The sensor head has an internal humidity detector. The humidity reading can be checked on the indicating transmitter display, select 3 SENSOR STATUS from the Main menu. Check the humidity reading once in every three months.
Increasing humidity level indicates either condensate forming in the sensor head (if the process temperature is below ambient) or prism leakage. If the humidity reading exceeds 30 %, replace the dryer. If the reading exceeds 50 %, check the prism seals. Relative humidity exceeding 60 % produces a diagnostic message HIGH SENSOR HUMIDITY (see Troubleshooting messages).
The dryer (desiccant) is two pieces inside the sensor head cover, see Sensor PR-21-S (G). The dryer cannot be reactivated, it must be replaced. The spare part number for the dryer sachet is PR-9108.
For instructions on how to open the sensor head to replace the dryer, see Disassembling the sensor.