In-House vs Outsourced Calibration: UK Guide

"Can we just calibrate these ourselves?" is a reasonable question, and the answer is yes — with conditions that are more specific than most teams expect. UKAS policy explicitly recognises in-house calibration, and it also lists seven things that have to be in place before those calibrations count. The decision is less about cost per certificate than about whether you can meet those seven conditions repeatably.

This guide covers what UKAS policy actually says about in-house calibration, what makes a source of traceability acceptable, and how to work the decision for your own equipment. The relevant document is UKAS TPS 41, UKAS policy on metrological traceability, Edition 6, December 2022.

The Question Is Traceability, Not Permission

Nobody forbids you from calibrating your own instruments. TPS 41 section 4.1 recognises the practice directly:

"It is recognised that organisations accredited for calibration, testing, inspection or proficiency testing activities may choose to carry out some calibration activities in-house to support their measurement activities rather than seek the services of an external accredited laboratory."

Section 1.3 puts these calibrations in their own category: "For calibrations performed by an organisation in order to establish metrological traceability for its own activities, and which are not a part of its scope of accreditation, the policy in Section 4 is applicable. Such internal calibrations are sometimes known as 'in-house' calibrations."

So the real test is whether the results are traceable. Metrological traceability is the unbroken documented chain back to a national or international reference — and a calibration you perform yourself either extends that chain properly or breaks it.

What Counts as an Acceptable Source

Before deciding whether to do it yourself, it helps to know what "properly calibrated" means for the reference standard you would be measuring against. TPS 41 section 2.1 gives two primary acceptable sources of traceability:

1. A national measurement institute (NMI) or designated institute "whose service is suitable for the intended need and is covered by the CIPM MRA for those services." In the UK that is the National Physical Laboratory; the full signatory list is on the BIPM CIPM MRA site.
2. An accredited calibration laboratory "whose service is suitable for the intended need and which is accredited for those services by an Accreditation Body that is included in the ILAC Arrangement or by Regional Arrangements recognised by ILAC."

Two practical readings follow. First, it does not have to be UKAS — UKAS is one accreditation body in the ILAC Arrangement, and an equivalently accredited laboratory in another signatory country is an acceptable source. Second, and more often missed, accreditation alone is not enough: the service has to be "suitable for the intended need". Section 2.1.3 is explicit that "the extent of the calibration obtained should be sufficient in terms of quantities and coverage to demonstrate properly the performance of the equipment, otherwise traceability of results may not be assured." Checking that in practice means reading the laboratory's schedule of accreditation against your own measurement — quantity, range and capability — which the UKAS scope statement checklist sets out as a printable four-stage check.

That is the trap in supplier selection. A certificate from an accredited laboratory that only covers part of your instrument's working range does not give you traceability across the range you actually use. Checking coverage is part of reading the calibration certificate properly, and it applies whether you outsource or calibrate in-house against your own reference.

The Seven Conditions for In-House Calibration

This is the part worth printing out. TPS 41 section 4.5 sets the expectation, then lists what must be in place:

"It is reasonable to expect that in-house calibrations are subject to the same level of technical rigour that would be obtained if an external accredited laboratory or recognised NMI were used."

The seven requirements that follow are:

Condition (g) is where in-house programmes most often fall short. Producing a defensible uncertainty figure for each calibration type is real work — the method is set out in UKAS M3003, and the measurement uncertainty guide walks the calculation. If you cannot produce a budget, you are performing a comparison, not a calibration, and it should be recorded as such.

Note also section 4.4, which covers the "calibrate before use" pattern — where calibration of supporting equipment is built into a test procedure. TPS 41 requires that "the procedure shall contain sufficient information for proper conduct of the calibration" and that the equipment "should be labelled 'calibrate before use', or similar". It is a legitimate approach, not a shortcut around the criteria.

If You Hold Accreditation, You Have to Declare It

For accredited organisations there is an administrative obligation people miss. Section 4.6 requires that organisations doing in-house calibration in support of accredited activities "provide details of these calibrations to UKAS", and names the mechanism: "UKAS form F166 'In-house Calibration Declaration' can be used for this purpose." The details "will normally include information regarding the methodology involved, the traceability arrangements and the uncertainty budgets", and UKAS will normally request them at initial assessments, reassessments and extensions to scope. Section 4.7 adds that UKAS must be notified of changes to those details as soon as they occur.

There is a cost implication too. Section 4.8 states that assessment of in-house calibrations will be covered within normal assessment activities "wherever practical", but that "where significant additional assessment time or additional assessors are required, there will be an additional cost associated with this activity."

That belongs in the business case. A cost comparison that weighs outsourced certificate fees against staff time, but leaves out reference standards, their recalibration, procedure writing, uncertainty budgets and possible additional assessment cost, will make in-house look cheaper than it is.

"The Equipment Is Traceable" Is the Wrong Phrase

One genuinely useful correction from TPS 41, because it reframes the whole decision. Section 1.4.3:

"It is further noted that metrological traceability is an attribute of the result of a measurement. Consequently, commonly used expressions such as 'the equipment is traceable' are incorrect."

