Calibration of Turbine Meters
Introduction to turbine meters
Turbine meters can consist of a single rotor or dual rotors. The frequency at which the blades turn represents a given flow because each blade sweeps out a fixed volume of fluid. Shown is a dual rotor meter with its casing removed and a single rotor meter with casing and a magnetic pick-off to record the rotor’s frequency.
The standard for calibrating turbine meters
Turbine meters are calibrated using NIST’s volumetric based, 20−L piston prover with a 1.2 centistoke (20 °C) PG+W mixture. The prover has a 95 % confidence level uncertainty of 0.074 %. In the figure, MUT stands for "meter under test."
The following figure is a schematic of the 20−L piston prover. The motor−driven piston works like a syringe pump. It sweeps a known volume during known period of time through the cylinder generating a known flow through the meter. The frequency measured from the meter during the known volumetric flow gives the meter specific factor.
Presentation of data
For a given meter, the geometry and fluid properties do not change during a calibration. Therefore, dimensionless quantities: Strouhal (St) and Reynolds Number (Re) or Roshko (Ro) numbers are used for the y and x-axis respectively (see table below). This approach for turbine meter calibration curves results in the collapsing of the multiple kinematic viscosity calibration curves in the viscosity independent range of the meter. Therefore differences in fluid properties between the fluid(s) being metered and the fluid used for calibration do not effect how the calibration curve is used to determine flow.