To date there have been two recognised standard methods commonly in use for the calibration of POVAs – the gravimetric method (ISO 8655-6:2002) and the Australian Standard AS 2162.2:1998, which itself was based on an earlier draft of ISO 8655-1/2/3/4.
Noting a number of technical differences between the two standards and with ISO 8655-6 being the acknowledged source of many of the requirements and methods, a set of additional criteria to the Australian Standard was published in the 2019 NATA Specific Accreditation Criteria: ISO/IEC 17025 Application Document, Calibration – Annex, Mass and related quantities.
This set of additional criteria is to be considered when reporting compliance to the Maximum Permissible Errors (MPEs) published in Tables 1 and 2 of ISO 8655-2. This is often the case as most POVA manufacturers design the pipettes to these specifications. The additional criteria ensure the higher criteria in ISO 8655 Parts 2 and 6 are applied.
For example:
- for volumes at 10μL, a balance with minimum resolution of 0.001 mg (ISO 8655-6) is required instead of minimum resolution 0.01 mg (AS 2162-2);
- the MPEs stated in ISO 8655-2 are based on 10 aliquot repeats and not the option of fewer repeats contained in AS 2162.2;
- for variable volume apparatus, clause 7.1.1 of ISO 8655-6 requires at least three volumes to be tested, unless a reduced user range has been agreed to with the end user and subsequently reported;
- for multichannel apparatus, clause 7.5 of ISO 8655-2 requires each channel to be considered independently against the specifications.
When stating conformance to the systematic error and random error MPEs contained in Part 2 of ISO 8655, the procedure as stated in clause 8.4.2 and 8.5.2 of ISO 8655-6 shall be applied, that being the measurement uncertainty is not added or subtracted from the result. This follows the Accuracy Method of ISO GUM 98-4, clause 8.2.4, where the well characterised test method minimises sources of uncertainty by use of instruments with maximum permissible errors within specified limits (clause 4 of ISO 8655-6).
NATA has been advised that AS 2162.2 was withdrawn in October 2019. In view of the number of recognised technical differences with the more up-to-date ISO 8655 publication, NATA assessment staff have been advised to take this into consideration and remove reference to AS 2162.2 in scopes of accreditation at a facility’s next NATA surveillance visit. All facilities will be expected to comply with the ISO standard or equivalent method.
Should any facilities have any enquiries, they should contact their Client Coordinator in the first instance.
