Deposit build up on the prism surface disturbs measurement. Look out for the following indications of coating:
- Abnormally high concentration reading or upward CONC.
- CCD value is getting higher and QF value drops down.
- High LED current, especially LED = 100 (max). See Viewing sensor status.
- Prism wash (see Testing prism wash)
does not change optical image.
In most of the applications the prism will keep clean, but if coating occurs, check the following:
- Sensor mounted correctly in respect to flow direction (arrow on sensor head).
- Sufficient flow velocity, see Choosing sensor mounting location
- A temperature difference between process fluid and sensor probe may cause coating. This may happen for small flows if the thermal insulation is inadequate. In some cases it helps to insulate also the connection flange, Figure 2.
If there is a coating problem, it is recommended to try to increase the flow velocity, for example by installing a pipe portion with smaller diameter. If this cannot be done, the prism should be automatically washed at regular intervals, for example by steam or hot water. A prism wash nozzle (see Prism wash with integral steam nozzle) can be useful also in cases without coating problems, for example to clean the prism after a process stop using a manual wash valve.