Optical Level Switch Gives Incorrect Signals: Correctly Assessing Foam, Bubbles and Deposits

→ Product category: Point Level Detection

 

The tank is almost empty, but the optical level switch continues to indicate “full”. After cleaning, the measuring point works perfectly again. In another tank, the switching signal constantly fluctuates between “medium present” and “medium not present” while the pump is running.

Such behaviour with optical point level sensors is not automatically a sign of a defective sensor. Foam, air bubbles, droplets, product films or deposits on the optical tip can change the interface on which the measuring principle depends.

It is therefore particularly important to determine whether the sensor tip is actually completely covered by or completely free of the medium, or whether only a thin film, a foam bubble or a changing gas-liquid interface is present.

Suitable solutions can be found under Point Level Detection. Further measuring principles and products are grouped under Level Measurement.

How does an optical point level switch work?

An optical or optoelectronic point level switch normally has a transparent prism tip on the process side.

The sensor contains:

• a light source, often an infrared LED,
• an optical prism,
• a light receiver,
• evaluation electronics.

If the prism tip is surrounded by air or gas, the light is largely reflected within the prism back to the receiver.

If the tip is covered by liquid, however, the optical interface between the prism and the surrounding medium changes. A larger proportion of the light exits the prism into the medium and considerably less light reaches the receiver.

The electronics detect this change and switch the output.

The principle therefore normally requires:

• no float,
• no mechanically moving parts,
• no long measuring rod.

This makes optical point level switches particularly suitable for compact tanks and applications in which a defined single switching level is to be detected.

Why can an optical sensor switch incorrectly?

The sensor cannot determine why the optical conditions at its prism tip have changed.

For the sensor, the decisive factor is initially whether the tip optically corresponds more closely to the condition:

dry

or:

covered by medium

.

Conditions between “completely dry” and “completely immersed” can therefore also influence the measurement.

Typical causes include:

• foam,
• air bubbles,
• gas bubbles,
• droplets,
• adhering liquid films,
• oil or grease deposits,
• dried product residues,
• strong wave movement directly at the switching point.

Troubleshooting should therefore always begin with a visual inspection of the sensor tip and the installation conditions.

Correctly assessing clear and turbid liquids

Optical point level switches are often associated with “optical measurement” and are therefore incorrectly assumed to be suitable only for transparent liquids.

However, the measuring principle does not look through the entire tank.

What matters is the optical interface directly at the prism tip.

Depending on the specific device, the sensor can therefore detect, for example:

• water,
• oils,
• emulsions,
• coolants,
• other suitable liquids.

A turbid liquid is therefore not automatically problematic.

More critical can be media that:

• adhere strongly,
• coat the prism tip,
• drain only very slowly when the tank is emptied,
• foam strongly.

Suitability should therefore always be assessed based on the specific medium and the manufacturer’s approval for the sensor.

How does foam affect the measurement?

Foam consists of liquid films and enclosed gas bubbles.

This means that there is no clearly homogeneous condition at the sensor tip.

Depending on:

• foam density,
• bubble size,
• liquid content,
• wetting of the prism tip,
• foam stability

the sensor can react differently.

Dense, wet foam can behave optically more like a liquid than dry, coarse-bubble foam.

This can lead to two fundamentally different problems.

Foam is detected as medium

The actual liquid level is below the sensor, but dense foam has already reached the prism tip.

The sensor may then indicate:

Level reached

even though the liquid itself is still lower.

Liquid is present, but the signal remains unstable

Large gas bubbles or strongly moving foam can alternately wet and uncover the prism tip.

This can cause the output to switch repeatedly.

If the process regularly generates strong foam, the first question must therefore be:

Should the foam be detected or ignored?

This question has a major influence on whether an optical level switch is the appropriate measuring principle for the application.

Gas and air bubbles at the sensor tip

Even without visible foam, individual air or gas bubbles can influence the switching state.

