CAUTION: There are live circuits inside the controller even when the power switch is in the
OFF position! Servicing and troubleshooting the controller must be performed by
Qualified
Service Personnel.
Section 3 (A): WEL-PH Housing + Electrode Reads Low or High Versus a
Calibrated Handheld Meter: Cannot Calibrate the Electrode (continued)
If the resistance reading is incorrect, disconnect the WHITE/GREEN wires from the terminal
strip inside the controller and using a multimeter, set to resistance (ohms), measure the
resistance across the WHITE/GREEN and GREEN/WHITE wires coming directly from the
electrode. Reading should be 1000 ohm + 3.85 ohms per degree C above 0°C. If reading is
incorrect, the temperature element in the electrode is bad. If the resistance reading is good, yet
it still displays the temperature incorrectly, the problem is with the controller front panel.
If the problem is the controller front panel, please contact your supplier of Walchem equipment.
Step 3:
Calibration fails
• In the controller’s pH Electrode Input menu, look at the pH Raw Value and the Gain
Value.
• What is the calibration range that you are allowed to go to?
o The controller shows us a current pH reading and a raw pH reading.
o This raw value is in mV units.
o The range (referred to as gain) that the controller will allow you to calibrate to
uses the raw pH as a reference point.
o Gain is a value that the controller calculates, and it tells us how far you are
calibrating from the raw pH value.
o For pH electrodes, the calibration will fail if the adjustment to the gain is outside
of the 0.2 to 1.2 range, or if the calculated offset is outside of -140 to +140mV.
• What is Offset?
• Offset = this is the mV value that the pH electrode is measuring at pH=7
• Normal offset at pH=7 should be approximately +/-10mV
• The controller software allows this range to be -140 to +140mV
To re-establish a starting point, the controller allows you to reset the calibration values as
follows:
• Go to the pH Electrode Inputs menu. Press the Settings key, then the Down arrow until
you see Reset Calibration Values. Press on the Reset Calibration field, and then press
the Check mark to accept.
• You should now perform a pH electrode calibration in accordance with the controller
manual.
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