Water level measurements

Measuring liquid level is quite a common task.

There are numerous ways of doing this.

Probably the most known of them is the fuel level measurement in cars.
This post is about a tank water level gauges incorporating different types of sensors.
In principle the water level can be determined by measuring the water pressure at the bottom of the tank. The higher the pressure, the higher the water level.

Measuring water level with water pressure sensor can cause errors.

Unfortunately this method has some oversimplification. The environmental air barometric pressure hasn`t taken into account.

As weather continuously changes, so does the ambient air pressure. There is an average variation of 3 kPa, which corresponds to roughly 300 mmH2O measurement (even if the actual water level in the tank is unchanged).

This calls for measuring the barometric pressure. Instead of measurung the ambient air pressure, an alternative measurement method is used.

The ambient air pressure can be continuously taken into accout if a differential pressure sensor is used. One port of the gauge goes to the bottom of the water tank, while the other port stays open. This way the open port “monitors” every change in the ambient air pressure.
This is useful for measuring rainwater storage tanks.


Measuring water level using differential pressure sensor

Due to the fact that integrated differential pressure sensors have some long term drift, optionally the measurement system can be completed with water level switches. One at the bottom and one at the top of the tank. These two switches create a so called auto tune option: they provide zero and full range levels, the differential pressure’s output value can be adjusted if either switch operates (changes state).


More accurate measurement using level switches

An alternative method is the floating ball water level measurement. Basically the angle of the floating ball’s stick is directly related to the water level. The angle can be measured with a potentiometer, but the casing must be watertight, which is difficult to achieve because of the moving elements.


Floating ball water level measurement with potentiometer

Another solution is to use an acceleration sensor. The sensor has to be mounted to the moving rod, so the angle of the rod can be calculated. Having the angle information the water level can be determined.

There can be additional level switches in the system to provide more accurate measurements, or to serve “underdraining” or “overfilling” signals.


Floating ball water level measurement with acceleration sensor

These techniques don’t provide highly accurate measurements, but are accurate enough for rainwater of greywater tanks level measurements.