Traceability can be defined as an unbroken record of documentation ("documentation traceability") or an unbroken chain of measurements and associated uncertainties ("metrological traceability"). As used here, the word "traceability" always means "metrological traceability."
The National Institute of Standards and Technology maintains the U.S. national standards for temperature. In other countries, similar national standards laboratories perform the same function.
To establish and maintain traceability, the readings of a thermometer can be compared to a fixed-point temperature (e.g., ice-melting point) or a reference thermometer at a fixed temperature – this testing process is often called verification, performance validation or calibration.
Once a thermometer's accuracy is authenticated, it can serve as a reference to establish traceability for other thermometers. This process can be continued, providing an unbroken chain of measurements from the final thermometer all the way back to the NIST standards.
The final measurement will have traceability to NIST standards if the following conditions are met:
- An unbroken chain of measurements back to NIST standards is maintained.
- Each step of the chain has known and documented uncertainties.
- There is a quality system to ensure that the thermometers and associated measurement equipment maintain their measurement uncertainty (accuracy).
The figure below shows a common traceability path. With proper care, a thermometer can be used through many recalibration cycles beyond what is shown in the figure.
Measurement Assurance and Questions about Accuracy
Certain types of thermometers are quite fragile. Unfortunately, there are often no visible signs of damage, especially for digital thermometers! The only way to determine for certain that a thermometer's calibration results are still valid is to verify its performance.
For all of the measurements, an allowable tolerance or accuracy should be established. In regulated applications, the tolerance of the thermometer may be specified; the user can select a tolerance based on the required application accuracy; or in some cases the manufacturer specification is adequate. Thermometers that give results outside the allowed tolerance/application accuracy should be recalibrated or taken out of service.
Frequently Asked Questions
1. I have a "certified" thermometer that claims to be traceable to NIST. What does this mean?
There is no official definition of "certified." Often, a certified thermometer has been tested against standards traceable to NIST, but the user is given less information on the certificate than is typical for a calibration report. To be sure that the thermometer is truly traceable to NIST, we suggest asking the vendor if the certification followed a documented process, what was the measurement uncertainty, and how the reference standards are traceable to NIST.
2. Who has responsibility for ensuring that a measurement is traceable?
Ultimately, the user bears the responsibility of evaluating the traceability chain. NIST does not monitor claims of traceability. There are several factors relevant to evidence of traceability from the thermometer manufacturer/vendor:
- Calibration methods and procedures should be openly documented.
- Uncertainties of calibration should be clearly stated.
- Traceability records should not be claimed to be private or proprietary knowledge.
- Laboratory accreditation is not a guarantee of traceability, but accreditation does provide assurance that qualified assessors have looked at a laboratory's traceability procedures.
3. I have purchased a calibrated thermometer. How often must I have it tested to maintain traceability?
Initial testing/validation/calibration intervals should be based on manufacturer's recommendations or past experience with a type of thermometer (typically one year). Intervals may be adjusted based on the historical calibration results of a particular thermometer. If measurement history indicates significant drift, then recalibrate. If check measurements indicate large and sudden changes, remove the thermometer from service.
4. Can I do any of the calibrations myself?
You can perform the calibrations yourself if you meet all of the requirements for maintaining traceability and have the necessary laboratory equipment and skills. Users can perform in-house performance checks, such as checks in an ice-melting point and steam point. To be sure that you are performing in-house checks correctly, we recommend that users try out their performance checks on newly calibrated instruments that are known to be accurate.
5. My thermometer is traceable to the national standards of another country. Is that equivalent to traceability to NIST?
In some cases, legal or regulatory requirements will explicitly require traceability to NIST standards. If there are no requirements of this type, then the standards of other countries likely are equivalent. Many countries, including the United States, have signed an international Mutual Recognition Arrangement acknowledging the validity of each others' calibration certificates. NIST also compares thermometers among nations to be sure that our standards are equivalent. Records of the recognized calibration capabilities and of comparison results can be found on-line.
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