500 and 700 Instruments have an accuracy specification of ±0.1% full scale (FS) from -5 to 50° C for both absolute and gauged sensors. The BaroTROLL® Instrument has an accuracy specification of ±0.2% FS from -5 to 50° C.
An accuracy specification of ±0.1% FS represents a deviation from 100% accuracy; therefore, accuracy should be specified as 99.9%. However, convention dictates that ±0.1% is an accuracy specification rather than an inaccuracy (tolerance or error) specification.
Gauged Pressure Sensors
A gauged Level TROLL 500 or 700 measures true water level elevations within ±0.1% FS. Post-correction is not needed, but the user must ensure that the vent tube of the cable is venting properly. In order to maintain proper venting and to protect the cable connector from moisture and corrosion, In-Situ, Inc. recommends using a desiccant cartridge at the surface connector of the vented cable.
Absolute Pressure Sensors
An absolute Level TROLL 500 or 700 also reads ±0.1% FS, but it needs to be post corrected with barometric data. Ideally, a BaroTROLL Instrument should be used to monitor barometric data when using an absolute Level TROLL Instrument. A BaroTROLL Instrument has an accuracy of ±0.2% FS. Therefore, the post-corrected data for true water level has an accuracy of ±0.3% FS because the error of the absolute Level TROLL 500 or 700 and BaroTROLL instruments is cumulative.
Accuracy Comparison of Absolute Sensors vs. Gauged Sensor
Figure 5 compares a gauged Level TROLL 15 PSIG sensor to an absolute Level TROLL 30 PSIA sensor. This illustration demonstrates how the output from an absolute level instrument has an additional error of ±0.2% due to the cumulative error of post correction. The total cumulative error is ±0.3% when using an absolute Level TROLL Instrument and a BaroTROLL Instrument compared to ±0.1% when using a gauged Level TROLL Instrument.
Absolute sensors will work for all applications if a barometric record is kept for applications that need to be compensated for barometric pressure changes, especially during long-term monitoring. However, post correction of the data introduces cumulative error into the final results, which makes the data less accurate than the data from a gauged sensor. A gauged sensor is more accurate because the vented system eliminates barometric effects on the sensor, which eliminates post processing for water levels in unconfined aquifers.
Well water levels in confined aquifers are indeed sensitive to barometric pressure. Post correction of water level for barometric effect is needed for both absolute and gauged sensors.
Source: In-Situ, Inc. Technical Note, Comparing Absolute and Gauged Pressure Sensors, April 2011.