Deposit build-up on the prism surface disturbs the measurement. An abnormally high concentration reading, an upward concentration (CONC) drift, decreased QF value or increased LED value may indicate coating.
In most applications the prism keeps clean due to the self-cleaning effect. If coating occurs, check the following:
- Sufficient flow velocity
- Temperature. A temperature difference between the process fluid and refractometer probe may cause coating. This may happen with small flows if the thermal insulation is inadequate. In some cases it helps to also insulate the clamp connector.
In case of a coating problem, the preferred solution is to try to increase the flow velocity, for example by installing a pipe portion with smaller diameter. Installing a wash nozzle can be considered, if increasing the velocity does not provide a solution.