Why Does Fluid Viscosity Matter When Selecting a Flow Meter
Flow meters are calibrated and/or specified at the viscosity of water at around 25 deg. C. This condition for water is considered a kinematic viscosity of 1 centistoke (cSt). In SI units this is 1 m^2/sec. Different flow meter types react differently to viscosity. Paddlewheel flow meters have pros and cons relative to some other types of meters.
In higher viscosity fluids, the paddlewheel calibrated for water will have a slower rotation as related to density. The result is that the reading will be lower than the actual rate of flow. Conversely, if the viscosity is less than 1 cSt, the paddlewheel will turn faster, indicating a higher than actual flow rate. Depending on the actual viscosity and the accuracy required for your application, this may or may not be an issue. If so, it is recommended that the flow meter be calibrated at the viscosity of the planned fluid. Functionally, paddlewheel flow meters can generally operate up to 100 cSt with a viscosity compensated calibration.
Vortex meters on the other hand are little affected up to 2 cSt. After that, there are curves and formulas that provide the compensated reading. The effect of viscosity is that the higher it goes, the lower the upper flow range of the Vortex meter becomes. For most purposes, after 5 cSt the vortex meter is not a good option.
If you are using a fluid other than water in your application, it is highly recommended that you discuss the application with an application engineer of the flow meter supplier.