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Ultrasonic and radar level meters are both used for non-contact liquid level measurement, but they do not perform equally well in every application. An ultrasonic meter can work well in a wastewater wet well with a relatively clear water surface. Radar is often considered when gas composition changes or the process conditions make the ultrasonic sound path less predictable. Foam, vapor, installation geometry and measuring range all affect the final choice.
Wastewater treatment provides a useful example. A clean, relatively calm wet well is often suitable for an ultrasonic meter. In a sludge digester, changing biogas composition can affect ultrasonic measurement, so radar may be a better candidate. If the digester also has thick foam, however, the radar frequency and the ability to detect the liquid surface must be checked.
Silver Automation Instruments supplies both HL ultrasonic level meters and radar level meters. Model selection depends on the actual process conditions, including surface behavior, vapor, installation and measuring range.
| Application condition | What to check first | Practical starting point |
| Foam over liquid | Foam thickness, whether the foam is continuous or intermittent, and whether the meter needs to detect the foam surface or the actual liquid level. | Radar is often considered for foaming applications. However, thick foam can weaken the radar signal, especially at higher frequencies. Check the foam conditions and radar frequency before selection. |
| Acid tank | Acid type, concentration, operating temperature, and the materials of the antenna, sensor housing, seals and process connection. | Check the chemical resistance of all exposed materials. Select a model with suitable materials for the specific acid and operating conditions. |
| Open sump or pit | Maximum and minimum liquid levels, sensor mounting height, blind zone, and nearby pipes, ladders or other obstructions. | Ultrasonic is often a cost-effective choice for wet wells and rainwater pits with relatively calm water surfaces. Radar can be considered when vapor, pressure or other conditions affect ultrasonic measurement. |
Use the rows as screening guidance, not as a performance commitment for a specific model. A stable reading still depends on the surface echo, installation and the model’s published limits.
An ultrasonic level meter sends a sound pulse toward the surface and calculates the distance from the time taken for its echo to return. Because sound speed changes with temperature, the HL meter includes temperature compensation. Gas composition can also change sound speed, while vapor, foam, turbulence and obstructions can weaken or disturb the echo. Since the signal travels through the gas above the liquid, ultrasonic measurement is not suitable for vacuum service. Gases such as nitrogen, carbon dioxide and methane have different sound velocities from air, so changes in gas composition can introduce measurement errors.
The HL specifications define the available measuring ranges and installation limits.
A non-contact radar meter sends microwave energy toward the surface and calculates distance from the returned signal. It does not rely on the speed of sound in the gas above the liquid, but it still needs a usable reflection. The liquid’s dielectric properties, foam, antenna, nozzle and nearby structures can influence the echo. “Non-contact” describes the measurement principle; it does not remove the need to check seals, antennas, flanges and vapor exposure.
For either instrument, first define the level value the control system needs: the liquid surface, the foam surface, or a point that triggers an alarm. These are different targets, and a meter that consistently detects the wrong surface is not measuring the value the process needs.
Foam can weaken an ultrasonic echo because it absorbs or scatters sound. A radar echo can come from the foam surface or from the liquid beneath it, depending on the foam and the instrument’s signal processing. Foam thickness and density can change during filling and operation, so a single observation may not represent the full operating cycle.
In wastewater treatment, sludge digestion generates both biogas and, in some processes, foam. Changing gas composition can make the ultrasonic sound path less predictable, which is one reason radar is considered for digesters. Foam introduces a separate issue: it can also attenuate radar signals, depending on its thickness and properties.
Silver’s published radar frequency guide explains that foam can weaken radar signals, particularly at higher frequencies. It notes that lower-frequency radar is generally better suited to thick, dense foam, while high-frequency radar can be a poor choice for foamy applications. Therefore, a 26 GHz radar meter should not be presented as an automatic solution for a heavily foaming digester. Confirm the foam properties, required measuring surface and exact radar model with our engineers. See the radar frequency guide for more detail.
If the reading must represent the liquid beneath the foam, describe the foam at its thinnest and thickest, whether it persists, and whether it changes during filling or agitation. Also state whether the control system needs the liquid level or the top of the foam. This ensures that the instrument is evaluated against the correct measuring target.
For an acid tank, material compatibility is usually a key selection factor, regardless of whether the instrument uses radar or ultrasonic technology. Identify the acid, its concentration, normal and cleaning temperatures, and any vapor, splash or condensation that can reach the instrument. Then check each exposed part, including the sensing face or antenna, seal and process connection.
