Calibration of Measuring Equipment: How Often, by Whom, with What Proof?

A quality assurance audit. The auditor takes a torque wrench out of the cabinet and asks: “When was this last calibrated?” The label has worn off, and the calibration certificate can’t be found. A simple question turns into a nonconformity – and suddenly the question arises whether all the bolted joints of recent months are actually OK.

Calibration sounds like a formality. In reality, it determines whether you can trust your measurements. A pressure gauge that reads incorrectly, a thermometer that drifts or a vernier calliper with a deviation leads to wrong decisions in production and maintenance.

In this guide you will learn how calibration, legal verification and adjustment differ, how often you should calibrate measuring equipment, who is allowed to do it and what evidence the common standards expect from you as a user.

Calibration, legal verification, adjustment: three different things

In everyday operations, the three terms are often mixed up. Technically, they mean quite different things:

Term	What happens?	Intervention on the device?	Who?
Calibration	comparison with a more accurate standard; the deviation is determined and documented	no	in-house laboratory or external calibration service
Adjustment	setting or trimming the device so that the deviation is as small as possible	yes	qualified staff, manufacturer or calibration service
Legal verification	legally required test of whether a measuring instrument meets the requirements	no, testing and marking	verification authorities or state-approved testing bodies

Calibration

When calibrating, you compare the reading of your measuring instrument with a reference standard whose accuracy is known and traceable. The result is a documented deviation, ideally with the measurement uncertainty stated. Nothing is changed on the instrument itself. Whether the deviation is acceptable is something you decide based on your tolerance specifications.

Adjustment

If the deviation is too large, the instrument can be adjusted – i.e. set so that it reads correctly again. The order matters: first calibrate (“as found”), then adjust, then calibrate again (“as left”). Only then do you know how large the deviation was before the intervention and whether earlier measurements could be affected.

Legal verification

Legal verification is an official, sovereign act. In Germany, it is governed by the Measurement and Verification Act (MessEG) and the Measurement and Verification Ordinance, and it applies to measuring instruments used, for example, in trade – such as a scale whose reading is used for billing. For most measuring equipment in production and maintenance, legal verification is not required; calibration is.

What the standards require of you as a user

ISO 9001, clause 7.1.5

If your company operates a quality management system to ISO 9001, clause 7.1.5 “Monitoring and measuring resources” governs how measuring equipment is handled. Where measurement traceability is required, measuring equipment must, among other things:

• be calibrated and/or verified at specified intervals or prior to use – against measurement standards traceable to international or national measurement standards,
• be identified so that its status can be determined,
• be safeguarded from adjustments, damage or deterioration that would invalidate the result.

If a measuring instrument turns out to have been unfit for purpose, you must determine whether earlier measurement results have been adversely affected and take action where necessary. That was the real problem in the opening example.

EN ISO 10012

EN ISO 10012 sets out requirements for measurement management systems, i.e. for measurement processes and measuring equipment. It goes further systematically than ISO 9001 and is a good guide if you want to set up your calibration management in a structured way.

ISO/IEC 17025 and DAkkS

ISO/IEC 17025 (EN ISO/IEC 17025) specifies requirements for the competence of testing and calibration laboratories. In Germany, the German Accreditation Body (DAkkS) accredits laboratories to this standard. A DAkkS calibration certificate proves that the laboratory has demonstrated its competence for the measuring range concerned and that traceability to national standards is ensured.

Which calibration certificate is the right one?

• Accredited calibration certificate (DAkkS): highest evidential value, with measurement uncertainty and documented traceability. Useful for reference standards, critical measuring equipment and where customers or regulations demand it.
• Factory calibration certificate: issued by the manufacturer or a non-accredited laboratory. Sufficient for a lot of operational measuring equipment, provided traceability is documented in a verifiable way.
• In-house calibration: using your own references, which are calibrated externally on a regular basis. Cost-effective with many similar instruments, but requires trained staff and a documented procedure.

Which option you choose depends on how important the measurement is. A vernier calliper in final inspection needs different evidence from a pressure gauge that only serves as a rough guide.

