For example, if a 2 metre sensor is reading full scale and the atmospheric pressure changes from 1000 to 1020 millibar, both faces of the sensor experience the same pressure equivalent to approximately 200mm depth. Thus the sensor will continue to read 2 metres. Gauge sensors utilise a vent tube to the surface to ensure that the reverse face of the pressure sensor experiences the atmospheric pressure. Vent tubes are generally encased in the sensor cable that connects the pressure sensor to the measuring instrument on the surface. The use of vent tubes is a very practical means of balancing atmospheric changes.
Absolute sensors have their reverse face enclosed and fixed at zero pressure.
Others can be referred to one-standard atmosphere. These have an advantage in that the vent tube is not necessary, but the disadvantage in shallow depths is that the atmospheric pressure must be independently and accurately monitored to compensate for significant errors due to atmospheric changes. Greenspan sensors are available in both Gauge and Absolute' Water Density and Gravity. When pressure sensors are used for depth readings of any fluid, the fluid density becomes an important parameter. This difference should be considered when particular measurement accuracy's are required.