When it comes to the uncertainty associated with temperature calibration, be certain about the reported uncertainties.
It is important to understand what is included in the reported uncertainty values and the basis upon which the calibration lab operates as the data can have an important influence in your process measurement accuracy and confidence.
NIST traceable: This is the minimum temperature calibration lab capability. The statement “NIST Traceable” indicates there is an unbroken chain of comparisons to stated standards, from the lab instrumentation to NIST. Calibration performance reported this way is only required to include the uncertainty of the calibration equipment and process. It does not require the inclusion of the uncertainty (short term behavior) attributed to the sensor being calibrated.
ISO 17025: This ISO standard titled: General requirements for the competence of testing and calibration laboratories, like many ISO systems focuses on the quality system related to the calibration function. A Calibration Lab that states compliance with ISO 17025 insures traceability to the primary standard as well as broader system confidence as it addresses practices beyond the specific process related to calibration. ISO 17025 also addresses the management of the process and lab including training, document control, contract review and management oversight. The other very important difference is the inclusive analysis of the calibration uncertainty. ISO 17025 requires labs to include the sensor being calibrated as a component of the stated uncertainty.
Accredited: There are a few organizations that issue accredited status of metrology labs. The organizations such as NVLAP and A2LA monitor and audit the labs practices and performance in accordance with ISO 17025. Burns, (lab code: 200706-0) selected NVLAP, established by NIST in 1976 as the National Voluntary Laboratory Accreditation Program. The NVLAP accreditation requirements are described in NIST Handbook 150. NVLAP also includes the general requirements of ANSI/NCSL Z540. Uncertainties reported are inclusive of the sensor being calibrated, and validated through proficiency testing.
Here is a short paper on Understanding Calibration Uncertainties as reported by Burns Engineering.
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