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Large Dehumidifying Dryers Maintenance 59
5. Remove the restricted filter cartridge from the housing again. The alarm light should
stay off when the process blower operates if the switch is adjusted correctly.
6. Re-install the obstructed cartridge filter. The alarm light should go on again if the
switch is adjusted correctly.
7. Fine tune the switch setting so it consistently warns of a blocked filter and does not
falsely indicate a blockage of a clean filter.
8. Remove the restriction from the filter element and re-install the clean filter. The
alarm light should remain off.
Checking the Dewpoint Control System
The Dewpoint Switch option on dryers depends on the proper operation of the dewpoint
sensor and its control board. The dewpoint sensor is in the process air stream and is therefore
susceptible to contamination.
Dewpoint sensor life is dependent on:
Air temperature and flow passing over the sensor.
The amount of fines [dust] in the process air.
The amount of plasticizer vapor in the process air.
The dryer operator should monitor the initial dewpoint sensor readings and establish a
periodic replacement schedule as needed.
Caution! Do not attempt to check the continuity or resistance of a dewpoint sensor. The
sensor will be destroyed.
If you suspect that the dewpoint readings on the dewpoint meter are incorrect, a dewpoint
simulation calibrator is available from the manufacturer. This device simulates various
dewpoint outputs. It can confirm proper operation of the dewpoint sensor and dewpoint
system control board.
Use the following procedure to check the dewpoint sensor and dewpoint control board:
1. Remove the sensor cable from the sensor hex nut adapter.
2. Connect the cable to the plug on the rear of the simulator.
3. Turn the rotary knob on the simulator to each position and note the readings on the
dewpoint meter on the dryer's control panel. The readings should correspond within a
degree or two across the entire range.
4. If the readings agree, the control board is OK. Dewpoint sensor replacement is
recommended. If the readings do not agree, replace the control board and run the test
again to rule out the possibility of a bad sensor.
Note: Do not attempt to make any adjustments to the components on the dewpoint
control board.
5. When testing is finished, re-connect the control cable to the sensor adapter.