Key points
- Interval is set by measured drift, not by a fixed calendar.
- Use two-point calibration with buffers bracketing the process pH.
- Clean before calibrating; most drift is fouling.
- Slow response and low slope (<90%) indicate an ageing electrode.
Typical starting intervals
| Application | Starting interval | Main drift cause |
|---|---|---|
| Drinking water, clean process water | Monthly | Reference junction ageing |
| Cooling towers | 2–4 weeks | Scale, biofilm |
| Municipal wastewater | 1–2 weeks | Grease, solids coating |
| Chemical neutralisation, high temperature | Weekly | Chemical attack, poisoning |
Starting points only. Adjust using your drift log.
Two-point calibration method
- Remove and rinse the electrode; clean coatings with the recommended solution.
- Immerse in pH 7 buffer, wait for a stable reading, set zero (offset).
- Rinse; immerse in pH 4 or pH 10 buffer (bracketing the process), set slope.
- Record offset (mV) and slope (%). A healthy electrode is roughly 95–102% slope.
- Return to process and confirm against a grab sample.
The SUP-PH6.5 applies automatic temperature compensation (NTC10K, Pt1000 or Pt100) during calibration and measurement. Background on the measurement: pH meter.
When to replace the electrode
- Slope below about 90% after cleaning.
- Response to a buffer takes several minutes.
- Offset beyond about ±30 mV at pH 7.
- Cracked glass or a dry, contaminated reference junction.
Related questions
Can I calibrate in place against a grab sample?
Yes, as a one-point standardisation between full buffer calibrations, provided the lab measurement is reliable.
Does the dissolved oxygen sensor follow the same schedule?
Membrane DO sensors such as the SUP-DM2800 are typically air-calibrated on a similar interval, with membrane and electrolyte replaced periodically.
Sources and further reading
Product specifications quoted here are summarised from Supmea datasheets. Confirm the figures for your exact model before final design.