Gravity Filter Bed Level Monitoring: 80 GHz Radar Level Transmitter Case Study for Waterworks
2026/10/27
The application is a gravity rapid sand filter in a municipal waterworks, one of six identical cells. Above the sand media there is only 1.2–1.5 m of water at normal filtration level, and the level rises by 0.6–0.9 m during backwash. The filter gallery is permanently wet, relative humidity stays near 100 %, and chlorine vapour is present. The existing instruments are cleaned with a hose, and the walkway above the cells vibrates slightly when the backwash blower runs.
| Field parameter | Typical value | Engineering consequence |
|---|---|---|
| Water depth above media | 1.2–1.5 m (filtration), 2.1 m (backwash) | Short span demands a sensor with a small dead zone and millimeter resolution |
| Humidity | ≈100 %, chlorine vapour | Enclosure must be IP67/IP68 with corrosion-resistant housing |
| Surface condition | Scum, foam, floating algae in summer | Optical and float devices foul quickly; non-contact measurement is preferred |
| Access | Narrow walkway, handrail only | Installation must be done from above without entering the cell |
| Signal destination | PLC in the filter gallery, then SCADA | Modbus RTU or 4–20 mA is required; wireless only as a backup path |
- Level switches alone gave only "high" and "low" states, so the operators could not see the actual filtration head loss trend.
- A float device jammed against the wall after a backwash and reported a constant level for three days.
- Foam formed during backwash caused a false high alarm, and the sequence stopped mid-cycle.
- Cable trays in the gallery were already full, so pulling new analog cables for six cells was expensive.
The requirement is a non-contact, short-range, humidity-proof measurement that can be mounted from the walkway and can feed the existing PLC without new cable trays. An 80 GHz radar level transmitter with a small dead zone was selected, supplemented by a wireless remote module for cells where cabling was impossible.
| Selection item | Specified value | Reason |
|---|---|---|
| Measuring principle | 80 GHz FMCW radar, non-contact | Narrow beam ignores the wall and the handrail; no fouling contact with scum |
| Range and dead zone | 0–5 m range, 0.05 m dead zone | Covers backwash peak with margin above a 1.5 m working depth |
| Accuracy | ±3 mm | Resolves head-loss changes of a few millimetres across a filter run |
| Antenna | PTFE-encased lens | Resists chlorine and condensation; easy to wipe |
| Output | 4–20 mA with HART, Modbus RTU option | Direct replacement of the old analog input on the PLC |
| Protection | IP67, −40 to +80 °C | Wet gallery, warm summer air |
| Wireless option | Battery LoRa node where cabling is blocked | Only for signal back-up, not for control |
- A stainless bracket is clamped to the handrail so the antenna is centered over the cell and at least 300 mm from the wall — off-center mounting is the main cause of false echoes in a narrow cell.
- The antenna face is set at least 500 mm above the maximum backwash level to keep it out of splash.
- The beam is aimed vertically; a 1–2° tilt is enough to lose a weak echo off a rippled surface.
- The false-echo suppression routine is run empty and full during commissioning, and the resulting map is stored in the device.
- Foam during backwash is handled by enabling the foam filter and adding a 10-second signal hold in the PLC, so alarms do not chatter.
- The wireless node is mounted outside the wet gallery on the dry wall, with the antenna clear of the metal handrail.
- Continuous head-loss trending replaced switch-only indication, so backwash starts on measured head loss rather than a fixed timer.
- Foam-triggered false alarms stopped after the filter routine and signal hold were configured.
- Because the radar needs no contact with the water, cleaning dropped to a simple monthly wipe of the lens.
- Only two of six cells needed the wireless module; the rest reused existing analog inputs, keeping the retrofit cost low.
Is radar suitable for a 1.5 m depth? Yes — the dead zone matters more than the range. Confirm the antenna can be mounted 500 mm above the highest water level.
Will foam break the measurement? Foam attenuates the echo; a foam filter plus a short signal hold in the PLC is the practical answer. Extremely thick foam needs a guided-wave probe instead.
- Provide the exact distance from the mounting bracket to the lowest water level, not just the tank height.
- Confirm the PLC input type (4–20 mA, HART or Modbus RTU) before ordering the output version.
- Ask for the echo-map commissioning procedure in the manual; without it, startup depends on the supplier's technician.
- Specify IP67 minimum for the wet gallery, and a PTFE lens where chlorine is dosed.
