ISO 9001 : CALIBRATION OF MEASURING INSTRUMENTS
ISO 9001 certified organisations have to make decisions regarding where to send their measuring instruments for calibration. Nowadays, many calibration laboratories have got accredited to ISO 17025. Also, many accredited laboratories are not accredited for all the services they offer. The use of non-accredited calibration labs, or non accredited services of partially accredited labs, may reduce operating costs in the short term, but could turn out to be costly in the long term. Examination of ISO 9001 (2000) and ISO 17025 suggests that ISO 9001 certified organisations should select their calibration labs carefully and make sure that the labs they use are properly accredited for the services they provide.
Organisations certified to ISO 9001 are required to calibrate all their measuring equipment used to verify or control quality, and all such calibrations are required to be traceable to national or international standards (ISO 9001 1994 section 4.11, ISO 9001 2000 section 7.6). All the records of calibrations are required to be maintained properly and corrective action to be taken when measurement equipment is found to be out of specification. Some of the implications of calibration and traceability requirements for ISO 9001 certified organisations and for calibration and test laboratories begins with the investigation, regarding the meaning and components of the term `traceable’.
Many calibration laboratories claim accreditation to ISO 17025. Here we go for NABL for accreditation and in Australia NATA is the accrediting body. Accredited labs are entitled to use the NABL logo on their documents and web pages. ISO 17025 is an international standard that specifies quality and technical competence requirements for testing and calibration laboratories. ISO 17025 replaced ISO Guide 25 in 1999.
Hiring and keeping competent technical staffs, internal audits, maintenance of in-house quality checks and participation in proficiency testing programs will improves the likelihood of an error-free service but it alone can never guarantee complete absence of calibration or other errors. However, customers of reputable ISO 17025 accredited labs can expect to be informed promptly and fully of errors when they are discovered, and of the particular consequences related to the calibration of their equipment (as per sections 4.9, 4.10).
Calibration, uncertainty and traceability
The ISO 9001 requirement for traceable calibration of test and measurement equipment raises questions concerning the term `traceable’. When we examine definitions and components of traceability extracted from ISO 9001, ISO 17025 and other documents, we get a clear answer for it along with clear definitions.
Tractability is defined as, ‘the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties….’
The unbroken chain of comparisons is called a `traceability chain’. An unbroken chain of comparisons is a logical and easily understood component of traceability. The manager of a non-accredited lab might claim that his calibrations are traceable because he is able to trace only the calibration pedigree of the references and standards, which he uses.
This aspect can be analyzed by an example. Assume we keep a set of weights which we use to check balances in a chemical laboratory. The balance should have a resolution and repeatability necessary for the uncertainty required in the final result, when compared with set of calibrated weights. It must be properly serviced and maintained, mounted on an appropriately rigid and vibration-free bench in a temperature controlled environment. Air movement around the balance may need to be restricted. If the weights to be compared are of different density compensation for buoyancy might be necessary. Buoyancy compensation might require measurements of air temperature, humidity and barometric pressure. If the lab provides other calibration services then the presence of other equipment nearby may alter the environment in the vicinity of the balance, e.g. a temperature calibration oven might alter the mean radiant temperature in the vicinity of the balance.
If we appreciate the potential complexity of the calibration process then we should require that the lab calibrating our weights employ a technician with sufficient competence and training to appreciate all the potential sources of error in the calibration. He should be capable of setting up the equipment properly and deciding which errors are significant and which can be ignored for a particular calibration. Competence as a component of traceability is addressed in ISO 17025 section 5.6. The Section 188.8.131.52.1 states that traceability of measurement shall be assured by the use of calibration services from laboratories that can demonstrate competence, measurement capability and traceability. The use of the word ‘shall’ in a standard usually means that there is no other way to achieve compliance. ISO 17025 further states that, any calibration laboratories fulfilling the requirements of this International Standard are considered to be competent . A calibration certificate from a calibration laboratory accredited to this International Standard for the calibration concerned is sufficient evidence of traceability .
Uncertainty as an essential component of traceability
No measurement is ever true. There is always a difference between the true value of a measurand and the output of an instrument. Measurement uncertainty is a quantitative statistical estimate of the limits of that difference. The measurement uncertainty is ‘ a parameter associated with the results of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand’. Uncertainty estimates document the rationality and consistency of the comparisons. A traceability chain is a documented set of comparisons between consecutive pairs of instruments or measurement systems: A-B, B-C, C-D, etc. Usually instrument A is compared with instrument or standard B for the purposes of calibrating A, and the uncertainty estimated is that associated with that calibration process. The contribution of instrument or standard B to the overall calibration uncertainty is typically 4-10 times smaller than the contribution of A. Similar process to be handled between C & D also. Properly calculated and documented uncertainty estimates in a calibration chain indicate the `direction’ of traceability.
Everyone should view uncertainty estimates as confirmation that his instrument was calibrated against a reference of adequate performance and that all-potential source of error was under control during the calibration process. The essential component for traceable calibration is stated as below, Traceable calibration involves comparisons with traceable standards or reference materials. Only laboratories, which demonstrate their competence, can perform traceable calibrations, by accreditation to ISO 17025. A traceable calibration certificate must contain an estimate of the uncertainty associated with the calibration Organisations using non-accredited calibration labs do not conform to ISO 9001 and it should not claim conformance. At the same time, some calibration laboratories offer a wide range of calibration services but are accredited for only a subset of those services.
In some cases labs claim `ISO 17025 accreditation’ but are vague about exactly which services are accredited and which are not. ISO 9001 organisations should be careful to select calibration labs that are explicitly accredited for the services they are using. NABL keeps an up-to-date publicly, regarding the available list of accredited labs with details of the calibration services for which they are accredited and their least uncertainties of measurement. In a manufacturing environment it is often the case that more than one measurement system is used to monitor or control the quality of the product, and inevitably some measurements contribute more than others to uncertainty in product quality. ISO 9001 does not require all measurement systems to be calibrated – only those that contributes significantly to the control or verification of the quality of the product. One approach to this problem might be to perform uncertainty analyses on quality-related measurements using techniques similar to those outlined in the ISO to determine which measurement systems require calibration and the maximum associated uncertainties.
Thus, an ISO 9001 certified organisations should analyze the measurement systems they use to verify or control quality, make informed decisions on which instruments require calibration, and have these instruments calibrated by selected ISO 17025 accredited labs.