The short answer
Radar Level Measurement
Radar level transmitters measure distance by timing a microwave signal reflected from the liquid surface, then subtracting that distance from a configured tank height to report level. Because nothing touches the liquid and the measurement does not depend on air temperature, humidity, vapor, or foam density the way ultrasonic does, radar is the preferred technology for wet wells, chemical tanks, and any vessel with a difficult atmosphere.
Key points
- Measures distance by time of flight, then converts to level using a configured reference height.
- Unaffected by air temperature, humidity, vapor, and most foam, unlike ultrasonic.
- Higher frequency gives a narrower beam, which matters in a narrow or obstructed wet well.
- Guided wave radar sends the signal down a probe and handles low-reflectivity liquids and turbulence.
- Most field problems are mounting position and false echoes, not the instrument.
How it works
The transmitter emits a microwave signal toward the liquid surface and measures how long the reflection takes to return, either by direct pulse timing or by frequency-modulated continuous wave, which sweeps a frequency range and derives distance from the frequency difference between transmitted and received signals.
That distance is subtracted from the configured distance to the bottom reference to yield level. This means the reference measurement, usually called the tank height or empty distance, is the single most important configuration value. Get it wrong and every reading is offset by exactly that error.
Level = Reference_Distance − Measured_Distance
- Reference_Distance — configured distance from the sensor face to the zero level point
- Measured_Distance — the distance the instrument measures to the surface
Why it beats ultrasonic in a wet well
Ultrasonic level uses sound, and the speed of sound in air varies with temperature and, to a lesser degree, humidity and gas composition. A wet well in July at the top and cooler at the surface is exactly the condition that makes an ultrasonic reading drift. Heavy vapor, condensation on the transducer face, and foam all degrade it further.
Radar is an electromagnetic measurement. Its propagation speed does not meaningfully change with air temperature or humidity, and vapor is largely transparent to it. That is why a station that gets recalibrated every spring on ultrasonic frequently stops needing attention after conversion to radar.
| Technology | Strengths | Weaknesses |
|---|---|---|
| Non-contact radar | Unaffected by vapor and temperature; no contact with sewage | Cost; needs a clear mounting path; false echoes from obstructions |
| Guided wave radar | Works on low-reflectivity liquids, turbulence, and narrow spaces | Probe is in the liquid and can collect rag and grease |
| Ultrasonic | Lower cost; well understood | Drifts with temperature; degraded by foam, vapor, and condensation |
| Submersible pressure | Simple; unaffected by surface conditions | In the liquid; vent tube can plug; grease fouling |
| Bubbler | Tolerates very dirty service | Requires an air supply and maintenance |
| Float switches | Simple, cheap, independent | Discrete only; rags foul them; used for backup, not measurement |
Frequency and beam width
Radar transmitters are commonly available around 6 GHz, 26 GHz, and 80 GHz. Higher frequency with a given antenna size produces a narrower beam.
Beam width matters enormously in a wet well, which is typically a narrow concrete shaft with a ladder, pump discharge piping, guide rails, cabling, and a level float or two. A wide beam illuminates all of it and the transmitter has to distinguish the surface echo from the reflections off everything else. An 80 GHz instrument with a beam of a few degrees can often be mounted where a wider-beam unit would be unusable.
False echoes and what to do about them
A false echo is a reflection from something that is not the liquid surface. The classic symptom is a level that reads correctly through most of its range and then locks onto a fixed value as the surface passes an obstruction.
- 01Move the instrument if you can. A relocation of eighteen inches often solves what hours of configuration cannot.
- 02Perform a false echo mapping or suppression scan with the vessel empty, so the instrument learns the fixed reflections and ignores them.
- 03Set the near-field blanking distance so reflections from the nozzle and mounting are excluded.
- 04Use a stilling well or a guided wave probe where the geometry is genuinely hostile.
- 05Verify across the full range, not just at one level. False echoes hide at specific distances.
Guided wave radar
Guided wave radar sends the signal down a probe, either a single rod, a twin rod, or a coaxial element, which concentrates the energy along the probe instead of spreading it. Far more of the transmitted energy returns, so it works on low dielectric liquids that reflect poorly, in narrow spaces, and through turbulence and foam that defeat non-contact units.
The trade-off is that the probe is in the liquid. In municipal wastewater, rag and grease accumulation on a probe is a real maintenance item and is the main reason non-contact is usually preferred for raw wastewater wet wells. In a chemical day tank or a narrow standpipe, guided wave is often the better answer.
Commissioning checklist
- Measure and record the actual distance from the sensor face to the zero reference. Do not take it from the drawing.
- Confirm the configured range matches the 4-20 mA scaling in the PLC. Three places must agree.
- Run a false echo scan with the well as empty as it can safely be made.
- Verify at a minimum of three points across the range against a physical measurement.
- Set the fail-safe direction deliberately and write it in the control narrative.
- Confirm the PLC treats a fault current as a fault rather than scaling it into a plausible level.
- Record the echo curve if the instrument supports it. It is the baseline for every future diagnosis.
Frequently asked questions
- Does foam affect radar level?
- Light foam is usually transparent to radar and it measures the liquid beneath. Dense, wet foam can absorb enough energy to weaken the return. Guided wave radar handles heavy foam better than non-contact.
- Why does my radar level freeze at one value?
- Almost always a false echo. The instrument has locked onto a fixed reflection from a ladder, a pipe, a bracket, or a build-up on the wall. Run a false echo suppression scan with the vessel empty, and check the mounting position against obstructions.
- Can radar measure through a plastic tank wall?
- Sometimes, for some frequencies and materials, but it is not a general-purpose solution. It is sensitive to wall thickness and material, and it degrades unpredictably. Mount through a proper nozzle where the application matters.
- What is the minimum measurable level?
- Set by the near-field blanking distance, which excludes reflections from the antenna and nozzle. It is typically a few inches to a foot depending on the instrument. Account for it when you set the low level pump-off point.
Related topics
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- Ultrasonic Level MeasurementNon-contact level by time of flight: how the sensor works, the blanking zone and beam angle, temperature compensation, the surfaces and vapors that defeat it, mounting rules, and what to do when it loses its echo.
- Wet Well Level ControlThe measurement that runs a lift station: choosing setpoints, protecting the pumps, validating the signal, and the failures that flood a station.
- Level Reading JumpsA level that steps, spikes, or flips to a fixed value while the water moves smoothly: the false echoes, foam, turbulence, condensation, cable faults, and controller-side causes behind each pattern, and how to tell them apart from the trend before opening a hatch.
- Hydrostatic Level MeasurementMeasuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
- Differential Pressure LevelMeasuring level in a closed or pressurized vessel with a DP transmitter: high and low side connections, dry legs and wet legs, zero suppression and elevation, the range calculation with a worked example, remote seals, and the density dependence.
Direct contact
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