Applications for measuring and monitoring water levels (in wells, tanks for irrigation, etc.) can utilize a variety of sensors for continuous or point level measurement.
We recommend using a hydrostatic level sensor for continuous water level measurement.
Hydrostatic sensors of surface level are basically pressure transducers, which measure the water level by measuring the hydrostatic pressure. The measurement is very accurate and stable over time; the thermal volume expansion can cause some inaccuracy, however, it is totally negligible for standard level measurements.
Hydrostatic level measurement has several advantages
The sensor has no moving parts, and the measurement is not influenced by impurities and solid particles on the surface or at the bottom. Measurements of the level of polluted liquids is as exact as measuring in a very clean environment.
For open tanks there are available sensors which measure hydrostatic pressure as a gauge pressure against the atmosphere. Venting to the atmosphere is done via a tube with an open end (usually it is installed together with the power cable), which needs to be installed so as to prevent clogging (e.g. the free end of the tube can be placed below the tank cover and bent downwards).
If you need to monitor both the minimum and maximum water level in the reservoir (such as the well), you can also use point level sensors, which are cheaper than the continuous type, but only give you information whether a certain predefined level has been reached. If two levels must be monitored (e.g. MIN and MAX), the price of good-quality point level probes is close to that for continuous measurement devices, which are more advantageous. Before you buy some cheap and dilettantish solutions, you should first take into account the extreme conditions on the water level - humidity, condensation, the influence of flowing current on corrosion, etc. Therefore it is advisable to obtain a better quality product in spite of a higher acquisition price.