Contrary to popular opinion silicon sensors can also be used to measure liquid levels. The use of transmitters or ready-to-use sensors with a standardized output in a package for direct installation is recommended here.
However, as this application involves liquid the media compatibility must be taken into account. Here, it is possible to use sensors with underside pressurization or fully media-insulated sensors. Underside pressurization* means that there is contact with the liquid on one side only.
Sensors such as these are used both to monitor (tank) volumes and to control the liquid inlet and outlet. In all applications the suitability of the sensors for the liquids to be sensed must be taken into account (media compatibility).
Possible areas of applications include, for instance:
- Water tanks
- Open silos and tanks
- Closed tanks
In all fields of application a distinction must be made between open and closed vessels.
In this scenario pressure sensors are used as relative pressure sensors. They sense the fill level regardless of the external pressure, thus yielding a fill height which can be determined when the liquid density is also known.
When determining the liquid level in wells, it must be noted that the relative pressure sensor is in the form of an immersion probe (transmitter* with a hose connection to the ambient air).
Open silos and tanks
In open silos and tanks the fill level is sensed by relative pressure sensors which are usually fixed to the outside of the vessel. The transmitters* used here are relative pressure sensors whose pressure range is dependent on the fill level. 10 meters of water = 1 bar.
In closed tanks the fill level must be determined using absolute pressure sensors. With pressure tanks in particular two sensors are required, as in the gas space above the liquid a process-dependent pressure can exist which must be taken into account when ascertaining the fill height. The absolute pressure sensors are dimensioned depending on the process pressure and fill level. The use of pressure transmitters* or ready-packaged sensors for direct installation is also recommended here.
10 meters of water = 1 bar + internal tank pressure
*Transmitters are calibrated, amplified, ready-to-use sensors with a standardized output in a package for direct installation.