Capacitance sensing probe devices for measuring liquid level and converting it to an electrical signal to send to other instrumentation.
A capacitive liquid level sensor is an instrument for measuring the height of a liquid and converting it to an electrical signal. The liquid level signal output can then be utilised by other instrumentation to display, monitor, log or control the liquid level.
Capacitive liquid level sensors are measurement devices which typically consist of a single probe which acts as an immersed electrode inside a tank. This probe interacts with an opposite electrode, usually the the wall of the tank, and the two are connected together to form a capacitor.
The liquid contents of the tank act as the dielectric medium, which will affect the capacitance created between the two electrodes. If the liquid level rises and falls, this will change the capacitance measured by the probe in a predictable way, and therefore it can be used very effectively as a measurement of liquid level.
The change in capacitance can be measured by applying an alternating voltage and current between the two electrodes, with a frequency typically in the radio frequency (RF) range.
A capacitive liquid level sensing probe makes direct contact with the liquid to be measured, and therefore material compatibility has to be a consideration, but for most applications a stainless construction is more than adequate for most fluids. Despite the material considerations, capacitance liquid level measurement is very reliable due to there being no moving parts to contribute to the wear and tear of the device.
Since the capacitive probe is essentially a stainless steel rod fitted to a tank, it is possible to construct the device to withstand high pressures so that it can be used on sealed pressurised tanks.
Also since there is no sensitive components immersed in the fluid it is possible to use on higher temperature media that could not be tolerated by other types of liquid level sensor.
Capacitive liquid level sensing probes are generally mounted through the top of a tank with probe protruding vertically into the tank and extending down to the minimum required level.
- Works well on dielectric fluids
- Simple mechanical construction
- No moving parts requiring regular inspection and maintenance, or leading to wear and tear
- High temperature fluid compatibility since no sensitive components make contact with the fluid
- Sealed pressurised or evacuated vessels will not affect performance
- Doesn’t work with conductive fluids without insulated outer tube
- Thick or sticky media will leave a coating on the probe creating an insulation layer causing malfunction, or affect performance
- Requires calibration to distinguish difference between liquid and air.
- Sensitive to changes in media which cause dielectric constant to change
- Probe protruding into liquid causing obstruction, attracting fouling and hygienic cleanliness issues