Application Cautions for Positive Displacement Flow meters

Avoid using positive displacement flow meters in dirty fluids unless the dirt can be effectively removed upstream of the flow meter. Operating these flow meters in dirty fluids can cause plugging and increase maintenance costs. Be careful when selecting bearings because the non-lubricating nature of some fluids, impurities, and dirt can increase bearing wear and maintenance costs. Note that bearings usually do not necessarily fail catastrophically — they can slow down and adversely affect accuracy before they stop working. 

Avoid liquids with gas bubbles unless the bubbles can be effectively removed. As viscosity increases, be sure to ensure that the pressure drop across the flow meter is acceptable. Make sure that the viscosity of the operating fluid is similar to that of the calibrated fluid, because the different amounts of slippage exhibited by different fluids can cause measurement error.