A Certified NIST-reference thermometer is defined as which of the following?

• A. A thermometer manufactured by NIST.
• B. A thermometer that has been calibrated against NIST standards without documentation.
• C. A thermometer that has been tested against standards traceable to NIST, with a certificate documenting traceability.
• D. A thermometer used only in home laboratories.

Answer: (C)

The key idea here is traceability and documented calibration. A certified NIST-reference thermometer is one that has been calibrated against standards that are traceable to NIST, and there is an accompanying certificate that documents that traceability. This certificate provides the formal evidence that the thermometer’s readings can be linked back to NIST through an unbroken calibration chain, with the stated uncertainties.

Why this is the best description: it includes both parts needed for true traceability—testing against standards that themselves are linked to NIST and a certificate that confirms and records that link. Without the certificate, there’s no formal proof of traceability, even if the thermometer was compared to NIST standards. Without standards traceable to NIST, there’s no guaranteed linkage to the national reference framework. And the use in home laboratories doesn’t define the certification status; a device could be used in any setting, but what makes it a NIST-reference thermometer is the documented calibration traceability to NIST.

In short, the defining feature is the documented chain of calibration to NIST standards and an accompanying certificate of traceability.