Positive displacement flowmeters, also known as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid.
The helical screw principle uses 2 counter rotating gears to form a continuous constant volume cavity. As fluid flows through the meter the screws / gears are forced to rotate. The flowrate is derived from the known volume within the cavity per revolution. In most helical screw manufacturing the gears are machined as a matched pair keeping clearances and very tight tolerances. Relatively little force is required to rotate the screws ensuring the pressure drop is kept to a minimum. As a result they can be selected for very high viscosity fluids and flow rate uncertainties are down to approx 0.1%.