How does the CTD sensor measure water level?

The CTD sensor uses an absolute pressure sensor located at the tip of the sensor housing to measure the weight of the water column above it, which is then converted to a water level (depth) reading. For compensated water level data that accounts for changes in barometric pressure, a separate barometric pressure reference is required — this can be provided by a second MX802 logger with a termination plug installed, or a U20 series pressure logger deployed at the surface.