How to calibrate sensors on site — step by step

Short answer: Calibration means showing the sensor a known solution and telling it "this is 7" or "this is 1,413 µS/cm", so that it reads correctly afterwards. It should be done monthly, and it can be done on site with nothing more than buffer solution and ten minutes.

Without calibration even the most expensive sensor reads incorrectly — and decisions based on wrong readings can be very costly.

Why calibration is necessary

Sensors drift over time. Something that read 7.0 yesterday may read 7.3 today, without anyone noticing.

The real damage is that you then make decisions on a wrong reading — overdosing chemical, or replacing a membrane that was actually fine.

How often

In difficult water such as an ETP, more frequent calibration may be needed.

pH sensor — two-point calibration

1. Prepare: buffer 7 and buffer 4 (or 9.2), clean water, tissue
2. Remove the sensor, rinse with clean water and blot gently — do not rub
3. Place in buffer 7 and allow one to two minutes to settle
4. Enter calibration mode and set 7.00
5. Rinse, then place in buffer 4 (or 9.2)
6. Set that value once settled
7. Refit the sensor in the line

Which second buffer: whichever side you work on. Buffer 4 if you dose acid, buffer 9.2 if you dose alkali.

Conductivity and TDS calibration

1. Take a standard solution, usually 1,413 µS/cm (or 1,000 ppm TDS)
2. Rinse and dry the sensor
3. Immerse in the standard and let it settle
4. Set that value on the controller

ORP calibration

ORP sensors need calibrating less often. Place in an ORP standard solution, typically 470 mV, and set that value.

If the reading differs from the standard by more than 30 to 40 mV, clean the platinum tip — gently, never scrubbed.

DO calibration — in air

This is the simplest of all:

1. Remove the sensor and blot it gently
2. Leave it in air for 5 to 10 minutes (in humid air is better — near a damp cloth)
3. Set "100% saturation" or "air calibration" on the controller

Because the oxygen content of open air is constant, it serves as a reliable reference.

About buffer solutions

• Check the expiry date — old buffer gives a wrong calibration
• Do not use straight from the bottle — pour a little into a separate cup. Dipping the sensor into the bottle spoils the whole bottle
• Discard used buffer — never reuse it
• Keep it closed — open buffer degrades in air, buffer 9.2 particularly
• Store cool, away from sunlight

Five common mistakes

1. Rubbing the sensor — the glass membrane is thin, and a scratch ruins the sensor
2. Rushing the calibration — allow one to two minutes for the reading to settle
3. Expired buffer — gives a wrong value
4. One-point calibration only — buffer 7 alone is incomplete; the second point is essential
5. Not recording the calibration — note the date and values each time, so you can see the trend

Keep a simple record

Note the following in a book:

This record tells you in advance when a sensor is about to fail, so you can order a spare and the plant is never held up.

When to replace instead

Calibration itself gives you the answer:

• The controller will not accept the calibration at all
• The adjustment required grows month on month
• It reads around 6 or 7 even in buffer 4
• Readings take two to three minutes to settle

Prices and options are in our pH sensor guide.

Frequently asked questions