Camper tank level sensors: an accurate water tank level gauge

The tank gauge in a motorhome is notorious: it jumps from “full” to “half” to “empty”, shows nothing but nonsense for grey water after a few weeks, or only has three little lights. That's almost always down to the measuring principle. This article explains which camper tank level sensors are available, how accurately they measure, what matters for grey and black water and how to retrofit an accurate water tank level gauge for fresh water, grey water and black water.

Tank level sensors: the measuring principles compared

Principle	How it works	Display	Typical weakness
Electrodes or probe rod	Metal pins at different heights. When water touches a pin, a small current flows.	In steps, often 3 or 4	Coarse steps, deposits in grey water distort the reading
Float	A float moves with the water level and switches contacts or changes a resistance.	In steps or continuous	The mechanism can stick, especially in grey and black water
Capacitive sensors on the outside	A strip on the outer wall of the tank detects the water through the wall.	In steps or continuous	Depends on wall thickness and material, not for metal tanks
Ultrasonic or radar from above	A sensor in the tank lid measures the distance to the water surface.	Continuous	Foam, condensation and sloshing interfere, needs a clear line of sight
Pressure sensor at the bottom of the tank	The sensor measures how hard the water column presses on it. The higher the water, the higher the pressure.	Continuous	Must be drained when there's frost, needs calibration for the tank shape

For an accurate, continuous reading in litres or percent, measuring the pressure at the bottom is one of the most reliable ways. It has no moving parts, doesn't depend on the tank material and isn't affected by foam. Sloshing while driving is smoothed out in the evaluation.

Why the tank shape matters

Hardly any motorhome tank is a simple box. Many are sloped, stepped or shaped around the wheel arch. But a sensor that measures height doesn't know how many litres there are in each centimetre. That's why calibration is important: you fill the tank step by step and tell the system how much is in it. After that it converts height into litres, even for irregular shapes.

Fresh water, grey water, black water: what matters for each

• Fresh water: Here you want an accurate reading in litres, so you know how many more days you can stay.
• Grey water: Grease and soap residue build up on pins and floats. A sensor with no moving parts and a smooth surface lasts longer here.
• Black water: What matters is a timely warning before the tank is full. With the urine container of a separating toilet, measuring is harder because urine scale can build up. We can't currently make any promises for that.

How Fluidy Pro measures

Fluidy Pro uses level sensors that hang in the tank and measure the water pressure at the bottom. Inside the sensor cable is a thin air tube, which the sensor uses to compare against the air pressure. The sensors are typically accurate to 1%, relative to the full measuring range.

• Up to 4 tanks on one module, for example fresh, grey and black water plus one in reserve.
• Any tank material: plastic and stainless steel are no problem.
• Installation with a 17mm hole and the supplied waterproof cable gland (M20, IP68). Installation is the same for all three tank types.
• Guided calibration in the dashboard, with no maths needed. Upfitters export the calibration and reuse it on the next identical vehicle.

The step-by-step instructions are in Installing a water level sensor.

The fresh water tank in the dashboard: litres, percent and water temperature.

The fresh water tank on the SmartKnob.

More than just the tank level

Fluidy Pro also measures up to 5 temperatures, the water pressure after the pump and leaks.

• Water pressure: If the pressure keeps dropping and the pump keeps kicking in, a fitting is leaking somewhere.
• Leak detection cable: It lies where water runs first and reports moisture before the floor swells. How it works is explained in Detecting water leaks.
• Temperatures: boiler, heating circuit or fresh water tank, with an accuracy of 0.5°C. That way you can also see when frost is on the way.

The leak widget shows whether everything is dry.

Build your own tank gauge?

You can build a simple tank level display yourself with a pressure sensor and a microcontroller. The measuring is the easy part. Three things get tricky: converting for irregular tank shapes, temperature drift over the winter and a cable entry that stays watertight for years. If you want to solve all of that properly, you'll quickly spend many evenings on it. If you just want an accurate reading, a ready-made system gets you there faster.

Important in frost

Frost protection: Level sensors and the pressure sensor must be completely free of water whenever there's a risk of frost. If water freezes in the sensor, it is permanently damaged. When you put the vehicle into storage, drain the tanks and empty the fresh water system.

How the tank gauge works together with power, lighting and heating is shown in the guide Camper van smart home.