What Equipment Needs Calibration?

The test is narrower than most equipment lists imply: an instrument needs calibrating if its reading has a significant effect on the accuracy or validity of a result you rely on. That covers measurements used to decide whether a product conforms, and it covers the supporting measurements that feed those decisions. It does not cover instruments that merely indicate something is happening.

Which means the question is not really "what type of instrument is this?" but "what is this instrument's reading used for here?" — and the same model of gauge can land on either side depending on the answer.

The Rule, Stated

Most guidance on this offers a list of equipment types, which is unhelpful because the answer depends on use. The published rule is better, and it is short. UKAS TPS 41, Edition 6, section 1.2:

"Metrological traceability is required for all equipment used for tests and/or calibrations, including equipment for subsidiary measurements (e.g., for environmental conditions) having a significant effect on the accuracy or validity of the result of the test, calibration or sampling. Calibration may not be required for subsidiary equipment where the result is unaffected or where the equipment is monitored by calibrated equipment during the measurement."

Two sentences, and they do most of the work. The first extends the scope beyond the obvious: not just the instrument taking the measurement, but anything measuring a condition that materially affects it. The second gives two specific exemptions.

Three Categories

In scope, obviously. The instrument that takes the measurement a conformity decision rests on. Micrometers and calipers checking a dimension against a drawing tolerance. Pressure gauges on a test rig where the recorded pressure is the result. Torque wrenches where the applied torque is specified. Balances weighing to a tolerance. If the number goes on a certificate, a report, an inspection record or a release decision, the instrument is in.

In scope, less obviously — the subsidiary measurements. This is the category the TPS 41 wording exists to catch, and the one most commonly missed. If your measurement is temperature-sensitive and you record the ambient temperature to correct for it or to demonstrate conditions were within limits, that thermometer is feeding the result. So is the humidity sensor where humidity matters, and the timer where elapsed time is part of the method. The test is the phrase "significant effect on the accuracy or validity of the result" — not whether the instrument is the star of the procedure.

Out of scope — indication only. Instruments whose readings nothing depends on. A gauge that tells an operator a system is pressurised, without the value being recorded or relied on. A wall clock. A thermometer in an office. These are genuinely outside the programme, and treating them as inside is its own problem: an inflated register costs money and dilutes attention from the instruments that matter.

The Two Exemptions, and Which One Is Useful

TPS 41 gives two circumstances where subsidiary equipment may not need calibration.

"Where the result is unaffected." The straightforward one. If ambient temperature genuinely does not influence what you are measuring, the thermometer on the wall is not part of the measurement chain. The judgement to make — and to write down — is that it genuinely does not, rather than that nobody has considered whether it does.

"Where the equipment is monitored by calibrated equipment during the measurement." The more useful one, and the more often applicable. If a calibrated logger is recording the conditions during the measurement, an uncalibrated indicator sitting alongside is not carrying any weight — the calibrated device is the evidence. This is what lets a workshop have a wall-mounted temperature display for convenience while the actual environmental record comes from a calibrated probe. It is worth being clear which device is which, because the exemption only holds while the calibrated one is the one being relied on.

Neither exemption is a blanket permission. Both are decisions about a specific instrument in a specific use, which is why they belong in writing.

Four Decisions, Worked

The rule is easier to apply against examples than in the abstract. Each of these is the same instrument type landing differently depending on use.

A pressure gauge on a test rig, where the test report records the pressure reached. In scope. The number is the result. Straightforward.

The same model of gauge on a compressor, showing the operator that the system is up to working pressure. Out of scope, indication only — nothing records or relies on the value. Worth labelling as such, because it is physically identical to the one above.

A thermometer in a dimensional inspection room, where measurements are corrected for temperature. In scope, as a subsidiary measurement. It is not measuring the part, but its reading feeds the correction applied to the result, which is squarely "significant effect on the accuracy or validity of the result".

The same thermometer, in a room where a calibrated data logger records conditions throughout the measurement and the logger's record is the evidence. Exempt under the second TPS 41 case — monitored by calibrated equipment during the measurement. The wall thermometer is convenience; the logger is the record. This exemption holds only while that stays true, so if the logger is removed the thermometer's status changes with it.

The pattern across all four: the instrument type told you nothing. What the reading is used for told you everything.

Write the Exclusions Down

The most common way this goes wrong is not including too little equipment. It is excluding equipment correctly and leaving no trace of the decision.

From an assessor's side of the desk, an instrument in use on your floor and absent from your register has two possible explanations — a considered exclusion, or a gap — and nothing distinguishes them. So the exclusions belong on the calibration register with an explicit status: indication only, not calibrated. One row, and the question is answered before it is asked. The reasoning behind the class of exclusion belongs in the calibration plan, which is where the scope of the programme is defined.

The physical counterpart matters too. An uncalibrated instrument that looks like a calibrated one will eventually be used as one. Labelling indicating-only devices clearly — distinctly from calibration labels — is what stops a well-reasoned exclusion becoming an incident.

Scope Drifts, and Usually Quietly

The decision is made once, when an instrument enters service, and use changes afterwards. A gauge bought for indication gets pressed into service taking a reading someone writes down. A process changes and a measurement that never mattered starts mattering. Neither event prompts anyone to revisit the register, because nothing about them looks like a calibration decision at the time.

The practical guard is to make scope part of an existing review rather than its own task. When intervals are reviewed, ask whether anything has changed about how each instrument is used — and when a new measurement is introduced into a process, ask what is taking it. The ISO 9001 Clause 7.1.5 requirements cover the wider control this sits inside, and the calibration audit checklist covers what gets examined.

Once an instrument is in scope, how often it needs calibrating is a separate question with its own method — determining calibration intervals covers it, and how often test equipment should be calibrated gives typical starting points by equipment type. Where the calibration is done is covered in in-house versus outsourced calibration.

How CalProof Fits

CalProof's register holds every instrument you calibrate, with its interval, due date and full history in one place. A dedicated status and reason for instruments you've deliberately excluded from the programme isn't yet a field it captures — until it is, keep that decision written down alongside the register, so when an assessor asks why something is not calibrated, the answer is still on record even if it isn't inside the tool.

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The calibration interval recommender gives a starting interval once an instrument is in scope, and the ISO 9001 Clause 7.1.5 self-assessment walks the wider requirement. To see the shape of the output, the sample audit pack is ungated and downloads as a single PDF.

Sources

• UKAS TPS 41, Edition 6 (December 2022) — UKAS policy on metrological traceability (section 1.2)
• United Kingdom Accreditation Service

This guide applies to UK manufacturers and laboratories working to ISO 9001 or ISO/IEC 17025. It is general guidance based on published UKAS policy; which of your own equipment falls in scope depends on your processes, your measurements and your assessor's interpretation. Verify against your certification body or UKAS assessor. This is not legal or compliance advice.