Managing calibration of measuring equipment: register, interval and certificates

What is calibration, and how does it differ from legal verification and verification?

Calibration is comparing a measuring instrument with a reference of known accuracy, so you know how much it deviates. You record the measured deviation, usually in a calibration certificate. Calibration itself does not adjust the instrument. That is called adjustment, and it can follow directly after calibration if the deviation is too large.

Legal verification, known in Dutch as ijken, is something else: a statutory check of measuring instruments used for trade or other legally regulated purposes, such as shop scales and fuel pumps. It is governed by the Dutch Metrology Act, with supervision by the Dutch Authority for Digital Infrastructure (RDI). If you have such instruments, ask your supplier or the supervisor which rules apply.

Verification is checking whether an instrument still meets the set requirements, for example with a reference weight or with ice water, without a full calibration. Where measurement traceability applies, ISO 9001 leaves the choice between calibration, verification or both. Keep them apart in your register. Calibration is about the correctness of a measurement, inspection about the safe use of work equipment.

Which measuring equipment goes into the calibration register?

The criterion is simple: does the result of the measurement drive a decision about the quality of your product, the safety of people or compliance with a requirement? Then the instrument belongs in the register. If you use measurements as proof that products and services meet requirements, the outcomes have to be trustworthy, which is also the thinking behind the ISO 9001 requirement on measuring equipment.

Typical examples are listed below. Instruments that only give an indication, such as a room thermometer in an office, you do not include. Mark them visibly as not subject to calibration and record the basis on which you include or exclude an instrument, so you can explain that choice to an auditor or client.

• Core temperature thermometers and temperature sensors of cold rooms, freezers, ovens and pasteurisation, with which you monitor critical limits from your HACCP plan
• Scales and dosing equipment
• Calipers, micrometers, dial gauges and other dimensional measuring tools
• Torque wrenches and screwdriving machines
• Pressure gauges, flow meters and pH meters
• Gas detectors, sound level meters and lux meters for safety and environmental measurements

What does a calibration register contain?

A calibration register answers three questions per instrument at a glance: what is it, when was it last checked and may it be used now? Record fixed fields. The measured deviation matters more than just the word approved, because it is what you later use to justify your interval.

• Identification: unique number, description, make, type and serial number
• Location and user, for example the department or the employee who has the instrument
• Measuring range, accuracy and the permitted tolerance for your application
• Interval and its basis: manufacturer, use, earlier deviations or client requirement
• Last and next calibration date
• Internal or external, with the name of the performer or the laboratory
• Certificate number, with the certificate itself attached
• Outcome: within or outside tolerance, with the measured deviation
• Status: in use, out of use or rejected, plus the label on the instrument

How do you determine the calibration interval?

There is no fixed interval that applies to all instruments. You start with the manufacturer's recommendation, often one year, and weigh in the use: how intensive, in what environment and with how much risk of shocks, moisture or dirt. The importance of the measurement counts too, and your client, standard or certification scheme may set a maximum.

After that you adjust based on your own data. If an instrument stays well within tolerance at every calibration, you can extend the interval with justification. If the deviation is close to the limit or the instrument is rejected, you shorten it. The guideline ILAC-G24 / OIML D 10 describes methods for determining recalibration intervals this way. Also calibrate after a drop, an overload or a repair, regardless of the date.

Traceability: why the calibration certificate matters

Traceability means that a calibration can be traced, through an unbroken chain of comparisons each with a known uncertainty, back to national or international measurement standards. In the Netherlands, VSL is responsible for the national measurement standards. Accredited calibration laboratories, in the Netherlands accredited by the Dutch Accreditation Council (RvA) to ISO/IEC 17025, can demonstrate that traceability.

Check a certificate for the following: does the serial number match, are date, measuring points and results stated, is the measurement uncertainty given, are the reference standards and conditions named, and is it signed? For an accredited calibration the accreditation mark belongs on it, and the calibrated quantity must fall within the scope of the accreditation. Whether the deviation is within your tolerance is something you assess yourself.

Also look at the relationship between the uncertainty of the calibration and your tolerance. As a rule of thumb it is often said that the reference should be four times more accurate than the instrument you are checking. That is a guideline, not a fixed requirement, and a standard or a client may set other requirements.

Calibrate internally or externally?

External calibration, preferably at an accredited laboratory, suits high accuracy requirements, complex instruments such as pressure and temperature equipment, a requirement from the client or scheme, and cases where you have no traceable reference yourself. You receive a certificate with traceability and measurement uncertainty.