Typical causes include:

• return lines,
• pump suction,
• strong circulation,
• air entrainment during filling,
• degassing of the medium,
• cavitation.

If the sensor is installed directly in a strongly flowing area, a larger gas bubble can briefly displace the liquid from the prism tip.

For a short moment, the sensor then sees a condition similar to “dry”.

This can result in a short switching pulse.

Such behaviour should not immediately be concealed by configuring a very long PLC time delay.

First, it should be checked whether the measuring point can be moved away from the highly turbulent area.

Droplets, films and deposits

After a tank has been emptied, the prism tip must return clearly to the dry condition.

With liquids that drain well, this normally happens quickly.

Problems can occur with:

• viscous oils,
• sticky media,
• emulsions with residues,
• greases,
• crystallising liquids,
• dried process residues.

If a sufficiently thick liquid film remains on the prism, the optical interface can remain altered.

The tank may already be mechanically empty while the sensor tip is not yet optically fully uncovered.

A typical fault pattern is:

The sensor continues to indicate “full” after emptying and only switches back after cleaning.

If deposits repeatedly occur, it is not sufficient merely to clean more frequently.

It should be checked whether:

• the mounting position can be improved,
• the medium can drain more effectively from the tip,
• regular automatic cleaning is possible,
• another point level measuring principle would be more suitable.

Selecting the correct mounting position

Depending on the version, optical point level switches can be installed in different orientations.

For the actual application, however, not every position is equally favourable.

Especially with media that tend to adhere, the mounting position can influence how quickly the sensor tip becomes free again after the level falls.

Unfavourable measuring points are those where:

• liquid remains permanently,
• spray hits the prism directly,
• a return flow is directed onto the sensor,
• foam preferentially collects,
• deposits are promoted by the design.

With side mounting, it should also be checked whether the medium can drain cleanly from the prism tip.

With top mounting, however, it must be ensured that the desired switching level is reached unambiguously and that permanent droplets or condensate do not influence the sensor.

Reducing waves and turbulent liquid movement

In a moving tank, a switching level is not a mathematically exact horizontal line.

In small machine tanks, significant waves can already be caused by:

• pump return flow,
• agitators,
• moving machine components,
• rapid filling.

If the average liquid level is exactly at the height of the prism tip, each wave alternately covers and uncovers the sensor.

A fast-reacting optical sensor reproduces this condition correspondingly quickly at its output.

Possible corrective actions include:

• moving the measuring point to a calmer area of the tank,
• increasing the distance from the return line,
• providing a stilling area,
• using a short switching delay.

Correctly defining switching logic and time delay

After the mechanical installation has been optimised, the electrical evaluation should be considered.

For a point level signal, it must be clearly defined:

• What does output ON mean?
• What does output OFF mean?
• How is a wire break evaluated?
• What response occurs in the event of a power failure?
• What delay is permissible?

The PLC should not document only the logical designation “0” or “1”, but the actual process meaning.

Example:

Condition	Process Meaning
Sensor dry	Switching level not reached
Sensor wetted	Switching level reached
No signal	Check measuring circuit or power supply

A time delay can suppress brief bubbles or waves.

However, it must match the protective function.

For dry-running protection, for example, the delay must not be so long that the pump runs for several seconds without sufficient medium.

Checking temperature, pressure and media compatibility

In addition to the optical behaviour, the normal process data must also be considered.

These include:

• medium temperature,
• ambient temperature,
• process pressure,
• process connection,
• material of the prism tip,
• seal materials,
• housing material.

An optical level switch must not be used solely because the measuring principle is generally suitable for liquids.

All wetted materials must be suitable for the process.

With changing temperatures, it must also be taken into account that the following properties of the medium can change:

• viscosity,
• foaming behaviour,
• wetting behaviour.

A measuring point may therefore release perfectly with cold oil and behave differently with warm, more strongly adhering medium – or vice versa.

