The specific gravity of a liquid is the ratio of its density to that of water at 4B C (39.2B F) and is dimensionless. While changes in specific gravity do not affect the average PRECISION TURBINE FLOWMETER (PFT) K-factor value, the overall linear range of the PTF is changed. The linear range represents the minimum to the maximum flow rate within which the linearity of the flowmeter is specified.
The rotor turns or spins due to pressure differential across the rotor blades. Specific gravity is one of the factors affecting this pressure differential. As the specific gravity decreases, the pressure differential decreases. On a fluid with a low specific gravity and a low flow rate, the pressure differential across the blades is low, leaving almost no energy for turning the rotor. Consequently, the rotor cannot turn in proportion to the liquid throughput and the K-factor drops. Therefore, the angle of the rotor blades is changed to help compensate for the change to a lower specific gravity. This allows products with lower specific gravity to be measured accurately by the PTF.