Continuous measurement or switching: what is the difference?

There are two main functions in level measurement: continuous measurement and switching measurement.

A continuous level sensor provides an analogue signal (usually 4–20 mA or 0–10 V) corresponding to the actual fill height of the liquid. This allows you to calculate the contents of a tank or control a regulating valve. This type is used when you need to know the exact fill level at all times, for example for batching or inventory management.

A level switch only outputs a signal when a preset liquid level is reached. This can be a minimum level (e.g. for pump protection) or a maximum level (e.g. as overflow protection alongside a continuous measurement). Level switches are simpler and less expensive, but provide no information about intermediate levels.

What types of level sensors are available?

Ebora supplies the following measuring principles for liquid level measurement and level switching applications:

• Ultrasonic level sensors
• Capacitive level sensors and level switches
• Hydrostatic level sensors
• Radar and guided wave radar level meters
• Float switches
• Vibrating fork level switches
• Optical level switches
• Magnetostrictive level sensors
• Conductive level switches
• Magnetic level gauges

Selection guide: which measuring principle suits your liquid application?

Unsure which type is right for you? Provide us with the process parameters — medium, temperature, pressure and tank geometry — and we will advise on the most suitable type.

Ultrasonic level measurement: suitable for open liquid applications

An ultrasonic level sensor emits acoustic pulses that reflect off the liquid surface. The internal processor calculates the level from the travel time, corrected for temperature. This principle works well for clean liquids in open or lightly enclosed tanks and is not sensitive to changes in the dielectric constant or conductivity of the medium.

Note when selecting: in the case of heavy foaming, severe turbulence or intense evaporation above the liquid surface, ultrasonic measurement may be less reliable. In vacuum conditions this principle cannot be applied.

Capacitive level sensors: also suitable for aggressive liquids

Capacitive level sensors measure the change in capacitance when the electrode is submerged in the liquid. The measurement is not sensitive to pressure, temperature or vapours above the surface, making this principle suitable for closed tanks and aggressive media.

Choose a capacitive level sensor when the liquid has a stable dielectric constant. When the composition varies significantly — for example with changing mixing ratios — calibration or an alternative measuring principle is advisable. Capacitive level switches are also widely used as simple point detection for minimum or maximum levels in liquid applications.

Hydrostatic level sensors: direct pressure measurement in the tank

A hydrostatic level sensor measures the pressure at the bottom of the tank. Because hydrostatic pressure is directly dependent on the height of the liquid column (p = h · ? · g), this provides reliable continuous measurement — even with foaming or turbulent liquids. This type is not sensitive to surface disturbances.

This type is used when the liquid has a stable and known density. When the liquid density varies, correction of the output signal is necessary. Hydrostatic sensors are widely used in water treatment, the chemical industry and storage tanks. They are closely related to the pressure sensors in our instrumentation range.

Radar and guided wave radar: for demanding liquid processes

Radar-based level meters operate on the basis of microwaves and are completely independent of the atmosphere above the liquid. Pressure, temperature, steam and foaming have no influence on the measurement.

With guided wave radar (TDR — Time Domain Reflectometry), the electromagnetic pulses are guided via a probe to the liquid surface. The travel time of the reflected pulse determines the level. This principle is suitable for liquids with a low dielectric constant and for tanks with internal structures that make free radiation difficult.

Choose radar or guided wave radar when ultrasonic or capacitive measurement is insufficiently reliable due to high pressure, high temperature or heavily foaming liquids.

Level switches for liquids: float switch, vibrating fork and optical

Level switches are used for point detection in liquid applications: signalling a minimum or maximum level. Ebora supplies several versions:

• Float switch: a mechanical principle in which a float rests on the liquid and closes a contact at a specific level. Reliable and proven in straightforward applications.
• Vibrating fork level switch: vibrates at resonant frequency and detects damping upon contact with the liquid. No moving parts, suitable for clean and lightly contaminated liquids.
• Optical level switch: operates on the basis of infrared light and refractive index. Compact and without moving parts, suitable for clean, transparent or lightly coloured liquids.
• Conductive level switch: detects conductive liquids via an electrical circuit. Simple and reliable for water, acids and alkalis.

Selection criteria for the right level sensor

Several technical and practical factors play a role when selecting a level sensor for liquids:

• Medium: dielectric constant, density, conductivity and aggressiveness
• Process conditions: temperature, pressure or vacuum, foaming, turbulence
• Tank or vessel: geometry, connection points, presence of agitators or internals
• Measurement: continuous value or switching point, required accuracy, response time
• Installation: mounting options, calibration, environmental classification (e.g. ATEX)

Level sensors in combination with other instrumentation

In many installations a level sensor works together with other measuring instruments. A hydrostatic level sensor is often combined with flow meters for mass balance or with temperature measurement for density correction of the liquid. For storage of hazardous liquids, leak detection can be applied as an additional measure. For processes where liquid quality also needs to be monitored, Ebora additionally offers conductivity measurement and turbidity measurement.

Select the right level sensor based on liquid type, process conditions and required accuracy, or request technical advice directly via our contact page.

Frequently asked questions

What is the difference between a level sensor and a level switch?

A level sensor provides a continuous analogue signal representing the actual fill height of the liquid, such as 4–20 mA. A level switch only outputs an on/off signal when a preset liquid level is reached. Level switches are often used as a safety measure alongside a continuous measurement, for example as overflow protection or pump protection.

Which level sensor is suitable for aggressive liquids?

For aggressive or chemical liquids, capacitive level sensors and hydrostatic sensors with a suitable material selection (PVDF, Hastelloy, stainless steel) are most commonly used. Guided wave radar is also an option, as the probe is the only part in contact with the liquid.

Can a level sensor be used under pressure or vacuum?

This depends on the measuring principle. Capacitive and hydrostatic level sensors and guided wave radar are suitable for closed tanks under pressure or vacuum. Ultrasonic level meters cannot be used in vacuum conditions, as sound does not travel in a vacuum.

How do I choose between radar and ultrasonic for liquid applications?

Choose ultrasonic for clean liquids in open or lightly enclosed tanks without extreme process conditions. Choose radar or guided wave radar when there is high pressure, high temperature, heavy foaming or steam above the liquid surface — conditions that interfere with ultrasonic measurement.