pH Electrode Troubleshooting

This presentation is a systematic procedure to isolate problems in your pH measuring system.

Troubleshooting Equipment

The following tools are recommended to assist you in problem identification:

1. Portable pH/mV meter with interconnect cable
   The meter and buffers can verify operational status of a pH electrode or ORP electrode.
   
2. A DVM (digital volt meter)
   If your pH system has Automatic Temperature Compensation (ATC), the DVM will verify operation of the ATC element. Refer to the ATC page or your meter's instruction manual for the appropriate ATC resistance reading (typically 100, 1000, 3K, 10K, or 30K ohms).
3. pH Simulator
   Simulators similar to Sensorex pH Checker can quickly identify or eliminate the or controller as the cause of the problem. Check under high impedance condition--an option on the pH Checker
4. pH Buffers
   Use 7 and either 4 or 10 buffer depending upon at which end of the pH scale you normally operate in.

Meter, Preamplifier, Controller Troubleshooting

The meter or controller is the easiest component to eliminate as a possible cause of your problem.  Follow the stepwise instructions below for quick problem solving.

1. Disconnect the pH electrode from the meter, preamplifier (if you have an external preamp), or controller.
2. Connect a pH Simulator (like the Sensorex pH Checker) in the position where the electrode was connected. 
3. Set the pH simulator to 7 with high impedance and verify the meter, preamplifier, or controller output .
4. Reset the pH simulator to 4 with high impedance and verify the output. If you did not get 7.00 and 4.00 pH, recalibrate or troubleshoot the meter, preamplifier, or controller in accordance with its instruction manual.  Note that miswired preamps usually give negative readings or constant zero reading regardless of electrode or simulator input

pH Electrode Troubleshooting

Remove the electrode from the sample or process and disconnect it from the meter or controller. Connect to the portable pH/mV meter and test electrode in 7 and 4 pH buffers. Compare results to the following table:

If you are still having problems after trying the above diagnostics, fill out our Application Questionnaire and e-mail it to us for assistance.

Speed of Response*
New pH electrodes from Sensorex with spherical pH measuring surfaces will respond to within 95% of their final value in less than 1 second. New flat surface electrodes (due to their higher initial impedance) have 95% response in less than 5 seconds. As electrodes age, their impedance will increase and their Speed of Response will become slower. Coatings on the pH glass will also give a slower response time.

Offset*
A perfect electrode will read 7.00 in 7.00 buffer. Offset is defined as the difference between the electrode's reading in 7.00 buffer and 7.00. For example, when an electrode reads 7.15 in 7.00 buffer, its Offset is 0.15 (7.15-7.00 = 0.15). The factory specification for a new electrode is 7.00 +/-0.20 pH since all modern meters and controllers are easily capable of correcting such offset.

As electrodes age, their Offset will change. Usually offset changes are corrected by calibrating the pH meter or controller with the electrode together. An electrode is usable as long as the meter or controller can be adjusted to correct the offset and span and the Speed of Response is acceptable. Changes in an electrode Offset of more than 0.50 pH in a month or less may indicate an application problem.

Span***
A perfect electrode will have 100% of the theoretical Span which means that when an electrode is calibrated to read 7.00 in 7.00 buffer and is place in 4.00 buffer, it will read 4.00. Sensorex specification for a new electrode is at least 97% of theoretical Span. So when an electrode is placed in 4.00 buffer, it can read between 3.90 and 4.10. Similar to Offset, most meters and controllers have Span adjustments to correct for span errors. Normal electrode aging will reduce the Span. As long as the pH meter or controller has sufficient adjustment to correct of the reduced span, the electrode is usable, providing the speed of response is acceptable to your process. Coating on the pH sensitive glass can also reduce the Span.

Ground Loops***
When a pH system is unstable, erratic, or the offset drifts, the most common problem is an electrical ground loop in the system, particularly if the tank and/or pipes are plastic. To verify this problem, remove the electrode and calibrate it in a known buffer in a beaker. If the electrode measures within specification (stable and adjustable offset) when calibrated, place a copper wire in the beaker and the other end in your system. If the reading becomes unstable or shifts, a ground loop is your problem. 

           ELECTRODE CALIBRATES            THEN         ELECTRODE READS HIGH

                    IN BUFFER                                            OR OUT OF RANGE IN PROCESS

The sources of the ground loop could be any mixer motor, pump, conductivity probe, or other electrically powered device in the media with the pH electrode. Do not attach the conductivity probe or any other electrically powered device to the same ground on your meter or controller as the pH electrode. You can also try placing a large (12 or 14 AWG) copper wire into the media and the other end to the meter or controller ground terminal to draw the ground loop away from the pH electrode. Sensorex also offers a Ground Loop Interrupt electrode to solve this problem.