Considering cleanability of the measuring point

The optical surface is the actual functional element of the measuring point.

It should therefore be considered in maintenance planning in the same way as a measuring diaphragm or electrode.

Important factors include:

• accessibility,
• suitable cleaning method,
• suitable cleaning agents,
• avoiding damage or scratching of the optical surface.

A sensor installed deep behind a difficult-to-access cover may function technically, but if deposits occur regularly it will cause unnecessarily high maintenance effort.

For heavily contaminating processes, cleanability should therefore already be considered when selecting the measuring point.

Systematically testing an optical level switch

For an implausible switching signal, the following procedure is useful:

1. Check the process condition: Is the sensor tip actually wetted or dry?
2. Inspect the prism: Look for foam, droplets, oil film or deposits.
3. Clean the sensor: Use an approved cleaning method.
4. Test dry operation: Check the output condition with a clean, dry tip.
5. Test wetting: Cover the tip with the actual process medium.
6. Check the switching state: Verify sensor LED or output.
7. Observe the installation: Check return flow, bubbles and waves during operation.
8. Compare the PLC input: Verify that the electrical sensor state is processed correctly.
9. Repeat the test: Wet and uncover the sensor several times.

If the sensor works reliably after cleaning, the cause is often not the electronics but the actual wetting condition at the optical tip.

Optical, capacitive or float switch?

Measuring Principle	Typical Strength	Points to Consider
Optical point level switch	Compact, fast, no moving parts	Assess foam, bubbles and deposits on the optical tip
Capacitive or impedance-based point level measurement	Depending on technology, targeted differentiation between medium and deposits may be possible	Consider dielectric properties and parameterisation
Float switch	Simple direct mechanical principle	Consider moving parts, viscosity, contamination and jamming

There is therefore no universally best measuring principle.

An optical sensor is particularly attractive for:

• small tanks,
• clean to moderately contaminated liquids,
• compact machines,
• fast point level detection,
• applications without mechanically moving sensor components.

With strong foam or persistent deposits, however, a sensor principle that can specifically distinguish between liquid, foam and deposits may be more suitable.

Practical example on a coolant tank

A machine tool has a small coolant tank equipped with an optical point level switch.

The sensor is used for minimum-level monitoring of the coolant pump.

After several months, the control system sporadically indicates:

Coolant level sufficient

even though the tank is almost empty.

The electrical test shows:

• supply voltage correct,
• sensor output switches,
• PLC input works correctly.

During the mechanical inspection, however, the prism tip is found to be covered by a viscous film of coolant emulsion and fine machining residues.

After cleaning, the dry sensor switches back correctly immediately.

During operation, it is also found that the return jet is directed directly at the measuring point.

As a result, the prism remains constantly wetted even when the average liquid level has already fallen slightly below the sensor.

The measuring point is therefore moved to a calmer area of the tank.

Condition	Before Modification	After Modification
Tank sufficiently filled	correct	correct
Tank almost empty	sporadic false indication	clear release
Pump running	strong wetting caused by return flow	calm measuring point

The sensor itself was therefore not defective.

The cause was a combination of deposits and an unfavourable mounting position.

Typical faults and corrective actions

Fault	Possible Consequence	Suitable Corrective Action
Dense foam reaches the sensor	Switching level is indicated too early	Determine whether foam should be detected or ignored
Large air bubbles at the tip	Brief switching back	Move the measuring point away from the turbulent area
Liquid film remains on the prism	Sensor continues to indicate medium after emptying	Optimise cleaning and mounting position
Return jet hits the sensor directly	Permanent or changing wetting	Select a calmer measuring position
Switching point located directly in a strong wave zone	Fluctuating signal	Calm the measuring point and, if necessary, use a short delay
Long PLC delay conceals the problem	Real level change is detected too late	Correct the mechanical cause first
Deposits are never checked	Switching behaviour changes over time	Include visual and functional inspection in maintenance
Incorrect switching logic in the PLC	Wetted and dry states are reversed	Test the actual switching condition during commissioning
Sensor selected only according to connection thread	Temperature, pressure or medium may be unsuitable	Check the complete process specification

Which devices are suitable?

