How to Read a Calibration Certificate

A calibration certificate is only useful if someone reads it. In many audits, certificates are filed without anyone checking whether the results are acceptable for how the instrument is actually used. This guide explains what an ISO/IEC 17025 certificate must contain, how to review one in a few minutes, and the warning signs that something is wrong.

What the certificate must contain

The content is set by clauses 7.8.2 and 7.8.4 of ISO/IEC 17025:2017. In the order you will usually find it:

# Element What to look for Clause

1 | Title and identification | A title (“Calibration Certificate”), a unique certificate | 7.8.2.1 a, d | number on every page, page numbering (“page 2 of 4”) and | a clear end-of-certificate mark

2 Laboratory Name and address of the laboratory 7.8.2.1 b

3 | Location | Where the calibration was performed: in the laboratory, or | 7.8.2.1 c | on site at your premises

4 Customer Your company name and contact details 7.8.2.1 e

5 | Item | Description, manufacturer, model, serial number or ID, and | 7.8.2.1 g | its condition when received

6 Dates Date of calibration and date of issue 7.8.2.1 i, j

7 | Method | The procedure used, often referencing a recognized | 7.8.2.1 f, n | guideline such as EURAMET or DKD, plus any deviations | from it

8 | Conditions | Ambient temperature, and humidity where relevant, during | 7.8.4.1 b | calibration

9 | Results | Reference values, instrument readings and the error or | 7.8.2.1 m | correction at each point, with units

10 Measurement uncertainty Uncertainty for each result, with the coverage factor 7.8.4.1 a

# Element What to look for Clause

(usually k = 2, about 95 % confidence)

11 Traceability statement How the results are metrologically traceable to the SI 7.8.4.1 c

12 | Validity statement | A statement that the results relate only to the item | 7.8.2.1 l | calibrated

13 | Authorization | Name and function of the person who approved the | 7.8.2.1 o | certificate

As found and as left. If the instrument was adjusted or repaired, the certificate should show results before and after, where available (clause 7.8.4.2). The as-found data is the evidence of whether your earlier measurements were affected.

The accreditation symbol. This is not an ISO/IEC 17025 requirement. Its use is governed by the accreditation body’s rules: EGAC, in line with ILAC P8. An accredited laboratory may show the symbol only for calibrations within its accredited scope. Any results outside that scope must be clearly identified.

What the certificate does not contain (unless customer requirement)

• A pass/fail statement. This appears only if you requested a statement of conformity against a specification and agreed the decision rule. See Decision Rules.
• A recommended calibration interval. Under clause 7.8.4.3, this appears only if agreed with you. Setting the interval is the instrument owner’s responsibility. See Calibration Intervals.

What to do with it: a five-minute review

• Identity. Check that the ID and serial number match the instrument.
• Range. Check that the calibrated points cover the range you actually use.
• Units and corrections. Check whether results are reported as errors or as corrections, and in which units.
• Fitness for use. For each point, add the error and the uncertainty, then compare the total with your tolerance: error ± U against tolerance.
• Impact. If the results were out of tolerance, start an impact assessment of all results since the last calibration.
• Record. Record the review, update the equipment record, and update the status label under your own calibration programme.

Red flags

• No measurement uncertainty stated
• An uncertainty smaller than the laboratory’s published CMC for that range. Under ILAC P14, this should not happen.
• An accreditation logo, but the instrument type or range is not on the laboratory’s accredited scope
• Only one or two points checked across a wide range
• A “traceable to” claim, with no reference standards or traceability route identified
• No calibration location stated for work done on site
• A serial number or ID that does not match the instrument