Gauge measurement

This covers the majority of liquid level applications and is used where the storage tank is freely vented to atmosphere. Since the "back" of the diaphragm is itself referenced to atmosphere, its displacement is due only to the applied liquid head.

In certain applications, for example on board a ship, even though the tank is vented it may be difficult to provide a satisfactory atmospheric reference for the sensor , and a completely sealed construction is more appropriate. In such cases an Absolute sensor is used.

Absolute Measurement

During construction the chamber behind the diaphragm is evacuated to absolute zero and then sealed. In this case the diaphragm is displaced by both the liquid head and the atmospheric pressure. Calibration of the sensor may either be from absolute zero, or, more normally , with a fixed offset - usually equal to the lowest anticipated atmospheric pressure.

To determine the actual tank level, Barometric pressure must also be measured and its value subtracted.

These notes are based on our previous experience and can only offer general guidance. PSM will be pleased to discuss your particular needs in detail.