Depth of water is measured by a non-vented strain gauge. A differential strain gauge transducer measures pressure with one side of the transducer exposed to the water and the other side exposed to a vacuum. Depth is calculated from the pressure exerted by the water column minus atmospheric pressure.
Factors influencing depth measurement in water include barometric pressure, density, and temperature. Calibration in the atmosphere zeros the sensor with respect to the local barometric pressure. A change in barometric pressure will result in errors unless recalibrated or compensated.