BrauMo BrauMo ← Blog

Food safety · 6 min read ·

Calibrating the pH meter: the record behind every reading

Your food-safety case rests on one number, and that number comes out of one instrument. A reading from an uncalibrated meter is a guess with decimals after it. The calibration register is what turns the reading back into evidence.

In a kombucha brewery the critical limit is written against pH, so the meter is not a convenience. It is the measuring device for the control point that carries the safety argument. Auditors know this, which is why the question after "show me the reading" is almost always "show me the calibration record for the meter that took it". This guide covers what calibration actually does, how often to do it, the procedure that avoids the common mistakes, and what to write down afterwards.

What calibration actually does

A pH electrode does not measure pH directly. It produces a small voltage that changes with acidity, and the meter converts that voltage into a number using two values it learned the last time you calibrated: an offset, which is where the scale sits, and a slope, which is how steeply voltage tracks pH. Both drift. The glass bulb ages, the reference junction clogs, and a meter that was right last month reads confidently wrong today. Calibrating with solutions of known pH is how you re-teach it those two values.

This is also why "the meter says 3.4" is not a finding you can defend on its own. It is defensible when you can show the meter was calibrated that day, against which buffers, and that it accepted the calibration.

Which buffers, and how often

Buffers are solutions with a known, stable pH. For kombucha the working range runs from roughly 4.5 down to 2.5, so the two that matter are pH 7.00 and pH 4.01. A two-point calibration with those brackets the range and is what most small breweries do. The pH 10.01 buffer used for a three-point calibration is above anything you will ever measure in a kombucha tank and adds nothing here.

If you routinely read below pH 3, and especially if you run a vinegar or acidifier line, add a pH 2.00 buffer as a third point or at least as a check. A meter calibrated at 7.00 and 4.01 is being extrapolated when it reads 2.6, and extrapolation is where an electrode's age shows first.

On frequency, the practice that holds up is: calibrate on every day the meter is used, before the first reading of the day, and always before a reading you will use to release a lot. Between calibrations, a single check against a fresh 4.01 buffer takes a minute and tells you whether you have drifted. Keep the distinction clear in your records, because a check is not a calibration and an auditor reads them differently.

The procedure, and the mistakes to avoid

  1. Bring buffers and sample to the same temperature. Buffer values are temperature-dependent, and the printed value on the bottle is for a stated temperature. Automatic temperature compensation corrects the measurement, not the buffer's own shift, so let cold buffers reach room temperature before you start.
  2. Rinse the electrode with distilled or deionised water between every solution. Tap water carries its own ions and its own pH.
  3. Blot, never wipe. Touch the bulb gently against a lint-free tissue to remove the drop. Rubbing it builds a static charge on the glass that makes the next reading wander for minutes.
  4. Calibrate in 7.00 first, then 4.01. The meter uses the neutral point to set the offset before it works out the slope.
  5. Wait for a stable reading, do not chase it. Most meters show a stability symbol. An electrode that takes much longer than usual to settle is telling you something, and the answer is not to accept the number early.
  6. Never pour used buffer back into the bottle, and note the expiry date. An opened bottle of pH 4.01 goes off quietly, and a calibration against a spoiled buffer is worse than no calibration, because it is wrong and documented.
  7. Verify against a fresh buffer after calibrating. Put the electrode back in 4.01 and confirm it reads 4.01. That single line is the proof the calibration took.

Slope and offset: the meter's own verdict

Better meters report a slope percentage after calibration, and it is the most useful number your instrument will ever give you. Around 95 to 105 percent means a healthy electrode. Drifting down through the high 80s means it is ageing and you should have a replacement on the shelf. Below roughly 85 percent, most manufacturers consider the electrode finished, and no amount of recalibrating will rescue it.

Write the slope down every time. A single value tells you little; the sequence over months is what shows you the electrode dying with enough warning to order another one before a brew day.

Looking after the electrode

Kombucha is unusually hard on a pH electrode. It is acidic, it is sugary, and it carries culture and cellulose that film the glass. Three habits cover most of it.

What goes in the calibration register

One line per calibration is enough, as long as it is complete.

Keep this register for at least as long as you keep the batch records that depend on it. A batch record from March and a calibration register that starts in June do not add up to a defensible reading.

What an auditor asks for

The chain an inspector follows is short, and it runs backwards from the product.

  1. Show me the release reading for this lot.
  2. Which meter took it?
  3. Show me that meter's calibration record for that date.
  4. Which buffers, and were they in date?
  5. What did you do the last time a calibration failed?

A brewery that can answer those five in a minute is in good shape. The usual break is between the second and third question: the readings exist, the calibrations exist, and nothing connects a given reading to the state of the instrument that produced it.

Frequently asked questions

Can I use pH strips instead of a meter?
Not for a critical limit. Strips are a useful rough guide when you want to know whether a ferment is roughly on track, but they cannot be calibrated, they are read by eye, and their resolution is far too coarse for a limit written at a specific value. For the reading your safety case rests on, use a meter you can calibrate and document.
Two-point or three-point calibration?
Two-point with pH 7.00 and 4.01 covers the kombucha range and is what most small breweries run. Add a pH 2.00 buffer if you regularly read below 3, or if you also make vinegar, because at that end the meter is otherwise extrapolating past its calibrated span. The 10.01 buffer is irrelevant for these products.
My meter will not hold a calibration. What now?
Work through it in order: fresh buffers first, since an old bottle is the most common cause, then clean the electrode with a proper cleaning solution, then recondition it if the manufacturer offers a procedure. If the slope is still low after that, the electrode is finished. Record the failure and the replacement in the register, because a documented failure with an action beside it reads better than a gap.
How long do I keep calibration records?
At least as long as the batch records they support. Retention periods vary by country, from six months to two years, and Brazil's producer decree asks for 18 months. Match the longest period that applies to you, and keep the calibration register and the batch logs together, because separately they prove much less.
BrauMo keeps the calibration register beside the readings it backs: each meter, its calibrations with buffers, slope and result, and the batch readings taken with it, so a reading has a provenance instead of a decimal point. Documentation-only: it records and reports, it never blocks a batch. See how the records are kept →

Related reading

See how BrauMo handles this: Take the product tour → · See pricing →