Online pH Analyzer Shows 4.3 While Laboratory Measurement Shows 6.9
A wastewater tank normally operates at pH 7.0. The online pH analyzer suddenly indicates 4.3, while laboratory testing shows 6.9. The transmitter output is stable, temperature compensation works correctly, and there is no process upset. The electrode has been operating for 14 months.
What is the most likely cause of this incorrect pH measurement, and what should an instrumentation engineer check first before replacing the pH electrode?
Online pH Analyzer Shows 4.3 While Laboratory Measurement Shows 6.9
Most Likely Cause
Transmitter failure is unlikely. The most probable cause is pH electrode fouling or reference junction contamination. The glass membrane or the reference junction may become coated with particles, biological debris, scale, or chemical deposits in wastewater service. This can have a large effect on pH even if the process is very near neutral.
The 14 month operating time is very important. pH electrodes are consumables and performance can deteriorate with age, contamination, dehydration and reference poisoning. If it reads steady but wrong , then the sensor should be inspected and replaced .
What to Check First
First, put the online sensor in a clean sample or fresh laboratory buffer. Inspect the glass bulb and reference junction for coating, deposits, blockage or damage. Compare the transmitter reading in pH 7 and pH 4 buffers. Check the electrode slope and offset if available.
Then verify:
- Reference junction condition & electrolyte (if appropriate)
- Cracks the glass or coating of an electrode.
- Sensor cable, connectors and pre-amplifier connections
- Calibration and buffer condition history.
- Depth of installation, flow of sample and wetting
- Temperature measurement and compensation, even if temperature is functioning well.
If the cleaning recovers the predicted response and the calibration is still within acceptable limits, the electrode may be used. If the slope still indicates poor or excessive offset, delayed response or unstable calibration, replacement is warranted.
For wastewater applications, routine cleaning and a sensor that resists junction clogging can increase the reliability of online pH measurement, prolong the life of the electrode, and minimize unnecessary electrode replacement in the plant.
