pH Measurement – Field Reality (For Technicians)

pH measurement looks simple in textbooks, but in real plants it is one of the most failure-prone liquid measurements. This guide explains what actually causes unstable, drifting, or incorrect pH readings in the field.

Core truth: If a pH reading is wrong, the problem is almost always the sensor or the process, not the analyzer electronics.

Basic Working Principle (Short & Practical)

A pH sensor measures the voltage difference between a measuring electrode and a reference electrode. This voltage is converted into pH by the analyzer using calibration data and temperature compensation.

  • The signal is very small (mV level).
  • Any contamination, moisture, or leakage severely affects accuracy.
  • The analyzer only interprets the signal — it cannot fix a bad sensor.

Why pH Measurement Fails in Real Plants

  • Sensor fouling: coating from solids, oil, bio-growth.
  • Reference poisoning: sulphides, heavy metals, proteins.
  • Dry electrode: poor wetting, long shutdowns.
  • Temperature effects: slow response in cold processes.
  • Installation issues: air bubbles, dead zones, poor flow.

Calibration – Reality vs Expectation

Calibration does not repair a bad sensor. It only aligns the analyzer to the sensor’s current condition.

  • Frequent calibration usually indicates sensor aging.
  • Good sensors hold calibration for weeks to months.
  • Always use fresh buffers and allow full stabilization.
  • Slope and zero outside limits = sensor replacement.

Correct Troubleshooting Order

  • 1) Inspect sensor physically (glass, junction, fouling).
  • 2) Check installation (immersion depth, bubbles, flow).
  • 3) Verify temperature measurement.
  • 4) Perform buffer check (not full calibration).
  • 5) Only then check transmitter, wiring, grounding.
Common mistake: Replacing analyzers repeatedly without fixing sensor or process issues.

Technician Advice