Lynette de Vries
21 April 2026
Accurate level indication for unusually shaped water tanks
For an independent stay in a caravan or motorhome, three things are essential: electricity, gas and water. Almost all caravans and motorhomes have a drinking water tank that complies with current drinking water standards. If you maintain the water system properly, you can safely use this water for cooking, brushing your teeth, showering, washing up and flushing the toilet. However, it is important to change the water regularly and to clean the fresh water tank thoroughly. We have summarised the basic questions regarding the correct use of the water system in our water blog.
Here we look in more detail at the question: How do you determine the water level in the tank?
When using the water tank, it is often difficult to know exactly how much water is left in it. In caravans and motorhomes, water tanks are usually hidden behind or under furniture. So you can’t simply look into the tank without, for example, opening a bench seat or part of the bed. That’s a bit of a faff, yet it’s particularly important to keep an eye on the water level. After all, nobody wants to be standing in the shower, covered in soap, when the water suddenly runs out.
Contents
1. Traditional methods for measuring water levels
Modern caravans and motorhomes often have a level indicator on the vehicle’s central control panel, usually near the entrance door. The fill level is displayed there based on a wired sensor in the fresh water tank. This measurement is usually shown in increments of 25% or 33%.
Figure 1 – Traditional step counting
Source: Grand Design Owners Forum / “Black tank sensor reading ‘full’ when it isn’t”.
The measuring probe consists of three or four measuring rods of different lengths, which operate on a simple principle. In a system with three measuring rods, for example, the following applies: if the water does not touch any of the rods, the tank is shown as empty. If the water touches only the longest measuring rod, the system indicates 33%. If the water also touches the middle rod, 66% is displayed. As soon as the shortest rod is also touched, the tank is considered full. In systems with four measuring rods, the intermediate levels are empty, 25%, 50%, 75% and full.
Figure 2 – Standard probe assembly in a tank wall
Source: FaratFelicia / “RV Fresh Water Tank Sensor Wiring Diagram”.
With a 100-litre fresh water tank, a reading of 25% means there is just over 25 litres of water left. But as soon as the level drops just below that – for example, to 24 litres – the water no longer touches the dipstick and the indicator immediately jumps to ‘empty’. In practice, this creates uncertainty. Is there still enough water for a day? Or is the level already much lower? The traditional measuring method does not provide an exact answer to this. As a result, many campers top up sooner than necessary, just to be on the safe side. And they would rather have too much than too little. Even at the end of the holiday, the tank is often filled almost to the brim, whilst a large proportion of that water is ultimately drained away unused.
2. The E-Trailer method for measuring the water tank fill level
E-Trailer aims to make camping safer and more comfortable. That’s why we’ve developed the E-Waterlevel: a module that measures the water tank’s fill level continuously and with an accuracy of up to 1%. You can easily view this information in the E-Trailer app on your smartphone.
The app also converts the percentage into litres of fresh water and rounds this off to whole litres, depending on the tank size. You can also set up a notification that alerts you as soon as the water level drops below a value of your choice. Even whilst filling, you can see exactly how the level rises in the app. This allows you to easily monitor when there is enough water in the tank whilst standing next to the tap and stop filling in good time.
How does the E-Trailer measurement method work?
The E-Trailer sensor uses capacitive measurement via two measuring rods. One metal measuring rod is exposed, whilst the other is encased in plastic. Together, these two rods form an electrical capacitor, which generates a very weak electric field. If the material surrounding that capacitor changes – for example, because air is replaced by water – the capacitance also changes. This is due to the difference in dielectric constant between air and liquid.
The sensor detects exactly at what height along the measuring rods this change occurs and can thus determine precisely how high the water level in the tank is. This enables a continuous and accurate display of the fill level to within 1%.
The final challenge
This works perfectly with uniformly shaped tanks, such as rectangular, cubic or cylindrical tanks. This is because the same volume of water corresponds to every height of the measuring rods.
With irregularly shaped tanks, such as T-shaped, L-shaped or conical tanks, the situation is different. Here, the number of litres per height varies. In other words: a 1% rise at the bottom of the tank may correspond to a different number of litres than a 1% rise at the top of the tank.
It is precisely in compact motorhomes, such as campervans and small semi-integrated motorhomes, that such non-standard tank shapes are often found. After all, manufacturers make optimal use of every available space and adapt the shape of the tank accordingly.
Figure 3 – E-Waterlevel module
3. The E-Trailer calibration function for highly accurate water level measurement
To ensure the most accurate display possible, even for irregularly shaped fresh water tanks, E-Trailer has added a calibration function to the E-Trailer app with software update 2.3.085. This update is free of charge.
To calibrate, fill the tank step by step with small quantities of water as indicated by the app. For a 100-litre tank, for example, this means 20 steps of 5 litres. Add 5 litres of water and confirm this in the app. Then add another 5 litres and confirm this again. Continue in this way until the tank is completely full.
During this process, the app records which fill level on the dipsticks corresponds to the amount of water added. This data is saved so that the E-Waterlevel sensor is calibrated specifically for the tank in which it is installed. This calibration can also be carried out retrospectively on E-Waterlevel sensors that have already been installed.
After this one-off procedure, the fill level of an irregularly shaped fresh water tank is displayed with an accuracy of 1%. This means you can also rely on the conversion to litres. In practice, this offers significant advantages. You can see exactly how much water you are using day by day, set the alarm threshold in the app precisely, and always top up in good time. What’s more, at the end of your holiday, you only need to top up with as much water as you actually need, so you no longer have to drive home with an almost full water tank.