The 26 GHz range includes different liquid models. The SKRD91 is listed for highly erosive or corrosive liquids, with a maximum range of 20 m, process temperature from −40 to 120 °C, pressure from −0.1 to 0.3 MPa, and stated accuracy of ±5 mm. Its product documentation lists a PTFE antenna material. These figures describe the model’s listed limits; they do not prove compatibility with every acid or concentration.
The nitric acid level application is a separate configuration example, not a general chemical compatibility rating.
The SKRD92 is listed for liquid level measurement under specified conditions, including mildly erosive liquids. Its maximum range is 30 m, process temperature is −40 to 250 °C, process pressure is −0.1 to 4.0 MPa, and stated accuracy is ±3 mm. Antenna, flange and sealing materials must be checked against the exact order code and process conditions. Do not infer that the SKRD92 is suitable for a particular acid simply because it uses radar or because another model has PTFE parts.
An open sump often presents a geometry and range problem. Measure the distance from the planned sensor face to both the lowest and highest liquid levels. The highest liquid level must remain outside the model’s blind zone, while the lowest level must remain within its measuring range.
The beam also needs a clear path. A ladder, wall, pipe, inlet stream or pump structure inside the beam can create a competing echo. Waves and splashing can make the surface return fluctuate. Compare the installation drawing with the pit layout and avoid mounting directly over an inlet or a structure that can reflect the signal.
If the required output is flow rate from an open channel, level alone is insufficient. The channel geometry or a weir/flume and a configured level-to-flow relationship are also needed.
For a calm, atmospheric water sump, such as a rainwater basin or wet well, the HL ultrasonic range may be a practical option when its measuring range, blanking distance and beam clearance fit. Across a mechanical screen, two ultrasonic sensors can measure the level difference: when the screen is clean, the levels on both sides are similar; when the screen becomes blocked, the upstream level rises. For wet-well service, also check the sensor face for condensation and dirt, which can change the echo over time.
For a vessel with significant vapor or pressure, compare the exact radar model’s process limits and installation requirements instead of assuming that an open structure makes the application simple.
The HL and SKRD92 figures below describe different models and should be compared against the same measurement span and installation conditions. The HL is an ultrasonic series; SKRD92 is the liquid model in the 26 GHz pulse radar family.
| Specification | HL ultrasonic level meter | SKRD92 26 GHz pulse radar |
| Listed level range | 0–4 m to 0–40 m, depending on version | Up to 30 m for liquids |
| Blind zone | 0.20 m on the 0–4 m version; 1.5 m on the 0–40 m version | Confirm the selected antenna and drawing for the installation |
| Stated accuracy | ±0.2% of span | ±3 mm |
| Process pressure | 0.1 MPa rating | −0.1 to 4.0 MPa |
| Process temperature | −40 to 75 °C; HLT1: −20 to 75 °C | −40 to 250 °C |
| Configuration details | Thread or flange options; ABS/PVC or PTFE listed in model selection | Screw or flange connections; confirm antenna and sealing materials for the selected order code |
For the HL 0–4 m version, the 0.20 m blanking distance means the liquid surface must stay at least 0.20 m below the sensor face. On the 0–40 m version, that distance is 1.5 m. Its ±0.2% of span specification corresponds to ±80 mm over a 40 m span, so a 40 m range is not the same as millimeter-level accuracy.
For SKRD92, the 30 m maximum range applies to the liquid model. The 70 m figure on the broader family page belongs to solid-level models, not SKRD92. Its stated ±3 mm accuracy is a model specification; the antenna, nozzle and target still need to produce a usable echo. The ultrasonic blanking-distance guide shows why the minimum sensor-to-liquid distance matters in a short sump.
An open tank holding a sodium chloride solution needed level measurement at atmospheric pressure, with a maximum operating temperature of 55 °C. We supplied an ultrasonic level meter sized for a measuring range of up to 4 m to a system-integration customer.
The choice leaned toward ultrasonic for three reasons. The tank was open and atmospheric, the liquid surface provided a clear reflection for the ultrasonic pulse, and the 4 m measuring span fitted the HL 0–4 m version. Its 0.20 m blanking distance left most of the required range usable, while the 55 °C operating temperature was within the meter’s stated operating range.
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