The level control does not ensure a constant pressure, but a constant level.
This is important, for example, for water reservoirs, tanks, pits and for lowering groundwater levels. Depending on the technology, pump control with level control achieves a high degree of automation, prevents damage or reduces the energy requirements of a system. With the level probe, the ultrasonic probe, the radar measurement and the float, there is a suitable level meter for many scenarios.
Level probes measure the hydrostatic pressure of the liquid column above them. Since the hydrostatic pressure depends only on the height above the sensor and the density of the liquid to be measured, the level can be determined via this. To do this, the level probe is lowered into the liquid, usually through a level tube, to just above the bottom of the vessel. The absolute pressure at this point is made up of the atmospheric air pressure and the pressure of the liquid column.
To ensure that changes in the air pressure above the liquid do not falsify the measurement, this must be subtracted again from the absolute pressure. For this purpose, the external air pressure is passed on to the relative pressure sensor in the level probe via an air hose in the sensor cable; alternatively, a combination of two absolute pressure sensors can provide the pressure difference sought.
This type of sensor emits ultrasonic pulses that are reflected from the surface being measured and recaptured by the sensor. By measuring the transit time from the sensor to the surface and back, the level can be calculated. However, due to the decay behavior of the transmitter, pulses can only be received after a certain minimum running time; the minimum distance to the surface to be measured corresponding to this running time is called “blocking distance”.
A moving surface, for example directly after filling, also affects the results. Correct values can therefore only be obtained when the product is at rest.
An advantage is that physical and chemical properties of the liquid or the product generally do not falsify the measured value. Therefore, the ultrasonic level sensor is also suitable for reactive or highly viscous chemicals.
Radar sensors are mainly used for distance measurement over longer distances and for speed measurement.
The sensor emits a carrier frequency which is reflected by the surface to be measured. The reflected frequency and the propagation time are recorded so that precise statements can be made about the speed and the distance to the sensor. Radar sensors are extremely precise even in poor visibility conditions.
When used as a level sensor, the echo of the vessel bottom can be evaluated in addition to the echo of the surface. Since the propagation speed of the radar signal depends on the medium, the signal propagation time changes proportionally to the level, which means that the level can be calculated.
The two metrologically independent measured values for the height above and below the surface can be used to check the plausibility of the other value and to increase the accuracy of the level control.
Float switches consist of a float that experiences buoyancy from the liquid to be measured and triggers a switching operation due to the buoyancy force. Their disadvantage is that a change in their switching threshold is only possible by changing the mounting position. The system is therefore not particularly flexible.
Floats are often used as additional sensors for redundant dry-running or overflow protection or for the simplest controls where no particular accuracy is required, for example in the level control of a WC cistern.
Due to their simple mechanics, floats as level sensors are also considered to be particularly robust and insensitive, which is why they are also used in mine drainage.