The difference between traceable and accredited calibrations is a common question customers have when trying to determine what their products require. Knowing the significance of each can help ensure all calibration needs are met.
A NIST traceable calibration is a type of calibration in which a manufacturer or calibration laboratory can certify that the standards used to calibrate a product or device are traceable to the International System of Units (SI) through an unbroken chain of comparable measurements to the National Institute of Standards and Technology (NIST), an agency of the U.S. Department of Commerce. A more general statement of traceability, that is not limited to NIST, recognizes that metrological traceability can be to SI through any recognized national metrology institute. This type of calibration does not indicate or determine the level of competence of the staff and laboratory that performs the calibrations. It mainly identifies that the standard used in the calibration is traceable to NIST or another recognized metrology institute (RMI).An ISO/IEC 17025 accredited calibration is recognized internationally. ISO 17025 accreditation is a statement to the competence of the calibration laboratory. According to the ISO 17025, any number of national metrology institutes, like the NIST, shall be recognized for acceptable measurement traceability through international approval. One such method to demonstrate metrological traceability is through a third party organization such as A2LA, NAVLAP, or any number of other ILAC-MRA signatory organizations. These organizations audit the laboratory and its processes to determine the laboratory competent to perform calibrations and issue the calibration results as accredited. The certificate is combined with the Scope of what the laboratory has been accredited to calibrate. The laboratory is audited periodically, to ensure continued compliance to the ISO/IEC 17025 standard.
The ISO/IEC 17025 accredited calibration could be considered a step above a NIST calibration because the discipline of calibration is reviewed in addition to the traceability of the standards. Additionally, the ISO/IEC 17025 calibration not only includes the measurement traceability, but it also includes the measurement uncertainties of the calibration results.
How do I decide between a NIST traceable (or equivalent) calibration or an ISO/IEC 17025 calibration?
The choice is often decided by a company’s quality manual or the quality department. The decision can also be made depending on how the equipment that requires periodic calibration will be used. For example, if the equipment is being used in a critical application and there could be some risk of liability to the company if something were to go wrong, a ISO/IEC 17025 calibration might be worth considering. If the device is used more for reference purposes in a low risk setting in which the measurement just needs to be “close enough,” a NIST traceable calibration is most likely all that is required.
Some examples of critical measurements would be altimeters in planes, temperature in nuclear plants, flow measurements in which flow or volume is associated with some monetary value, or medical equipment that measures air pressure.
Examples of non-critical, low risk measurement devices that might be suitable for NIST calibration could be devices like tire gauges, process gauges that indicate good or bad (usually these might have green and red areas on the dial face), pressure switches and pressure dial gauges used for air compressors, or torque wrenches used in auto repair shops.
It is ultimately the responsibility of the owner of the device needing calibration to determine the appropriate type of calibration required - NIST or ISO/IEC 17025 accredited - to ensure the calibration meets the needs required in their industry.
More detailed information about the ISO/IEC 17025 standard can be found on the International Organization for Standardization (ISO) webpage and at the International Laboratory Accreditation Cooperation (ILAC) webpage.