The section goes on to list what has to be true before results can be demonstrated as metrologically traceable: the equipment must be "used correctly by trained staff in a suitable environment to defined procedures", properly maintained, itself subject to "a calibration of suitable coverage", and subject to "an appropriate level of intermediate checks and quality control."

This is why buying a calibrated reference standard does not, by itself, give you an in-house calibration capability. The certificate makes the reference traceable at the moment it was calibrated. Traceability of your results depends on everything in that list — which is the same list as the seven conditions, stated from the other direction.

Section 1.4.2 adds a detail worth checking on every certificate you receive: each certificate in the chain "must contain a statement of uncertainty (or, exceptionally, a statement of compliance with an identified specification) otherwise the results presented therein do not exhibit metrological traceability." A certificate with no uncertainty statement is not doing the job.

Working the Decision

Nothing above says outsourcing is better. It says the two options carry different obligations. A reasonable way to split them:

In-house tends to make sense when:

• The calibration is simple and high-volume — routine dimensional or weight checks where sending items out means losing them for a week each time
• You already own a suitable reference with a valid accredited certificate and adequate uncertainty
• The environment is genuinely controlled
• One or two named people can own the procedure and stay trained on it
• The measurement is not the one your product's conformity hinges on

Outsourcing tends to make sense when:

• The quantity is specialised — pressure, temperature, electrical — where a suitable reference plus the expertise costs far more than the certificates
• Volume is low, so per-certificate cost never catches the fixed cost of capability
• The instrument is critical enough that you want an independent accredited result behind it
• You do not have anyone who can own an uncertainty budget
• You hold accreditation and would rather not add declarations, assessment scope and cost

Most UK SMEs land on a mix: outsource the reference standards and the critical or specialised instruments, verify the high-volume simple items in-house against those references. That hybrid is normal and defensible — as long as the in-house half is documented to the section 4.5 conditions and the records show which is which.

Whichever way it splits, the calibration interval question still applies to both halves, and in-house references need their own intervals reviewed just as bought-in calibrations do.

If You Work to ISO 9001 Only

Scope matters here, and it is easy to over-read the above. TPS 41 is UKAS policy addressed to accredited organisations and to in-house calibration supporting accredited activities. If you hold no accreditation and work to ISO 9001 alone, section 4.6's declaration requirement does not apply to you, and there is no UKAS form to file.

What does still apply is the substance. ISO 9001 requires measuring equipment to be calibrated or verified against traceable references at defined intervals, and an auditor will ask how your in-house calibrations achieve that. TPS 41's seven conditions are the clearest published statement of what competent in-house calibration looks like, which makes them a useful benchmark to design against even where they are not binding. Meeting most of them and documenting the gaps is a stronger position at audit than not having considered them.

The Short Version

• In-house calibration is explicitly permitted — TPS 41 §4.1 recognises it. Competence, not permission, is the test.
• Seven conditions apply (§4.5): environment, competent authorised staff, traceable references with suitable uncertainties, documented procedure per type, data recording, quality control, and an uncertainty budget per calibration type.
• Condition (g), the uncertainty budget, is what most in-house programmes lack. Without it, record the activity as verification, not calibration.
• Acceptable external sources (§2.1): an NMI covered by the CIPM MRA, or a lab accredited by an ILAC-Arrangement body. Not UKAS-specific — but the service must suit the need and cover your working range.
• Accredited organisations must declare in-house calibrations to UKAS (§4.6, form F166) and may incur additional assessment cost (§4.8).
• "The equipment is traceable" is incorrect (§1.4.3) — traceability is an attribute of a measurement result.
• A hybrid — outsource references and critical items, verify simple high-volume items in-house — is the normal SME answer.

How CalProof Fits

A mixed in-house and outsourced programme is harder to keep straight than either one alone, because the records look different. Outsourced instruments have a supplier certificate; in-house ones have your own readings, your reference's certificate, and your uncertainty budget. Kept in a spreadsheet, the link between an in-house calibration and the reference standard it was performed against is usually the thing that goes missing — and it is the first thing an assessor asks for.

CalProof holds every instrument in one register regardless of who calibrated it, stores the certificate or the in-house record against each calibration event, and keeps each instrument's interval and history in the same place. Your reference standards are instruments in the register too, with their own due dates — so a reference going overdue is visible before it invalidates the in-house work done against it.

GBP pricing from £29 a month. No per-user fees on Pro and above. UK data hosting. No long-term contract.

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
• UKAS M3003 Edition 6, March 2024 — The expression of uncertainty and confidence in measurement
• BIPM — CIPM Mutual Recognition Arrangement (CIPM MRA)
• ILAC — signatory search (accreditation bodies in the ILAC Arrangement)
• NPL — National Physical Laboratory

This guide summarises UKAS TPS 41 Edition 6 (December 2022) as published. TPS 41 is UKAS policy addressed to accredited organisations and to in-house calibration supporting accredited activities; if you hold no accreditation, its declaration requirements do not apply to you, though its criteria remain a useful benchmark. Section references are to Edition 6. Verify against the current published edition and your UKAS assessor or certification body for requirements specific to your scope. This is not legal or compliance advice.