Measurement and Control Equipment

Capacitive Level Transmitter: Continuous Liquid Level Measurement with 4-20 mA

Learn how capacitive level transmitters measure liquid level in tanks and boilers continuously and convert it into a 4-20 mA signal, and how they are calibrated and used.

ORFA Technical TeamUpdated: Oct 30, 20263 min read

Capacitive level transmitter for continuous 4-20 mA liquid level measurement

In process control, a simple “full” or “empty” signal is often not enough; to manage pump speed, feed valves or inventory correctly, the level must be known at all times. A capacitive level transmitter measures the liquid level continuously along its probe and converts it into a standard 4-20 mA analogue signal.

Suitable for both conductive and non-conductive liquids, these transmitters have no moving parts and therefore run for long periods with little maintenance. Below you will find the working principle, calibration and typical applications.

How Does a Capacitive Level Transmitter Work?

The capacitance of a capacitor depends on the plate area, the distance between the plates and the dielectric constant of the material between them. In a capacitive level transmitter, the electrode rod and the tank wall (or a reference electrode) form this capacitor. Since the electrode geometry is fixed, what changes the measured capacitance is how much of the probe is surrounded by liquid and how much by air or vapour.

Because the liquid has a higher dielectric constant than air, capacitance increases in proportion to the level along the probe. The electronics measure this change and convert it into a linear current signal, typically 4 mA for an empty tank and 20 mA for a full one. The signal is used directly by PLCs, indicators, control panels or proportional controllers.

Calibration: Zero and Span

Measurement accuracy depends on calibrating the transmitter for the liquid being measured:

Zero: Set 4 mA with the probe uncovered or at the lowest measuring level.
Span: Set 20 mA with the liquid at the highest required level.
Check: Verify that the signal changes linearly at intermediate levels.

Tip: If the measured liquid changes (for example a different oil or a solution of different concentration), its dielectric constant changes too, and zero and span must be set again.

Advantages

Continuous measurement: The level is tracked along the whole probe, not just at fixed points.
Conductive and non-conductive liquids: With the right probe (insulated or bare) it works with water, oils, fuels and chemicals.
Standard 4-20 mA output: Connects directly to automation systems; current signals are robust against interference over long cable runs.
No moving parts: No mechanical wear, so maintenance is minimal.
Compact and easy to install: Mounted from the top of the tank through a single process connection.

Selection Points

Probe length: Must cover the required measuring range.
Probe insulation: Conductive liquids require a fully insulated probe.
Temperature and pressure: Hot, pressurised applications such as steam boilers need models designed for those conditions; ratings depend on the model.
Tank material: Plastic tanks require a probe with a reference electrode.
Supply and output: Must match the supply voltage and signal type of the control system.

Applications

Sector / application	Typical tanks
Heating and steam systems	Steam boilers (proportional level control), high-temperature tanks
Fuel and oil	Fuel tanks, oil tanks
Food and beverage	Food machinery, concentrate tanks, brine tanks, wine storage
Cooling	Glycol and coolant tanks, liquid CO2 tanks
Marine	Ship tanks and marine applications
General storage	Clean water tanks, plastic tanks, low-conductivity liquids

Frequently Asked Questions

Why does a 4-20 mA signal start at 4 mA rather than 0 mA?

With a “live zero”, 0 mA indicates a fault such as a broken cable or loss of supply, so an empty tank can be distinguished from a failure.

Can a capacitive level transmitter be used on a steam boiler?

Yes. Capacitive electrodes rated for boiler conditions are used to measure the water level continuously and modulate the feed pump or valve. Low-water safety must still be provided by separate, independent safety devices.

What if the measured liquid changes?

Because the dielectric constant changes, the zero and span settings must be repeated with the new liquid.

Conclusion

Capacitive level transmitters monitor liquid level continuously with a standard signal, making it easy to automate pumps, valves and inventory management. The right probe type, a suitable pressure-temperature rating and correct on-site calibration determine reliable measurement. Browse our Level Control Devices page and contact the ORFA technical team for help choosing a transmitter.

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