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Measuring principle
When metering flowing liquids, either the volume V is recorded over a given time t or the momentary flow rate q is determined.
The relationship between these variables is V = q · t.
In accordance with these two measuring principles, a differentiation is made between:
- direct volumetric meters, also referred to as positive displacement meters. These include rotary piston meters.
- indirect volumetric meters such as velocity meters, where the flow velocity v represents a direct measure of the flow rate q at a given cross section F according to the relationship q = v · A. Examples include electromagnetic flowmeters and flowmeters operating according to the differential pressure principle.
Rotary piston meters are direct volumetric meters. They operate according to the positive displacement principle. Their operation is based on the continuous limitation of defined portions of the volumetric flow in the mechanism by continuous filling and emptying of the measurement space. This consists of the walls of the measuring chamber and the moving part, i.e. the rotary piston.
The rotary piston is driven by the pressure difference in the metered liquid between the inlet and outlet. The meters are basically purely mechanical devices operating without a power supply.
The rotary piston (6) which has a double T-shaped cross section is guided by its gudgeon or guide pin (3) in an annular space in the base of the measuring chamber and also by its slot on the barrier (1).
The inlet port (2) and outlet port (7) are located on either side of the barrier. They are continuously sealed by the rotary piston and the barrier.
The incoming liquid fills the sickle-shaped spaces, attempts to enlarge them and thus turns the piston until the volumes V1 und V2 are reached in succession. With the further movement of the piston, this filled space is connected to the outlet and emptied. Since the two sickle-shaped spaces – the inner and outer – are displaced with respect to one another, no deadpoint occurs during the movement of the piston. The piston moves continuously according to the flow of the metered liquid.
The rotary movement of the piston guide-pin is picked up by a drive member and transmitted via a gland-free (industrial design only) permanent magnetic coupling to the register. One revolution of the piston pin corresponds to the passage of the capacity of the measuring chamber (V1+V2) through the meter. A gear unit converts the revolutions into a decimal value of e.g. 10 l, 100 l, 1 m3 or gallons.
Measuring process in the rotary piston meter