How long does a pH sensor last, and when to replace it

Short answer: A pH sensor typically lasts 12 to 18 months. It is a consumable — however well you look after it, the glass electrode gradually weakens. In difficult water such as ETP it may last only 6 to 9 months.

The clearest sign is that the sensor no longer calibrates properly, or that readings take longer to settle. That is when it needs replacing.

Why a pH sensor degrades

Inside the sensor there is a thin glass membrane and a reference solution. Over time:

• A film builds up on the glass and the response slows
• The reference junction becomes blocked
• The internal solution depletes or becomes contaminated
• The glass itself ages — this is simply its natural life

Five signs it is failing

1. Calibration fails: the sensor will not settle correctly in buffer 7 and buffer 4 — the surest sign
2. Slow response: a reading that used to appear in 10 seconds now takes a minute or two
3. Unstable reading: it never settles and keeps drifting up and down
4. Always around 7: whatever the water, it reads close to 7
5. Reads 7 even in buffer 4: the sensor is completely finished

Sensor types and prices

Why the ETP sensor costs so much more: it has a special junction that resists blocking, and a more robust body. A standard sensor rarely survives two to three months in an ETP, so the expensive sensor actually works out cheaper there.

Six ways to extend its life

1. Never let the sensor dry out. This is the most important rule. If the plant is shut down, remove the sensor and store it in its cap with KCl solution — not plain water.
2. Calibrate monthly — two-point, using buffer 7 and buffer 4. This also tells you in advance how the sensor is holding up.
3. Clean gently — with a soft cloth. Never scrub the glass membrane, which is very thin.
4. For oil or grease — soak for 15 minutes in a mild detergent solution, then rinse.
5. Fit it in the right place — where water keeps moving. A sensor in stagnant water blocks quickly.
6. Respect the temperature rating — a 0-60°C sensor used in hot water will die early. Use the 0-100°C version there.

How to calibrate

1. Rinse the sensor with clean water and blot it dry gently — do not rub
2. Place it in buffer 7, allow the reading to settle, and set "cal 7" on the controller
3. Rinse, then place it in buffer 4 (or 9.2) and set that value
4. Refit it in the line

Sensor only, or the whole controller?

The good news is that usually only the sensor needs replacing:

For controllers see the pH Controller PO 650 (₹9,975), or for display only the pH Indicator PO 350 (₹7,035).

Keep a spare, especially in an ETP

Where pH control matters for production or discharge, keep a spare sensor. If the sensor fails:

• ETP discharge norms may be breached — with a penalty
• Dosing goes wrong and chemical is wasted
• A new sensor can take several days to arrive

One common misconception

People assume a new sensor must read correctly. It does not — even a new sensor gives wrong readings until calibrated. Always calibrate immediately after fitting a new sensor.

Frequently asked questions