With the piston prover a known volume of water is „pressed" through the meter under test. This is performed via a dedicated calibration automated system.
When the calibration has been completely successfully, a calibration certificate is issued.
There are two critical parts of the piston prover: the diameter of the cylinder and
the position of the piston which displaces the water. The movement of the piston is determined by a ruler mounted on the piston rod.
The volume displaced through the piston is measured using the internal diameter of the cylinder and the movement of the piston. This volume is then compared with the volume which is measured by the meter under test.
Calibration with a piston prover is possible up to a diameter of DN 150. Bigger diameters are calibrated on a master rig or calibration rig.
In general the accuracy of a calibration rig must be 10 times better than that of the meter under test. Normally, the performance of the flowmeter is checked at two different volumetric flow rates.
