Calibration and maintenance

Awareness level only. This lesson explains the ideas so you recognise them. It does not qualify you to do this work. Before you use lab equipment or handle chemicals, biological materials or patient samples for real, you need the training your employer or institution requires, and you must work under supervision until they sign you off.

Why it matters

An instrument is only as trustworthy as its last calibration. A pH meter that drifted 0.2 units overnight, a balance that was knocked out of level, or a thermometer that reads 1 °C low can all produce results that look perfectly normal and are wrong. Calibration, verification and maintenance, and the records that prove they happened, are what let a lab stand behind its numbers. Auditors check these logs early in any inspection.

Three words that are often mixed up

• Calibration: comparing an instrument's readings with known reference standards, and usually adjusting it so it reads correctly. Example: calibrating a pH meter with pH 4, 7 and 10 buffers.
• Verification (or check): confirming that the instrument still reads within limits, without adjusting it. Example: weighing a certified check weight on a balance each morning.
• Maintenance: keeping the instrument working: cleaning, replacing parts, servicing. Some is daily (wiping a balance pan), some yearly by an engineer (preventive maintenance).

Traceability means each reference standard is linked, through an unbroken chain of calibrations, to national or international standards (in the US, often through NIST). Certificates for buffers, weights and thermometers state this traceability.

Calibrating a pH meter

A pH electrode produces a voltage that changes with pH. At 25 °C an ideal electrode changes by 59.16 mV per pH unit. Real electrodes age, so the meter is calibrated against buffers of known pH.

Typical steps (your SOP and the meter's manual come first):

1. Check the buffers are in date and at room temperature, and note their lot numbers.
2. Rinse the electrode with deionised water and blot (do not wipe) it dry.
3. Place it in pH 7.00 buffer first (this sets the offset, or zero point). Wait for a stable reading and accept.
4. Rinse, blot, and place it in pH 4.01 (or 10.01) buffer. This sets the slope. For samples across a wide range, use a three-point calibration with 4, 7 and 10.
5. Read the slope and offset the meter reports and compare them with your acceptance limits. A common rule of thumb is a slope of 95 to 105 percent and an offset within about ±30 mV, but follow your lab's limits.
6. Verify with a buffer not used in the calibration, if your SOP requires it.
7. Store the electrode in its storage solution (often KCl), never in deionised water, which damages it over time.
8. Log it.

Worked example: is this electrode acceptable?

At 25 °C the meter reads -2 mV in pH 7.00 buffer and +168 mV in pH 4.01 buffer.

slope = (168 - (-2)) / (7.00 - 4.01) = 170 / 2.99 = 56.9 mV per pH unit
percent of ideal = 56.9 / 59.16 x 100 = 96.1 percent
offset = -2 mV

The slope is within 95 to 105 percent and the offset is small, so the calibration passes. A slope of 88 percent would mean the electrode needs cleaning, reconditioning or replacement.

What goes in the log

A calibration record should let anyone reconstruct what happened. Typical fields:

When an instrument fails

• Do not use it. Label it out of service (a tag or sticker) so others do not either.
• Record the failure and what you did.
• Tell your supervisor.
• Ask the important question: what was measured since the last good calibration? Those results may need review or repeat testing. This is often handled as a deviation, which you will meet in the Lab Data & Quality course.

Preventive maintenance and service

Larger instruments (HPLC, centrifuges, biosafety cabinets, spectrophotometers) have scheduled preventive maintenance, often by the manufacturer's engineer, with a sticker showing the next due date. Biosafety cabinets and fume hoods are certified on a schedule. Check the sticker before you use any instrument; an overdue one may not be used for reportable work.

Common mistakes

• Calibrating with expired or contaminated buffers.
• Wiping a pH electrode (static charge and scratches) instead of blotting it.
• Storing the electrode in deionised water.
• Writing the log at the end of the day from memory.
• Using an instrument with an expired calibration sticker "just this once".

Quick recap

• Calibration adjusts against traceable standards; verification checks without adjusting; maintenance keeps it working.
• pH: offset at pH 7, slope with 4 or 10; accept a slope around 95 to 105 percent (per your SOP).
• Log every calibration completely and at the time you do it.
• A failed instrument is tagged out, and results since the last good calibration are reviewed.

For traceability and calibration services, see NIST: https://www.nist.gov/calibrations (Checked on 2026-10-23.)

Try it

Open the Do tab to judge calibration data and design a calibration log on paper.