Calibration Certificates

Incorrect Assumptions

For many organisations, calibration is treated as a simple tick-the-box quality requirement:

• Check the due date
• Send the instrument away for testing
• File the certificate, and move on.

Once a calibration certificate is received — especially one carrying a NATA logo — it is often assumed that the instrument fully complies with all relevant standards.

Unfortunately, this assumption is often incorrect.

A calibration certificate does not automatically mean an instrument complies with the applicable international standard. It is the responsibility of both the calibration laboratory and the customer to clearly understand what has actually been tested, what has not, and what the results mean.

Incomplete Disclosure

One common issue is incomplete disclosure. The front page of a calibration report — particularly when it is labelled “Page 1 of X” — rarely tells the full story. Customers should always check how many pages the report contains and ensure they have received all of them. If pages are missing, it is reasonable to ask what information may not have been provided.

For this reason, our calibration reports clearly state that they must not be reproduced unless copied in full, to avoid any misunderstandings. In practice, however, we often see only the first page of a much longer report being shared — for example, “Page 1 of 21”. This can easily hide important limitations, deviations, or failures identified later in the report.

To improve transparency, we provide a one-page calibration certificate that summarises the standard clauses tested and the results. If there is a failure, it will be clearly identified on that certificate. Despite this, we frequently encounter instruments that have previously been tested elsewhere and now fail certain requirements. In many cases, historical data shows these issues existed before, but they were not clearly highlighted or understood by the customer.

Incomplete Testing

Another area of confusion arises with certificates that refer only to an “acoustic test” while mentioning compliance with a standard. This situation often occurs with MEMS-based instruments, where it is not possible to apply electrical signals for testing.

All accredited calibration laboratories must operate to ISO/IEC 17025, the international standard that defines technical competence for testing and calibration laboratories. This is the standard under which laboratories are audited by NATA to maintain accreditation.

ISO/IEC 17025 states:

The laboratory shall ensure that it uses the latest valid version of a method unless it is not appropriate or possible to do so. When necessary, the application of the method shall be supplemented with additional details to ensure consistent application.

In practical terms, this may allow a laboratory to document limitations — such as the inability to perform certain electrical tests — and still issue a test report for older standards like IEC 60651, based only on acoustic frequency testing. However, this approach is unlikely to meet the requirements of newer standards such as IEC 61672-3, which replaced IEC 60651 and IEC 60804. These newer standards require specific checks of instrument markings and documentation before testing can even begin.

ISO/IEC 17025 further states:

Deviations from methods for all laboratory activities shall occur only if the deviation has been documented, technically justified, authorised, and accepted by the customer.

From this, it can reasonably be assumed that any omitted pages of a report should clearly explain that only acoustic testing has been performed, and that other required tests were not carried out.

Key Take Away

If a report claims compliance with a standard but only demonstrates compliance with acoustic testing, it does not prove full compliance with that standard. Electrical testing is far more comprehensive and rigorous, and it assesses critical aspects of instrument performance such as overload behaviour, low-level performance, linearity, and operational limits. Without these tests, a significant portion of the instrument’s functionality remains unverified.

In short, a calibration certificate should be read carefully and critically. Understanding what was tested—and what was not—is essential to knowing whether an instrument truly meets the standard it claims to comply with.