As a means of determining the depth of a sensor from the water surface, pressure measurement gives reliable and consistent results as there is a directly proportional relationship between depth and pressure. There are two primary methods to measure pressure underwater – the strain gauge, and the piezoelectric methods.
Strain gauges change their electric resistance when a force is applied to a spring body – changes in force (such as the tide coming in or going out) compress or stretch the spring and so differences in resistance can be measured through a bridge circuit. This method is most suitable for long term measurements where each successive measurement only varies slightly compared to the previous. Strain gauges used in applications such as permanently installed tide gauges can give highly precise, high accuracy results as they are stable over the longer term and are well able to be compensated for changes in temperature. Strain gauge tide gauges are typically designed to measure over a range of 0-10m.
Piezo-electric pressure measurement is used to capture pressure changes that can vary much more rapidly such as when gathering a vertical profile of measurements using a vessel deployed sensor package as it travels first down and then up through the water column. They also change in electrical resistance with pressure but this is on an atomic level through the whole crystal structure rather than mechanical changes in a spring. Pressure sensors of this type are typically packaged with other sensors such as those that measure temperature, conductivity, sound velocity, turbidity and/or chlorophyll and provide a depth reference for these other parameters.
Piezo-electric pressure measurement can be used for water level measurement in the open ocean (relative to the seabed) if they are suitably compensated for the effect of temperature and are mounted on a mooring or a seabed frame.