*Can a pressure measurement device be used to measure the volume in a rainwater harvesting tank?*

The volume of liquid in a tank can be determined using a pressure measurement device, however it cannot measure the volume directly and the pressure reading has to be converted into a volume using a conversion factor or formula.

For a tank that has a straight vertical shape (e.g. an upright cylinder), the volume calculation is very simple. A pressure sensor mounted in the bottom of the tank is used to measure the hydrostatic pressure of the water pushing down on the bottom. The hydrostatic pressure is directly proportional to the water level.

e.g. 100mbar ~ 1 metre of water

Since the cross sectional area of the cylindrical tank is constant all the way up, you only have to multiply the water level by the area in square metres to calculate the total volume of water in a vertical cylindrical tank.

e.g. 1 metre diameter tank with a hydrostatic pressure of 50mbar would contain approx. 0.39 cubic metres or 390 Litres

**Hydrostatic pressure measurement products**

- DCL 531 Modbus RTU RS 485 Submersible Stainless Steel Liquid Level Sensor
- LMP305 Borehole Level Transmitter
- 4-20mA Output Hydrostatic Level Transmitters & Sensors
- LMK458 Marine Approved Hydrostatic Level Transmitter

So when programming the display indicator, the signal output from the sensor that corresponds to the maximum hydrostatic pressure should be converted to read the maximum capacity of the water tank and all points between Empty and Full are assumed to be linear.

If the tank does not have a straight vertical shape in all directions (e.g. a horizontal cylinder), a variable factor will need to be applied dependent on the water level. This is a little more complicated, but many types of instrumentation will allow the use of non-linear functions to convert the hydrostatic pressure into a volume.

Normally the tank shape conversion formula is applied to the sensor output signal inside an intelligent display indicator. Typically the electronics of a typical hydrostatic pressure sensor will not include the maths functionality to configure a complex non-linear output signal.

**Tank level measurement products**

- DPT200 Pressurised Tank Level Differential Pressure Transmitter
- 18.605 G Low Cost Submersible Diesel Fuel and Water Tank Level Sensor
- LMK387 Cleanable Sewage and Sludge Pressure/Level Sensor
- Diesel Fuel Tank Level Sensors