Water level sensors for pump sumps and lifting stations: types, criteria, alarms.

Hydrostatic (submersible probe). The probe sits at the bottom and measures the pressure of the water column above it, which gives the height. No moving parts, and foam or dirt on the surface does not bother it. It has to be in the water, and the cable has to be routed cleanly. The usual principle for pump sumps.

Float switch. A float tips a contact at a certain level. Cheap, robust, in use for decades – but it only reports "full" or "not full", never a value. And it is the part that jams most often in a shaft: tangled cable, grease, deposits. More under float switch.

Capacitive. A probe measures the change in capacitance as water rises along it. Continuous and low-maintenance, but sensitive to build-up and changing media – in a sewage shaft with a film of grease, that is a weak spot.

Ultrasonic and radar (contactless). The sensor sits above the water and measures how long the signal takes to return from the surface. It never touches the medium, which is pleasant with sewage. Foam, vapour, narrow shafts and internal fittings distort the reading, though, and it needs a clear view downwards from the cover.

Conductivity (electrodes). Two or more rods report as soon as water bridges them. Simple and cheap, but it only gives point signals, and the electrodes scale up or foul over time.

The terms get mixed up, but they mean two different things.

A level alarm raises an alert when a certain point is reached – float or electrode, one threshold, done. You learn: it is too high. Nothing more.

A level sensor measures continuously and produces numbers. That shows you not just the moment but the trend: how fast the water rises, how often the pump starts, how long it runs. This is exactly where problems show up before they turn acute – a pump starting three times as often as last month is a signal; a level that no longer falls back to its old value after pumping is one too.

For a pure emergency alert, the alarm is enough. If you want to see a pump failure early rather than late, you need the measurement. The practical answer is both in one device: measure continuously, alert at a threshold.

What a measuring sensor reveals is shown by an analysis of 8 Drainbee pump sumps (March to October 2026, 1,616 device-days). On 62% of days the pump did not run at all. On days with pumping, the median was 4 pump cycles, the 95th percentile 22. A median pump cycle lasted about 3 minutes and lowered the level by around 10 cm. A detector with a single switching point stays silent through all of this until the level reaches its mark. Only the history shows whether a pump that normally switches four times a day has suddenly stopped starting.

• Measuring range. It has to match the depth of the shaft. For domestic pump sumps, 0–1 m is usually enough; deeper shafts need a different probe.
• Accuracy. ± 1 cm is fine enough at home to tell pump cycles apart. At ± 10 cm you only see coarse movements.
• Measuring interval. One value per hour is not enough if a shaft fills in 20 minutes. Adaptive devices measure more often as soon as something moves.
• Alarm paths. Local siren, push to the phone, call-style alarm – and who is reached when you are not looking at the screen?
• Power outage alert. Without power the pump stops. A device that reports the outage itself is worth more in a basement than an extra threshold.
• History. Without a history there is nothing to compare against "normal". Check how far the data goes back.
• Subscription costs. Some systems only work with an ongoing fee. Add it up over ten years before you compare purchase prices.
• Smart home and API. If you want the values in Home Assistant, ioBroker or Node-RED, check beforehand whether the device hands them out locally.

• Lowest point, but not in the sludge. The probe belongs on the shaft floor, a little way from the pump. Lying in sediment, the reading drifts.
• Distance to the inlet. Directly under the incoming jet, every sensor measures restlessly. Hang the probe on the opposite side.
• Route and secure the cable. The cable has to run taut and clear of the pump area, otherwise it tangles with the float cable – the most common cause of a jammed float.
• Set the zero point. Measure by hand once after installation so the displayed level matches reality.
• Threshold below the critical height. Do not set it at the overflow edge, but so that you still have time to act.
• Do not replace the existing control. The sensor monitors; the pump is still switched by the float switch or the level control.

Drainbee is a water level sensor built for exactly this job: a wall box plus a hydrostatic submersible probe, measuring range 0–1 m, accuracy ± 1 cm. It measures adaptively every 10 seconds to 5 minutes – often while the level moves, sparingly when nothing happens. The box sends data over Wi-Fi (2.4 GHz only) and MQTT to the cloud.

The app (iOS and Android) shows the current level, pump cycles, inflow and the rain and inflow history. Alerts cover threshold, power outage and Wi-Fi loss – as a push or a call-style alarm in the app. €298 one-off, no subscription.

Local interface (API) on the home network. Within your own network the device answers GET http://<device-IP>/status with JSON, for example {"measurement":{"cm":27,"ageMs":3200}} – read-only, no login. That lets you poll the level in Home Assistant (REST sensor), ioBroker or Node-RED and build your own automations there. The device does not push alerts into the home network itself, and there is no native integration with HomeKit, Matter, Bosch, FRITZ!Box or Homematic.

What Drainbee is not meant for: the probe measures up to 1 metre and is built for pump sumps and lifting stations. Deep cisterns, pools or pressurised tanks fall outside that; for DIY setups with a Raspberry Pi or 12 V electronics there are more suitable sensors.