How to Choose Between Ultrasonic and Radar Level Meters for Foamy Liquid Surfaces

At sites such as wastewater treatment facilities, fermentation tanks, cleaning solution tanks, chemical reactors, and mineral slurry ponds, the presence of foam on the liquid surface can easily cause level signals to fluctuate, jump, or even result in measurement failure. A common question from operators is: when dealing with foamy liquid surfaces, how should ultrasonic and radar level meters be selected?

The key is not simply determining whether “ultrasonic” or “radar” is more advanced, but analyzing foam thickness, foam density, medium dielectric constant, tank structure, steam, dust, and installation space. Different sensors receive echoes in different ways and therefore differ significantly in their ability to adapt to foam layers.

Xi'an Shenghongchuang Instrumentation Co., Ltd. has long provided sensors and transmitters for industrial measurement and control applications involving pressure, flow, displacement, weighing, temperature, and humidity. For level measurement projects involving foam, operating conditions should first be evaluated before determining a suitable solution, such as non-contact ultrasonic, radar, or hydrostatic level transmitters.

Generally, loose, thin, and short-duration foam has relatively little impact on some radar level meters. For continuously thickening and dense foam, guided wave radar, hydrostatic level sensors, or redundant measurement methods should be considered comprehensively. The more complete the site information before selection, the more stable subsequent operation will be.

I. First Understand Why Foam Interferes with Level Measurement

Foam Layers Change Echo Propagation and Reflection Conditions

Ultrasonic level meters calculate distance by transmitting sound waves and receiving echoes reflected from the liquid surface. Foam consists of a large number of bubbles. After sound waves enter a foam layer, absorption, scattering, and attenuation occur. In particular, foam thicker than 50 mm with a relatively loose structure often significantly weakens the effective echo.

Radar level meters use microwave signals for distance measurement, and their reflection performance is directly related to the dielectric constant of the medium. Water-based media generally provide good reflection conditions, but foam contains a large amount of air, reducing its overall dielectric properties. Radar waves may produce echoes of different strengths at the foam surface, inside the foam, or at the actual liquid surface.

When foam thickness and density continuously change, the instrument may lock onto an incorrect echo, resulting in slow level display drift, instantaneous jumps, or a fixed reading. If the control system uses this signal for pump start/stop, overflow alarms, or feeding control, unstable measurement can further affect process safety.

Therefore, for foamy conditions, it is not sufficient to consider only whether foam is present. The average thickness, maximum thickness, duration, whether it covers the entire liquid surface, and factors such as agitation, aeration, and spraying should also be recorded. It is recommended to observe at least 1 complete production cycle and, where necessary, continuously record level changes for 24 hours to 72 hours.

Common Foam Characteristics and Measurement Risks

II. Which Foam Conditions Are Suitable for Ultrasonic Level Meters

Advantages and Application Limits of Ultrasonic Measurement

Ultrasonic level meters feature non-contact installation, a relatively simple structure, and convenient maintenance. In storage tanks or pools under normal temperature and pressure, with relatively stable liquid surfaces and little foam, they can meet routine level monitoring requirements. For open containers with ranges of approximately 3 m, 5 m, and 10 m, good application results can be achieved with correct installation.

However, ultrasonic measurement is not the preferred choice under foamy conditions. Sound waves require a sufficiently clear interface for reflection. When a soft foam layer forms on the liquid surface, echo energy is dispersed. If high-temperature steam, condensate, dust, or strong airflow are also present, the measurement distance may be affected further.

For conditions where foam occurs occasionally and is typically less than 20 mm to 30 mm thick, display stability can be improved by properly setting damping time, echo threshold, and dead zone parameters. The damping time can be set according to the actual rate of level change. For example, storage tanks with slow level changes may use a signal smoothing time of 5 seconds to 20 seconds.

During installation, avoid the filling inlet, areas directly above aeration pipes, and agitator zones. The probe axis should be as perpendicular as possible to the normal liquid surface. For containers with a small diameter, prevent sound waves from striking the tank wall or supporting structures and producing false echoes. The high-level control point must not be set within the dead zone.

Key Points for On-Site Evaluation of Ultrasonic Solutions

When the on-site liquid is mainly clean water or low-viscosity liquid, foam appears only during feeding or short-term agitation, and the process permits a certain display delay, ultrasonic level meters can still be an economical option. In this case, on-site trial measurement should be prioritized rather than selecting a model solely based on the medium name.

If the site has experienced sudden level increases or decreases of more than 0.2 m, frequent lost-echo alarms, or repeated instrument fluctuations around the same level, the ultrasonic solution should be reassessed. Continuing to rely on software filtering can only alleviate surface symptoms and cannot eliminate insufficient physical echoes caused by foam.

For high-high level alarms requiring reliable interlocking, it is recommended not to use a single ultrasonic device as the sole basis. An independent high-level switch can be added, or a second type of level sensor based on a different principle can be used for cross-checking to reduce the risk of false alarms and missed alarms under foamy conditions.

III. What Type of Radar Level Meter Is More Suitable for Foamy Liquid Surfaces

Non-Contact Radar Generally Offers Better Environmental Adaptability

Compared with ultrasonic level meters, radar level meters do not depend on sound velocity propagation in air. Therefore, changes in temperature, pressure, steam, and airflow generally have less impact on distance measurement results. Radar usually offers more stable adaptability for level measurement in the presence of steam, condensation, higher temperatures, or enclosed vessels.

When dealing with light, thin, or intermittent foam, high-frequency radar such as 80 GHz radar usually provides a narrower beam angle, enabling it to better avoid interference from tank walls, ladders, and internal structures. For tanks with limited nozzle sizes and compact installation space, a smaller transmission angle also helps improve effective echo identification.

