Capacitive level measurement
For level measurement of different physical methods are applied. One of these is capacitive level measurement or detection, in which the dielectric constant or conductivity of the filling liquid, which differs from gases or air, is used.
Operating principles
The capacitive sensor is based on the change in the capacitance of a capacitor, which is partially formed by the liquid, when the level changes.
Probe in the tank
For the measurement, a probe located inside and the electrically conductive container wall form an electrical capacitor. If the probe is in air (1), a certain low initial capacitance is measured. If the container is filled, the capacity of the capacitor increases as the probe is covered. When the capacitive probe is designed as a limit signal transmitter, it switches when the capacity specified during calibration is reached. With capacitive probes for continuous level measurement, it is also possible to measure the level from a conductivity of 100 μS / cm, regardless of the dielectric constant (DK) of the liquid. This means that fluctuations in the DK value have no influence on the measured value display. Furthermore, errors caused by product build-up or condensate are avoided.
(1): CA = initial
capacitance (probe free) (2): R = conductivity of the liquid, C = capacitance of the liquid
(3): CE = end capacitance (probe covered), ΔC: change in capacitance
The probe electronics convert the capacitance into an electrical pulse train and amplify it. The evaluation electronics calculate the measured value from the pulse rate. The change in capacity of the medium is converted into a signal proportional to the level and enables the level to be displayed.
Mechanical construction
A capacitive probe usually consists of an electronics module in the housing with cable entry, the process connection and a probe rod or probe cable protruding into the container. The probe rod or probe cord are typically made of stainless steel and coated with a PTFE or PFA ( perfluoroalkoxylalkane ) insulation. The capacitive probe can often be combined with different process connections, housings and electronics according to a modular principle. This allows adaptation to the conditions of use.
Possible applications
The installation position of capacitive probes for continuous level measurement is always vertical, parallel to the container wall. In addition to continuous level measurement, these probes can also be used to measure the interface between e.g. B. Oil and water possible. In the case of probes for level detection, installation from above (rod or rope) or from the side (rod) is possible. In addition to pure point level detection in liquids and bulk solids, other possible applications are interface detection, foam detection or pump control. Modern capacitive probes are now able to compensate for the influence of the build-up on the probe, which means that they can also be used in highly viscous, adhesive media.
Sensors outside the tank
If the tank is non-conductive, the fill level can also be determined capacitively from outside the tank. For this purpose, capacitive proximity switches are often used, which are also able to detect non-conductive materials. They are attached on the outside at the point of the highest and / or lowest level and are calibrated in such a way that they only respond to one medium inside, but not through the container wall itself.
literature
- Rüdiger Settelmeyer: Process automation - From field device to automation solution , 2007, ISBN 3865223052
- Ellen Amberger: Level measurement technology. Basics and application examples , 1999, ISBN 3478930146
- Process automation - measurement, control and automation solutions for production and logistics in the process industry , Endress + Hauser , 2007