How pH probes measure acidity and alkalinity, what upsets them, and how to keep them calibrated.
A pH probe is a small battery whose voltage depends on the hydrogen ion activity of the water. It has two halves, usually built into one body (a combination electrode):
- Glass measuring electrode. A thin bulb of special pH-sensitive glass. A gel layer on the glass exchanges hydrogen ions with the water, and a voltage develops across the membrane that depends on the pH outside it.
- Reference electrode. A silver/silver chloride element in a salt (usually potassium chloride) electrolyte, which gives a steady voltage whatever the pH. It contacts the process water through a porous junction — a ceramic, PTFE, or wood plug, or an open annulus.
The transmitter measures the millivolt difference between the two halves:
| pH |
Ideal output at 25 °C (77 °F) |
| 4 |
about +177 mV |
| 7 |
about 0 mV |
| 10 |
about −177 mV |
- Slope. An ideal electrode changes by 59.16 mV per pH unit at 25 °C (the Nernst slope). A healthy electrode typically sits at about 95–102 % of that; as the glass ages the slope drops.
- Offset. At pH 7 the output should be close to 0 mV. A healthy electrode is typically within about ±30 mV; a drifting offset usually points to the reference.
- Temperature compensation. The slope changes with temperature (about 54 mV/pH at 0 °C, 66 mV/pH at 60 °C), so the probe carries a temperature element (commonly a Pt100, Pt1000, or thermistor) and the transmitter corrects for it. This corrects the electrode’s response only — it does not convert the water’s pH to a “pH at 25 °C”. The real pH of water does change with temperature.
pH is a logarithmic scale: one pH unit is a tenfold change in hydrogen ion activity.
- Coating and fouling. Scale, oil, biofilm, iron, and treatment chemicals coat the glass and junction. The reading becomes slow, then drifts or flattens out. This is the most common cause of a bad pH reading in cooling water.
- Reference poisoning. Sulphides, cyanides, heavy metals, and strong reducing agents can enter the junction and react with the silver reference, shifting the offset. A blackened junction is a sign of this.
- Clogged or dried junction. Precipitate in the junction, or a probe left dry, makes the reading noisy and sluggish.
- Low-conductivity water. Very pure water (for example RO permeate or condensate, below roughly 100 µS/cm) gives unstable, drifting readings because there are too few ions to complete the circuit through the junction. Use an electrode designed for pure water.
- Air bubbles trapped at the bulb, or a probe running partly dry, break the circuit and cause noise or a stuck reading.
- Temperature. A failed or disconnected temperature element gives an error that grows the further the water is from 25 °C and the further the pH is from 7.
- Electrical noise and ground loops. The glass electrode has a very high resistance, so its signal is easily disturbed. Keep sensor cable short and away from power cable, and look for pumps or heaters leaking current into the water if the reading shifts when they run.
- High pH and high temperature. Above about pH 12, sodium ions start to register as hydrogen ions (the “alkaline error”), and the reading reads low. Hot, caustic water also ages the glass fast.
- Cracked glass. A cracked bulb typically reads near a fixed value (often around pH 7) whatever the solution.
- Mount the probe tip down, at least about 15° above horizontal. This keeps the internal electrolyte against the glass and the air bubble inside the bulb away from the tip. Never mount a standard probe upside down or flat.
- Keep the bulb wet at all times. Choose a position where the probe stays submerged when flow stops — a flow cell or a pipe section that does not drain. A dried-out probe may recover after soaking, but often does not.
- Flow cell or insertion. A flow cell on a sample line makes cleaning and calibration easy without draining the system. Insertion fittings in the main line give faster response but need isolation valves or a retractable fitting to remove the probe.
- Steady, moderate flow. Too little flow gives a slow, unrepresentative reading; very high velocity can erode the glass and cause noise. Follow the manufacturer’s flow range.
- Sample after chemical has mixed in, not immediately after an acid or caustic injection point.
- Protect the glass. Avoid positions where the bulb can be struck, and do not overtighten compression fittings.
- Wire as specified. Use the manufacturer’s cable, and keep it away from VFD and power wiring.
¶ Maintenance
Clean when the response is slow, the slope drops, or deposits are visible.
- Rinse with clean water. Wipe gently with a soft, wet tissue only — never scrub or dry the glass, which damages the gel layer and builds static.
- Scale and hardness deposits: soak a few minutes in dilute hydrochloric acid (commonly about 0.1 M, or 5 %).
- Oil and grease: mild detergent solution in warm water.
- Biological film: a dilute bleach solution or an enzyme cleaner, per the manufacturer.
- After any chemical clean, rinse thoroughly and soak in storage solution or pH 4 buffer for at least 15–30 minutes before calibrating, so the glass re-hydrates.
Cleaning acids and bleach can burn skin and eyes, and mixing them releases chlorine gas. Wear eye protection and gloves, never mix cleaners, and follow Chemical Handling. Isolate and depressurize an insertion probe before pulling it — see Isolation and Pressurized Systems.
- Two-point buffer calibration. Calibrate with pH 7 first (sets the offset), then pH 4 or pH 10 (sets the slope) — choose the second buffer closest to the normal process pH. A three-point calibration (4, 7, 10) is used where the process swings widely.
- Use fresh, in-date buffers. Pour a little into a clean cup; never dip into the stock bottle or pour used buffer back. pH 10 buffer absorbs carbon dioxide from the air and drifts once opened.
- Rinse between buffers with clean water, and let the reading settle at each point.
- Buffers at a similar temperature to each other help. Buffer values change slightly with temperature; most transmitters correct for this automatically.
- Check the results. Note the slope and offset the transmitter reports. A slope below about 85–90 % or an offset beyond about ±30–50 mV typically means clean the probe, then replace it if cleaning does not recover it.
- Interval. Check monthly to start, then lengthen or shorten based on how far it drifts between checks. A one-point “standardize” against a trusted handheld can correct offset between full calibrations.
¶ Service life and storage
- Service life. pH electrodes are consumable. Expect commonly 6–18 months in cooling or process water; hot, dirty, or high-pH water shortens that. An electrode ages on the shelf too, so do not stock too far ahead.
- Store wet. Keep the protective cap on, filled with electrode storage solution (typically 3–4 M KCl), or pH 4 buffer if storage solution is not available.
- Never store in distilled or deionized water. It draws ions out of the glass and reference and shortens the electrode’s life.
Models documented in this wiki: