When the viscosities of various gases are similar and do not change significantly, it can be said that there is no effect on the performance of high-viscosity flow meters; if the viscosity of liquids differs considerably, it will have a certain impact on the performance of general-purpose flow meters. Some PDFs adapt to liquids with viscosities as high as 500 mPa·s by producing specialized models with design measures such as larger clearances. The PDF has more experience in applying flow meters to high-viscosity liquids. Although PDFs are affected by some high-viscosity liquids, compared with differential pressure, float, turbine, and other types of flow meters, the influence is much smaller. High viscosity affects the performance of PDFs in three aspects: measurement error, pressure loss, and flow range.
(1) Influence on Measurement Error
PDFs have a characteristic different from many other flow meters: as viscosity increases, the flow increases while the measurement error also increases. However, due to the reduction in clearance leakage caused by increased viscosity, the performance improves.
(2) Influence on Pressure Loss
If the viscosity of the liquid increases, the pressure loss of the PDF also increases. The relationship between pressure loss Δp and flow q can be expressed by the formula Δp = kqn (where k is the coefficient and n is the exponent). When the viscosity is below 0.005 Pa·s (= 5 mPa·s), n = 2; when above 0.5 Pa·s (= 500 mPa·s), n = 1; between these two values, n = 1.9 to 1.1.
When movable measuring elements are used for high-viscosity liquids, the load increases, leading to an increase in pressure loss. Specialized instruments for high viscosity usually adopt methods to increase the clearance, sometimes up to 0.5 mm. To reduce the squeezing load of the liquid between the gear teeth, elliptical gear flow meters often have several grooves cut into the gears for unloading (when ≥ 150 mPa·s), and when greater than 500 mPa·s, under-geared elliptical gears are used.
(3) Influence on Flow Range
As viscosity increases, so does the pressure loss, and for places where the use of pressure loss is limited, it is necessary to lower the upper limit of the flow, thereby narrowing the flow range. As viscosity increases, the lower limit of the flow decreases, expanding the flow range. As a rough estimate, when viscosity increases by 10 times, the lower limit of the flow drops to 1/10 to 1/3 of the original value.