However, radar cannot directly penetrate all types of foam. When the medium has a low dielectric constant, the foam is thick and dense, the foam has a high oil content, or the liquid surface is violently turbulent, the reflected echo from the actual liquid surface may still be insufficient. In such cases, attention must be paid to the instrument echo curve rather than only the instantaneous digital display result.

When selecting non-contact radar, the process connection size, antenna material, temperature and pressure resistance ratings, and protection requirements should be clearly specified. Common 4 mA to 20 mA outputs, RS485 communication, or fieldbus communication methods need to match the existing PLC, DCS, and alarm logic, preventing a correct instrument selection from being paired with incomplete system integration.

The Complementary Value of Guided Wave Radar and Hydrostatic Level Sensors

If foam remains thicker than 100 mm for extended periods and measurement of the actual liquid level is required, guided wave radar is generally worth serious consideration. Guided wave radar transmits electromagnetic waves along a probe rod or cable, concentrating energy near the waveguide structure. Compared with non-contact radar transmitted through space, it is more suitable for complex foam layers.

For applications with stable medium density and suitable bottom-tank installation conditions, submersible or flange-mounted hydrostatic level transmitters are also practical solutions. Hydrostatic measurement converts liquid column pressure into level. In theory, it will not mistake the top of foam for the actual liquid surface, but density changes, crystallization, blockage, and corrosion must be considered.

In production lines involving corrosive media, adhesive media, or hygienic requirements, the sensor wetted material and diaphragm structure are equally critical. The suitability of materials such as 316L stainless steel and PTFE should be confirmed based on medium composition, temperature, and cleaning methods, rather than being determined solely from experience with ordinary water-based media.

For critical storage tanks, a combination of “continuous radar measurement + independent level switch alarm” can be used. Continuous level measurement is used for process control, while independent alarm points are used for safety interlocking. When the two measurement units use different principles, they provide better isolation from the risk of foam affecting a single sensor.

IV. Practical Steps for Selecting and Installing Foam Level Meters

Collect Parameters First, Then Determine the Sensor Solution

The first step is to confirm the measurement range and installation height. For example, if the actual level range is 0 m to 6 m, the instrument range should not be selected based only on 6 m; installation dead zone and a safety margin should also be reserved. For top-mounted non-contact instruments, the maximum distance from the probe to the lowest level must be defined, as well as whether the highest level may enter the dead zone.

The second step is to record foam conditions, including foam thickness, duration, whether it is generated by aeration, and whether it contains oil or surfactants. When foam is less than 30 mm thick and occurs intermittently, non-contact radar can be evaluated first. When foam exceeds 100 mm for extended periods, guided wave radar or hydrostatic level transmitters should be included in the comparison.

The third step is to confirm medium temperature, pressure, viscosity, density, corrosiveness, and explosion-proof rating. High-temperature, high-pressure, flammable, explosive, or highly corrosive conditions impose higher requirements on sensor antennas, process connections, sealing structures, and electrical protection, and the configuration for ordinary water storage tanks cannot simply be used.

The fourth step is to clarify control accuracy. If the purpose is only overflow prevention or low-level indication, a certain degree of fluctuation is acceptable. If used for batching, metering, or reaction control, higher stability is required. Specifying allowable error, response time, and alarm setpoints in advance can prevent repeated parameter adjustments later that still fail to meet process requirements.

Installation and Commissioning Determine Final Measurement Performance

Radar or ultrasonic probes should be installed where the liquid surface is relatively stable, away from material feeding impact zones and vent outlets. For cylindrical tanks, the installation point should generally not face the center directly and should not be too close to the tank wall. The specific location must be determined in conjunction with the antenna beam angle, tank diameter, and internal structures.

During commissioning, it is recommended to verify actual values separately at low, medium, and high liquid levels, and observe echo changes before and after foam appears. After continuous operation for at least 24 hours, confirm the range corresponding to 4 mA and 20 mA, alarm thresholds, damping time, and fault output logic to avoid acceptance based only on static conditions.

For guided wave radar and hydrostatic level sensors, a regular inspection mechanism should be established. Scale buildup on the guided wave rod surface, submersible probes covered by debris, and blocked pressure ports can all cause long-term measurement deviations from actual values. Maintenance intervals can be set to 1 month, 3 months, or 6 months according to the cleanliness of the medium.

Xi'an Shenghongchuang Instrumentation Co., Ltd. recommends that users provide vessel dimensions, medium name, temperature and pressure range, foam photos or videos, existing mounting nozzle specifications, and control signal requirements before purchase. Based on this actual operating-condition information, level sensors, level transmitters, and supporting display and control instrument solutions can be matched more accurately to site requirements.

V. Conclusion: Do Not Simply Choose Between “Ultrasonic or Radar”

For ordinary level measurement with no foam or very little foam, ultrasonic level meters offer good practicality. For conditions involving steam, enclosed environments, thin foam layers, or limited installation space, non-contact radar level meters generally offer greater advantages.

When foam persists for a long time, is thick and dense, and the process requires identification of the actual liquid surface, guided wave radar and hydrostatic level transmitters should be compared as priority options. For important control points, a dual-measurement solution based on different principles is often more reliable than simply increasing the specifications of one type of instrument.

The key to stable level measurement is matching the sensor principle to on-site physical conditions. Only by fully considering foam, medium characteristics, vessel structure, and control requirements can fluctuations, false alarms, and maintenance costs be reduced while ensuring continuous production operation.

If your storage tank, reactor, or liquid tank has foamy liquid surface issues, you may compile information on the measurement range, medium, temperature and pressure, foam thickness, and installation site, and contact Xi'an Shenghongchuang Instrumentation Co., Ltd. for operating-condition evaluation and level sensor selection recommendations better suited to your site requirements.