Maintenance; Electrode Maintenance - Walchem WDP420 Instruction Manual

6.0

MAINTENANCE

The WPH/WDP control module itself needs very little maintenance. Clean the outside of the controller
enclosure with a damp cloth. Do not spray down the controller unless the enclosure door is closed and
latched. "Pigtails" should be protected from spray or wash-down. Check the cords and cables for
damage.
6.1

Electrode Maintenance

The pH or ORP electrodes require periodic cleaning and calibration. These electrodes are like batteries
and their voltage outputs will change with time even if they are not being used. After installation, the
rate of change increases, and factors such as temperature, extremes of pH, abrasion and chemical attack
will increase the required frequency of calibration. If the process solution contains oils, scale or other
solids, the electrode surfaces will tend to coat, its response time will slow down and cleaning will be
required.
The frequency of cleaning and calibrating will vary greatly depending upon the application, the factors
listed above, as well as the accuracy of control you require. The best way to determine the optimum
number of days between calibrations is to remove the electrode from the process periodically (weekly in
clean water applications, daily in dirty or hot applications) and check its accuracy in a buffer solution.
If using manual temperature compensation, remember to change the temperature from that of the
process to that of the buffer. If the accuracy of the reading is within your required tolerances, and the
speed of response is good, replace the electrode in the process. If not, clean the electrode and perform a
two point calibration.
The method of cleaning the electrode will depend upon the coating, as well as the materials of
construction of the electrode. Do not use a solvent that will attack the electrode! Care must be taken to
avoid scratching the pH electrode's glass, as this will shorten its life. An ORP electrode's platinum
surface may be cleaned with 600 grit silicon carbide paper, jewelers rouge or very fine steel wool.
Oily coatings should be removed with a mild detergent or isopropyl alcohol. Hard scales such as
calcium carbonate can usually be removed with a dilute hydrochloric acid solution. Soft coatings can
be removed using a soft cloth or soft toothbrush.
A two point calibration should always be performed after cleaning the electrode.
Because the electrode signal is so sensitive, the condition of the cable and connectors between the
electrode, preamplifier and controller is critical. Make sure that all electrical connections stay clean and
dry. Never splice the cable prior to preamplification. Replace the cable if there is any sign of damage.
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