What a calibration certificate should contain

A calibration certificate is only usable evidence if it clearly identifies the instrument and makes the results traceable. On receipt, check at least these details:

• unique identification of the instrument, such as serial number or inventory number
• date of calibration and the issuing laboratory
• standards used and a reference to their traceability
• environmental conditions, for example temperature
• measuring points with nominal and actual values and the deviation
• measurement uncertainty of the calibration
• in the case of adjustment: values before and after the intervention

Consider measurement uncertainty and tolerance together

A common misconception: the deviation is just within tolerance, so everything is fine. But if the measurement uncertainty of the calibration is large relative to the tolerance, you cannot say with certainty whether the instrument actually meets the requirement. So decide how you will handle results close to the tolerance limit – for example with a shorter interval or an adjustment.

How often does measuring equipment need to be calibrated?

There is no generally prescribed interval. The standards require “specified intervals” – and you have to specify them yourself. One year is a common starting value, but not a rule. What matters is:

• the manufacturer’s recommendation for the instrument in question
• frequency of use and stress, such as impacts, vibration and temperature fluctuations
• the importance of the measurement for product quality and safety
• calibration history: if the instrument has stayed stably within tolerance over several calibrations, the interval can be extended. If there were outliers, it should be shortened.
• requirements from customers, industry regulations or authorities

In addition to calibration, short intermediate checks make sense, such as checking a vernier calliper with a gauge block before important measurements. After a drop or visible damage, an instrument must be taken out of service immediately and checked.

Example: adjusting the interval based on history

A company initially calibrates its pressure transmitters once a year. Over several calibrations, almost all devices are stable and well within tolerance – except for two, which sit on a pump with severe pressure surges and end up close to the limit every time. The consequence: for the stable devices, the interval is extended with a documented justification; for the two heavily stressed measuring points, it is shortened. The justification is recorded in the measuring equipment register.

Calibration management in 6 steps

1. Record: enter all measuring equipment with a unique number in a measuring equipment register or software – with type, measuring range, location and the person responsible.
2. Classify: decide which instruments are subject to monitoring and which serve only as a guide. Label the latter clearly.
3. Set tolerances: define the permissible deviation for each instrument.
4. Plan intervals: set calibration dates and have reminders generated automatically.
5. Label: calibration label with the next due date on the instrument, calibration certificate stored in the system.
6. Handle deviations: if the limit is exceeded, quarantine the instrument, assess the impact on earlier measurements, then adjust, repair or replace.

Typical mistakes in practice

• Calibrating without a tolerance: a calibration certificate shows a deviation. Without your own tolerance specification, nobody knows whether it is acceptable.
• Adjusting without documenting: without the “as found” value, you can’t assess whether earlier measurements are affected.
• Personal measuring tools: employees’ own tools are often not registered but are still used for inspections.
• Wrong measuring range chosen: a pressure gauge that is constantly read at the bottom end of its scale measures inaccurately. This also applies when choosing new instruments.
• Overdue instruments in circulation: clear rules on who collects quarantined instruments prevent accidental use.

When a measuring instrument has to be replaced

Sometimes adjustment is no longer possible or not economical. Then you need a new instrument – ideally an identical model, so that inspection instructions, connections and measuring ranges stay the same. With older or foreign makes, that isn’t always easy.

KTB Europe is a family business from Hamburg and has been sourcing spare parts and MRO materials of all brands worldwide since 1976, including measuring devices such as pressure gauges, thermometers and sensors. Send us the part number, a nameplate photo or a description. If the original device is no longer available, we look for suitable alternatives. Find out more on our page about MRO procurement. What to look for when buying a pressure gauge is covered in the article Buying a pressure gauge, and how to integrate inspection dates into maintenance is shown in the article How to create a maintenance plan.

Looking for a specific measuring device? Send us your enquiry via the contact page.

Conclusion

Calibration determines how accurately an instrument measures. Adjustment corrects it. Legal verification is a statutory test for certain applications. If you keep these terms clearly apart, set intervals on a justified basis and document every deviation, you will not only pass audits but can also rely on your measurements in everyday work. Start with a complete list of your measuring equipment – often that is already the biggest step.