The process refractometer is mounted in a pipe. Vaisala recommends a flow velocity of 1 ‑ 3 m/s (3 ‑ 10 ft/s). If the flow velocity exceeds 6 m/s (20 ft/s), there is a risk of cavitation. Cavitation may damage the refractometer and the piping. Flow velocity that is too slow may cause erroneous measurement readings due to coating of stratification (layering) of the sample on the prism.
The diameter and form of the pipe and the process temperature all affect the measurement and need to be taken into account.
- If the process pipe diameter varies, select the position with the smallest diameter (and accordingly highest velocity). Then the prism stays clean better.
- If the refractometer is used in a feedback control loop, make the time lag short. For example, when a dilution valve is controlled, mount the refractometer close to the dilution point. However, make sure complete mixing has occurred at mounting location.
- If the temperature varies along the process pipe, select the position with the highest process temperature. That minimizes the risk of coating, because higher temperature means higher solubility and also lower viscosity.
- Often the position with the highest process pressure (after the pump and before the valve) has favorable flow conditions without sedimentation or air trapping risks.
- The refractometer should be conveniently accessible for service.