How radar sensors measure distance to a liquid surface, and when to choose radar over ultrasonic.
A non-contact radar sensor mounts in the top of a tank, looking down. It sends microwaves at the liquid surface and measures the reflection to find the distance, much as an ultrasonic sensor does with sound.
- FMCW radar. Most modern compact sensors use frequency-modulated continuous wave radar. The sensor sweeps its frequency continuously; by the time the echo returns, the transmitted frequency has moved on. The difference between the outgoing and returning frequencies is proportional to the distance.
- Pulsed radar. Older and some current sensors send short microwave pulses and time the echo directly, like ultrasonic but at the speed of light.
- 80 GHz sensors. Many current compact sensors work around 80 GHz (older designs commonly 6 or 26 GHz). The higher frequency gives a much narrower beam from a small antenna — commonly just a few degrees — so it is easier to aim past obstructions and tank walls, and it copes better with condensation and build-up on the antenna.
- Speed of light, not sound. Radar is not affected by headspace temperature, pressure, vapour, or gas composition in normal tank conditions. This is the main advantage over ultrasonic.
How much signal the liquid reflects depends on its dielectric constant (εr, also called relative permittivity):
| Liquid |
Dielectric constant (approx.) |
Reflection |
| Water and water-based chemicals |
about 80 |
Strong |
| Acids, caustic, most treatment chemicals |
high (water-based) |
Strong |
| Alcohols |
about 15–30 |
Good |
| Oils, fuels, solvents |
about 2–4 |
Weak |
Water-based treatment chemicals reflect strongly and are easy targets. Low-dielectric liquids such as oils reflect only a small part of the signal; the rest passes into the liquid and can reflect off the tank bottom. Check the sensor’s minimum dielectric rating before using radar on oils or solvents.
- Choose radar for chemical tanks with vapour or fumes, heated tanks, closed or pressurized tanks, tanks with condensation, and tall or narrow tanks where a narrow beam helps.
- Ultrasonic is often enough for clean, vented, room-temperature water tanks without foam, and is usually cheaper.
Agitators and foam are the usual problems in chemical and process tanks: a reading can be present but untrustworthy. Sensors that report stability or interference diagnostics make that visible.
- Foam. Dense or conductive foam can reflect or absorb the signal, so the sensor reads the top of the foam or loses the echo. Radar copes with light foam better than ultrasonic, but not with all foam.
- Agitators and turbulence break up the surface and scatter the echo; the blades themselves can give echoes as they pass through the beam.
- Obstructions in the beam — pipes, ladders, heating coils, baffles, welds — give false echoes.
- Low-dielectric liquids give weak echoes, and the tank bottom may show through.
- Heavy build-up on the antenna, especially conductive or wet crystal layers, weakens the signal. Light condensation is usually tolerated, particularly at 80 GHz.
- Fill streams crossing the beam give false echoes.
- Plastic tank tops and nearby radar sensors. Measuring through a plastic tank roof is possible with some sensors, but the material, thickness, and any condensation on it matter. Two radar sensors looking into the same tank can interfere.
- Point straight down, perpendicular to the liquid surface. Some tilt may be allowed on some models; follow the manufacturer.
- Keep the beam clear. Even with a narrow beam, avoid mounting directly over pipes, mixers, ladders, or the fill stream. Check the beam cone against the tank depth.
- Mount away from the tank wall by the manufacturer’s minimum distance, and not dead center on a domed or conical top, which can focus false echoes back.
- Nozzles. Keep nozzles short and wide enough for the antenna, as the manufacturer specifies; long, narrow nozzles cause strong near echoes. A smooth-walled nozzle with no weld beads inside is best.
- Blind zone. Radar also has a near limit close to the antenna, commonly much smaller than ultrasonic. Keep the highest level below it.
- Set up the tank. Enter the empty distance and span, and use the sensor’s false-echo suppression with the tank empty or at low level so fixed obstructions are learned.
- Seal and material. In closed or pressurized tanks, check the process connection and antenna material are rated for the chemical and pressure — see Material Compatibility and Pressurized Systems.
¶ Maintenance
- Radar needs little routine maintenance. There are no moving parts and normally no field calibration.
- Check against a known level — a dipstick, sight glass, or measured tape — at two different levels, especially after commissioning.
- Clean the antenna if heavy build-up forms, with a cleaner compatible with the antenna material. Do not scratch lens-type antennas.
- Watch the diagnostics. A falling echo strength or signal quality value warns of build-up, foam, or a new obstruction before the reading fails.
- Re-learn false echoes after changes inside the tank.
Opening a chemical tank fitting can release vapour, and a closed tank may be under pressure. Follow Chemical Handling and Isolation before removing the sensor.
Models documented in this wiki: