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What is Traceability?

Information about the Phase-Out of Mercury Thermometers Used in Industrial and Laboratory Settings

The word “traceability” when used in the context of measurement, including temperature measurement, is called "metrological traceability." Metrological traceability refers to how a thermometer’s measurement can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.  When we use the word “traceability” in this paper, we will always mean “metrological traceability.”

The National Institute of Standards and Technology maintains national standards for temperature.  In other countries, similar laboratories perform the same function. 

The readings of a thermometer can be compared to a known temperature standard through the process called “calibration.”  Once a thermometer is calibrated, it can serve as a standard at a lower level of accuracy to calibrate another thermometer.  This process can be continued, providing an unbroken chain of measurements from the final thermometer all the way back to the NIST standards.

When we compare one thermometer to another, the measurement has a probable error.  Sources of error could include how well we can read the thermometer, how close the two thermometers are maintained in temperature, and the repeatability of each thermometer.  The measurement uncertainty gives a measure of the probable magnitude of all of the combined sources of error.

The final measurement will have traceability to NIST standards if the following conditions are met:

  1. An unbroken chain of measurements back to NIST standards must be maintained.
  2. Each step of the chain must have known and documented uncertainties.
  3. There must be a system to ensure that the thermometers and other equipment used remain accurate between calibrations.

Example

The figure below shows a typical traceability path.  With proper care, a thermometer can be used through many recalibration cycles beyond what is shown in the figure.

Typical Traceability Path

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How to Ensure That a Thermometer Remains Accurate

Some thermometers are quite fragile.  Unfortunately, there are often no visible signs of damage, especially for electronic thermometers!  The only way to determine for certain if a thermometer’s calibration results are still valid is to check its performance.

Here are some methods that can be used to verify the performance of a thermometer.  See the guidance on Verification Methods {Insert hyperlink to WA_4-14_Task 4 Verification Methods.doc} for details on each of these methods, and recommendations on the methods to use for particular types of thermometers.  For all of the methods, an allowable tolerance for drift or variability of the thermometer should be established.  In regulated applications, the tolerance of the thermometer may be specified; otherwise, the user can choose a tolerance based on his or her judgment on the required accuracy.  Thermometers that give results outside the allowed tolerance should be recalibrated or taken out of service.

  1. Periodically have the thermometer recalibrated.  If recalibration indicates that the thermometer has drifted by a magnitude that is larger than an allowable tolerance, there are several options available.  Improved handling of the thermometer may reduce the drift; a thermometer with better stability can be used; or the interval between calibrations can be shortened.
  2. Check the readings of the thermometer at the ice or steam point.  The NIST Thermometry Group can provide simple procedures for preparing an ice or steam point.
  3. Compare the reading of a thermometer to another, recently calibrated thermometer.

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Frequently Asked Questions

1.  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 items that users can look for as evidence of traceability:

  1. Calibration methods and procedures should be openly documented.
  2. Uncertainties of calibration should be clearly stated.
  3. Traceability records should not be claimed to be private or proprietary knowledge.
  4. Laboratory accreditation is not a guarantee of traceability, but accreditation does provide assurance that qualified assessors have looked at a laboratory’s traceability procedures.

2.  I have purchased a calibrated thermometer.  How often must I have it recalibrated to maintain traceability?

Initial calibration intervals should be based on manufacturer’s recommendations or past experience with a type of thermometer.  Calibration intervals may be adjusted based on the historical calibration results of a particular thermometer.  If check measurements indicate significant drift, then recalibrate.  If check measurements indicate large and sudden drift, remove the thermometer from service.  See the section on Learning More for additional information.

3.  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-point bath.  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.

4.  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 a Mutual Recognition Arrangement that recognizes the validity of each others’ calibration certificates.  We also compare thermometers among nations to be sure that our standards are equivalent.  Records of the recognized calibration capabilities and of comparison results Exit EPA Disclaimer.

5.  I have a “certified” thermometer that claims to be traceable to NIST.  What is the difference between a “certified” and a “calibrated” thermometer?

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 were the reference standards traceable to NIST.

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Learning More

NIST provides a description of traceability and an extensive list of questions and answers on traceability Exit EPA Disclaimer.  This site is the source for official NIST policy on traceability.

The book Traceable Temperatures by J. Nicholas and D. R. White (Wiley, 2001) gives a good discussion of traceability for thermometer calibrations.

Learn more about modern uncertainty analysis

To learn more about methods to set calibration intervals:

“Guidelines for the determination of calibration intervals of measuring instruments,” ILAC-G24, (International Laboratory Accreditation Cooperation, 2007)

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