Internal calibration or verification suits simple checks, such as a thermometer in ice water or a scale with reference weights. That reference must itself be externally calibrated. Record a work instruction, appoint authorised employees and note every outcome. A common combination is, for example, an internal check every week or month and an external calibration once a year.

What do you do when an instrument is out of tolerance?

Then it is not just the instrument that is a problem, but everything that was measured with it. ISO 9001 wants you to investigate whether the measurements made before the rejection can still be trusted, and to step in where that is needed.

• Take the instrument out of use immediately, mark it as rejected and temporarily replace it with a calibrated one
• Determine the period: from the last calibration or verification that was good up to the moment of rejection
• Assess the measurements from that period: how large was the deviation relative to the tolerance, in which direction, and for which products or decisions was the instrument used? A deviation does not automatically mean that products must be rejected
• Investigate whether products may be affected: hold stock, re-inspect and, for delivered products, decide whether to inform the customer or recall. For food safety, involve the HACCP lead
• Repair or replace the instrument, recalibrate and only then put it back into use
• Find the cause and open a CAPA: wrong use, a drop, an interval that was too long or poor storage? Adjust the interval or the instruction and check later that it works

Example of a calibration register (illustrative)

Below is a small example register in list form. The instruments, tolerances, dates and intervals are made up and only show the structure. Your intervals and tolerances follow from your own application, the manufacturer and the requirements of your client or certification scheme.

• KM-001, core temperature thermometer, production. Tolerance ±1.0 °C. Weekly internal check in ice water, external calibration annually. Last calibration 3 February 2026, next 3 February 2027. Outcome: within tolerance, deviation 0.3 °C. Status: in use.
• KM-004, scale 30 kg, dispatch. Tolerance ±5 g. Monthly check with reference weights, external calibration annually. Last calibration 17 June 2026, next 17 June 2027. Outcome: within tolerance. Status: in use.
• KM-009, torque wrench 20 to 100 Nm, assembly. Tolerance ±4 %. External calibration annually and after a drop. Last calibration 9 January 2026. Outcome: outside tolerance, deviation 7 %. Status: rejected, CAPA opened, measurements since the previous calibration in January 2025 under review. Next calibration: after repair.
• KM-012, pressure gauge compressed air line, utilities. Tolerance ±1 %. External calibration annually. Last calibration 22 October 2025, next 22 October 2026. Outcome: within tolerance. Status: in use, calibration due soon.
• KM-015, caliper 150 mm, quality control. Tolerance ±0.03 mm. Six-monthly internal check with externally calibrated gauge blocks. Last check 4 September 2026, next 4 March 2027. Outcome: within tolerance. Status: in use.

Common mistakes when managing measuring equipment

Most mistakes are not in the calibration itself, but in what happens around it. The mistakes below are common in practice and can usually be fixed with fixed fields, an owner per instrument and a periodic check of the register.

• The register is incomplete: instruments in toolboxes, with field service or at other sites are missing, or it lists instruments nobody uses any more
• Only approved or rejected is recorded, not the measured deviation, so you have no basis to justify the interval
• The certificate is not checked: the serial number does not match, the measuring range falls outside the calibration or the deviation was not compared with your tolerance. The laboratory reports the deviation, you assess whether it is acceptable
• An expired instrument stays in use because the label is missing or nobody sees the date
• After a rejection, earlier measurements are not assessed, although ISO 9001 expects that
• Users can adjust the instrument themselves, for example via a screw or setting ring without locking or a seal
• The interval is never reviewed, not even after rejections or a change in use

How BrydgeOne supports this

In Calibration & measuring equipment you track all your measuring and monitoring equipment, such as scales, thermometers and dial gauges, with the full calibration history per instrument. You set the interval once, after which the next date moves on automatically with every recorded calibration, and colour badges show whether an instrument is in order, nearly due or overdue. If you attach the certificate as a PDF and AI is switched on, AI fills in date, result, certificate number, calibrating party and measuring points, which you check before saving.

If you record a calibration with the result Rejected, the instrument is set to Rejected and a corrective action is created automatically in Actions. If you enter the deviation, you see a drift chart with the tolerance band. Machines and tools subject to inspection are managed in Objects, and the monitoring of control points is recorded in HACCP & hazard analysis. BrydgeOne helps you collect the evidence. The assessment of the outcome stays with you.