How both work: time of flight
The gauge sends a microwave signal down to the surface and measures how long the reflection takes to return. Microwaves travel at the speed of light, so the time is tiny (nanoseconds), but the electronics measure it precisely:
distance = (c × t) / 2 level = reference height − distance
Because it measures distance with microwaves, radar is almost unaffected by density, pressure, temperature, vapour or dust changes that upset DP or ultrasonic gauges.
Side by side
Dielectric constant (DK, εr): why it matters
The strength of the reflection depends on the product's dielectric constant. Water and acids (DK ≈ 80) reflect strongly; oils and hydrocarbons (DK ≈ 2) reflect weakly; LPG and some powders are even lower.
- Low DK = weak echo. GWR handles low DK much better because the probe concentrates the energy.
- For interface (oil on water), GWR can see both the top surface and the oil/water interface, as long as the upper layer has a low DK and the lower one a high DK, with a clear boundary.
Which one to choose
| Non-contact radar | Guided-wave radar | |
|---|---|---|
| Contact with product | None | Probe immersed |
| Low DK products | Harder (needs high frequency, good set-up) | Very good |
| Interface measurement | No | Yes |
| Foam, turbulence, condensation | Can lose the echo | More robust |
| Sticky, coating, fibrous product | Good (no contact) | Build-up on probe can cause errors |
| Tall tanks, agitators | Good (no probe to break) | Long probes; agitators can damage them |
| Small nozzles, internals | 80 GHz narrow beam helps | Not affected by internals |
Frequency: modern non-contact radars use about 80 GHz for a narrow beam that avoids nozzles and internals; older 6–26 GHz units have wider beams.
Installation rules
- Non-contact: keep the beam clear of ladders, agitators, fill streams and the tank wall; avoid the centre of dome roofs (focused echoes). The antenna should extend past the nozzle end.
- GWR: keep the probe away from the wall and internals by the distance the manufacturer gives; anchor or weight long cable probes; use a coaxial probe in small chambers or very low DK.
- In a bridle or stilling well, size and smooth it as the manufacturer specifies; a GWR in a chamber is a common replacement for displacers.
- Check the blocking (dead) distance at the top: the gauge can't measure very close to its own flange.
Set-up and false echoes
- Enter the reference height (from flange to tank bottom) and the measuring range carefully. Most level errors are a wrong reference.
- With the tank empty or low, run a false echo mapping (interference suppression) so the gauge learns and ignores fixed obstacles.
- Check the echo curve with the configuration software: you should see one clear surface peak above the threshold.
- Compare against a manual dip or gauge glass at two levels and adjust the offset, not the range.
Echo curves are viewed with the manufacturer's DTM: see DTM & device software.
Troubleshooting
| Symptom | Likely cause |
|---|---|
| Level stuck at a fixed value | Locked on a false echo (internal, nozzle); redo mapping |
| Reads full when empty | Antenna fouled or condensation; ringing in the nozzle |
| Echo lost (fault current) | Foam, heavy turbulence, low DK product, coating |
| Constant offset vs dip | Wrong reference height or offset |
| GWR reads high near the bottom | Probe end/weight echo, build-up on the probe |
| Interface wrong | Emulsion layer, wrong upper-layer DK entered |
Q&A: test yourself
Related: Level Measurement · DP Level calculator · Interface & Density