Submersible level sensors and transmitters make it very easy to measure the level
of a tank, depth of a river, level of water in a borehole for instance. You simple suspend the sensor by its strain relieved cable to the bottom of the tank or borehole or river (or just off the bottom to prevent silt, rust and other grime found at the bottom from building up on the sensor) and it will measure the hydrostatic pressure
created by the liquid above the sensor (as a rough guide approx. 1m of water = 100mbar of pressure) and convert this to an electrical signal, we use the electrical signal to calibrate an accurate level measurement in the tank / river / reservoir / borehole for instance or if you know the dimensions of the tank you can easily calculate the volume of the tank. We have instruments
that will do that for you too in the indicators and data loggers section. The SRP-73
for instance all have 20 point linearisation or pre-configured tank shapes to calculate the square, cylindrical, dished or cone ends of tanks which makes life easier for installing.