Calibration Register: Fields Auditors Check

A calibration register is the controlled list of every instrument in your calibration programme: one row per instrument, carrying what it is, where it is, who owns it, how often it is calibrated, when it was last done and when it is next due. It is the answer to "what do you have, and what state is it in?" — which is the first question an assessor asks and the one that most often takes longest to answer.

It is not the record of what happened at an individual calibration. That is a different artefact with different fields, and keeping the two separate is what stops a register turning into an unreadable sprawl.

Register, Schedule, Record: Three Things That Get Merged

Most published templates for this fold three artefacts into one spreadsheet, which is why they grow columns until nobody maintains them. Separating them makes each one usable.

• The register is the master list of instruments and their current state. One row per instrument. It answers "what do we hold?"
• The schedule is the forward view of the same data — what is due, and when. It answers "what is coming up?" In practice it is a filtered view of the register rather than a separate document.
• The calibration record is per-event: what happened on the day this instrument was calibrated, what was found, what was left, who accepted it. Many rows per instrument, accumulating over years.

Mixing the third into the first is the failure. Once each calibration event adds columns to the instrument row — cal_date_1, cal_date_2, result_2024 — the register stops being a list of what you own and becomes an archive nobody reads.

The Fields

Identification first, and it carries more weight than it looks.

• Unique identifier. Your own asset number, not the manufacturer's serial number. This is the single most important field, and the one most often done badly. It has to be unique, permanent, and physically present on the instrument.
• Description, manufacturer, model, serial number. What the thing is. Serial number sits alongside your identifier rather than replacing it — two instruments can share a model, and serial numbers get transcribed wrongly.

The reason identification carries weight is that it is the link between the physical instrument, its label, and its paperwork. UKAS LAB 5, Edition 5 sets out at section 4.4 what should be indelibly inscribed on a calibration label, and instrument identification is on the list alongside the UKAS accreditation symbol, the date of calibration, the certificate number and space for the recalibration due date. LAB 5 also notes at section 4.3 that superseded UKAS calibration labels should be removed or cancelled — a small point with a real failure mode behind it, since an instrument wearing two labels is an instrument whose status nobody can determine at a glance.

If the identifier on the register does not match the one on the instrument and the one on the certificate, none of the three is evidence of anything. Everything else in the register is downstream of getting this right.

Then location and ownership.

• Location. Where the instrument normally lives. This is what makes a physical audit possible — an assessor asks to see three instruments from the register, and the location field is what turns that into a two-minute walk.
• Owner or custodian. The person or team responsible. Instruments without an owner are the ones that go missing.

Then the calibration control fields.

• Calibration interval, and the basis for it. The interval is a number; the basis is why. A short code against a class — manufacturer recommendation, use frequency, history — is enough, and it is the difference between a register and a defensible one. The methods for determining intervals cover how the basis is arrived at.
• Date of last calibration.
• Next due date.
• Current status. In service, out for calibration, quarantined, out of tolerance pending assessment, retired. This field is what makes the register operationally useful rather than historical.
• Certificate reference for the latest calibration. The link from the row to the evidence.

Then, where the instrument is used against a specification:

• Range and tolerance. The measuring range in use and the permissible error. These are what make a past result interpretable — without the tolerance, "as found: 0.03 mm high" cannot be judged as a pass or a fail two years later when someone is assessing the impact of an out-of-tolerance event.

The Field That Is Not Yours to Set

The next-due date on the register is a decision you own, and it is worth knowing that the published guidance says so directly. LAB 5 section 3.7:

"Any recommendation on the calibration interval must have been agreed with the customer, therefore it shall be clear from the certificate that if a calibration due date is reported it is at the customer's request, e.g. 'Customer requested calibration due date'."

The laboratory measures and reports. The interval — and therefore the due date on your register — is yours. This matters in practice because it is common for a due date printed on a returned certificate to be treated as authoritative, and it is not; it is either what you asked the laboratory to print, or a recommendation you have not yet agreed to. A register populated from certificate due dates is a register whose intervals nobody has actually set.

What Assessors Ask First

Three questions, in this order, and the register either answers them or it does not.

"Is this list complete?" Usually tested from the other direction — walking the floor and picking instruments, then asking to find them on the register. Anything in use for a conformity decision and absent from the register is a finding. This is why deliberate exclusions belong on the register with a status rather than off it: an indicating-only pressure gauge marked "indication only, not calibrated" is a documented decision, and the same gauge missing entirely is a gap.

"How do I get from this instrument to its evidence?" Instrument to identifier to certificate, in a reasonable time. This is where registers with a certificate-reference column and a folder of PDFs named scan_0047.pdf fall apart.

"What is overdue?" The one most often answered badly, because it requires the register to be current rather than accurate-as-of-last-quarter. A register showing four overdue instruments is a normal operational picture. A register showing none, on a fleet of two hundred, usually means it has not been updated. The wider ground an assessor covers is in the calibration audit checklist, and the clause-level requirements are in the ISO 9001 Clause 7.1.5 guide.

Why the Register Decays

Registers do not fail by being designed wrong. They fail by going stale, and the mechanism is always the same: the register is maintained by hand, and the events that should update it — an instrument going out for calibration, coming back, being quarantined, moving bench — happen away from the spreadsheet. Someone updates it before the audit.

The tell is a register where every row's status reads "in service" and the due dates cluster suspiciously. That is not a register being maintained; it is one being reconstructed. In our experience this tends to show up once a fleet grows past what one person can hold in their head, which is also where teams start comparing spreadsheets against dedicated tools — not because the spreadsheet cannot hold the fields, but because nothing makes the fields change when reality does.

How CalProof Fits

The equipment register is the centre of CalProof rather than a report it produces. Each instrument carries its identifier, location, the person or department it is assigned to, its interval, due date and status, with its certificates attached to the record — so the instrument-to-evidence path an assessor asks for is one click rather than a search through a shared drive. When an instrument is handed out, its issue and return history records who or which department has it, the dates it went out and came back, and the condition it came back in, so the register also shows who is holding each instrument today. Status changes when the instrument moves through calibration, so the register reflects the fleet rather than the last time someone tidied it.

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If you are starting from a spreadsheet, the calibration schedule template has the register and due-date structure already laid out, and migrating a calibration spreadsheet covers moving the data across. To see the shape of the output, the sample audit pack is ungated and downloads as a single PDF.

Sources

• UKAS LAB 5, Edition 5 (January 2025) — Reporting calibration results (sections 3.7, 4.3, 4.4)
• UKAS TPS 41, Edition 6 (December 2022) — UKAS policy on metrological traceability (section 3.1)

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; the fields your own register needs depend on your scope, your equipment and your assessor's interpretation. Verify against your certification body or UKAS assessor. This is not legal or compliance advice.