Optoelectronic point level switches

Optoelectronic level switches are suitable for compact point level monitoring of liquids.

Typical applications include:

• machine tools,
• coolant tanks,
• hydraulic power units,
• pump systems,
• small process vessels,
• mechanical engineering.

A specific example is the WIKA OLS-C01. The compact optoelectronic level switch operates without moving parts and uses an optical prism tip for liquid detection.

Alternative point level technologies

Under Point Level Detection, other measuring principles are also available depending on the application.

These can include, for example:

• capacitive point level sensors,
• electronic point level sensors,
• float switches,
• vibrating forks,
• other application-specific solutions.

With strong foam, changing media or persistent deposits, it should already be checked during device selection how the respective sensor principle behaves under these process conditions.

ICS Schneider Messtechnik provides support in selecting the appropriate point level measuring principle and in assessing medium, temperature, pressure, mounting position and the required switching function.

Conclusion

An optical point level switch is compact, fast and requires no moving sensor components. However, its function depends on a clear condition being created at the optical prism tip between dry and covered by liquid.

Foam, individual gas bubbles and liquid films can make precisely this transition ambiguous.

In the event of a false indication, the electronics should therefore not be suspected first. The most important check is the actual condition directly at the sensor tip.

If the sensor indicates “full” even though the tank is empty, particular attention should be paid to adhering films, droplets or foam.

If the signal fluctuates, air bubbles, wave movement and a measuring point directly next to a return line or pump are typical causes.

A PLC delay can suppress short disturbances, but it should not be used to conceal a fundamentally unsuitable installation position.

With permanently strong foam or persistent deposits, another point level measuring principle may ultimately be more suitable.

The decisive question is therefore not only whether a sensor can detect liquid, but under which real operating conditions the switching level occurs in the respective system.

Frequently asked questions about optical point level switches

How does an optical level switch work?

An infrared LED sends light into a prism tip. In the dry condition, the light is reflected back to the receiver. When the tip is wetted by liquid, the optical interface changes and the receiver receives considerably less light.

Can foam trigger an optical point level switch?

This depends on the type and liquid content of the foam as well as the wetting of the sensor tip. Dense, wet foam can significantly influence the switching behaviour.

Why does my sensor indicate full even though the tank is empty?

A common cause is a remaining liquid film or deposit on the optical prism tip. Foam or permanent wetting caused by a return flow can also be responsible.

Why does the output fluctuate?

Waves, air bubbles or a liquid level exactly at the height of the sensor tip can cause rapid switching between the wetted and dry states.

Does a PLC time delay help against air bubbles?

A short delay can suppress individual brief signal changes. However, it should first be checked whether the measuring point can be moved away from the turbulent area.

Do optical sensors work only with clear liquids?

No. The decisive factor is the optical behaviour directly at the prism tip. Depending on the sensor version, oils, emulsions and other suitable liquids can also be detected.

Are optical level switches suitable for sticky media?

This depends on the application. If permanent residues remain on the prism tip, the switching behaviour can be affected. Cleanability and mounting position must therefore be considered particularly carefully.

Can I install the sensor directly next to a return line?

This is often unfavourable. A return flow can generate waves, bubbles or permanent local wetting and therefore simulate a level that does not correspond to the average tank level.

What alternative is available for strong foam?

Depending on the medium, capacitive or impedance-based point level sensors can be used, for example, which may be able to distinguish foam or deposits more effectively through suitable parameterisation.

How do I test an optical point level switch?

Clean the prism tip, check the dry switching state, then wet the tip with the actual medium and compare the sensor output with the PLC input. The measuring point should then be observed under real operating conditions with pumps, return flow and foam formation.