PD meters can achieve better than ± 0.05% repeatability accuracy. Linearities of ± 0.15% to ± 0.5% are typical over flow ranges of 8:1 to 12:1, with a normal range of 10:1. Figure 500-25 shows the accuracy of a commonly used PD meter.
The dominant factor affecting meter accuracy is the amount of slippage (“bypass”) through the meter clearances. Ideally, if the amount of slippage remains constant, no error would occur and the meter would be perfectly repeatable. In reality, this is often not the case because of the variation of fluid dynamic properties (e.g., viscosity) and operating conditions (e.g., flow rate, pressure and temperature).
To meet the specified linearity accuracy, a minimum flow rate is often required. Because of the viscosity effect, the lower the fluid viscosity, the higher the minimum flow rate.