Traceability of measurements and measurement standards is key when examining a cal lab for proof of compliance with national standards.

Manufacturers searching for a commercial calibration laboratory to calibrate their test and measurement equipment need answers to tough questions. One of the toughest is the traceability of measurements and measurement standards to the National Institute of Standards and Technology (NIST) and other international organizations.

In establishing the validity of a lab’s claim of measurement traceability to NIST, manufacturers must obtain the pertinent documents and ask the direct questions. Further, even if the calibration laboratory has achieved accreditation, the potential customer should not accept its claims of measurement traceability and calibration reliability at face value.

A prospective calibration laboratory will have, or should have, a file system containing the calibration history for each standard that is used to calibrate a customer’s measuring and test equipment. This will be the main source of information supporting a claim of traceability. The lab’s internal audit records also should show evidence of its own verification activities regarding the traceability link.

Consider this general definition of measurement traceability: “Measurement traceability is the property of a result of a measurement whereby it can be related to appropriate standards, generally national or international, through an unbroken chain of comparisons the uncertainty of which is known at each point in the chain of comparisons.”

Unbroken chain

“Unbroken chain of comparisons” is the key phrase in this statement. A calibration laboratory must be able to show an unbroken NIST-traceable link from the lab’s secondary, primary or reference standards to an interim lab or labs and on to NIST. In some instances, the link will go directly from the lab to NIST. If this link is broken or interrupted, the traceability is invalid.

All calibration laboratories involved in this link must demonstrate valid measurement traceability, including measurement uncertainty, in their own processes. Accreditation or ISO 9000:2000 certification notwithstanding, while searching for a reliable, calibration laboratory with bonafide measurement traceability, obtain a list of the lab’s standards that will be used to calibrate the equipment. Also, obtain copies of the NIST test reports, if directly traceable, or calibration file report numbers that directly relate to the traceability of that equipment plus any supporting documents. This may be a long list if many types of equipment are to be calibrated or if many reference or primary standards are involved. The NIST test report or calibration file report numbers must also be stated on the calibration report or certificate for examination. This will show the results of the measurements taken using those standards.

It should be noted that not all lab calibration standards must be sent for outside calibration, providing that the lab has the capability to calibrate its own secondary standards and maintain the traceability link through their primary or reference standard reports.

Typically, calibration laboratories comply with the traceability mandates of ISO 17025, which is the international standard that is rigorously enforced by accrediting organizations. The lab will calibrate its customer’s measuring and test equipment with the laboratory’s own working or transfer (secondary) standards. These are calibrated using primary or reference standards that are in turn calibrated directly by NIST or through an interim lab with measurement traceability to NIST.

If the calibration lab does not strictly follow this process and reinforce it with a thoroughly verified paper trail, then the traditional traceability claim is in doubt.

Reliable calibration

In addition to traditional traceability, another important aspect to consider is reliable calibration. Assuming that the prospective commercial calibration laboratory has successfully jumped through all the traceability hoops, prospective clients need to turn their attention to the nuts and bolts that is the foundation of a quality calibration operation.

• Calibration technicians should be thoroughly trained in their calibration responsibilities and the lab should periodically verify the expertise levels of its technical staff by testing them.
• Clear procedures need to be followed during the calibration process, and these procedures should be documented and current.
• A measurement-uncertainty program must be implemented and statistical process control incorporated into the calibration operation.
• The lab must be able to detail how it deals with technical errors on the certificates of calibration.

These are some of the areas that auditors focus on when they audit a particular laboratory for accreditation, and something that manufacturers must address as well.

Three questions

In a nutshell, manufacturers must receive answers to three basic, but fundamentally important, questions:

1. Is the lab traceable through an unbroken chain of comparisons?
2. Does the lab provide reliable calibration, and will the documents be available for examination to ensure proof of compliance?
3. Is the lab accredited, and has the scope of accreditation been reviewed?

The first question is important because it verifies that the calibration laboratory is “dotting the I’s and crossing the T’s” relative to its measurement traceability. The second is critical because reliable calibration is the basis for a successful calibration system. The third shows that the lab has gone through the proverbial trial-by-fire with an accreditation organization and can provide reliable measurements within certain parameters.

If these three questions are answered satisfactorily, and the calibration laboratory is accredited, then by all means accept its traceability claim at face value. It will save valuable time.

But not all labs are alike in adhering to the above principles, sometimes not even accredited ones. Potential customers need to examine the history of reliability, dependability and professionalism of the calibration source. Issues such as length of time in business, customer references, validity of lab operations and professionalism of staff are all key. Visit the lab in person and observe the activities. Watch and ask questions. By doing all these things, an enlightened decision can be made regarding valid measurement traceability and reliable calibration.

Tech TIPs

• A calibration laboratory must have an unbroken NIST-traceable link. If this link is broken or interrupted, the traceability is invalid.
• All calibration laboratories involved in this link must demonstrate valid measurement traceability.
• A file system should contain the calibration history for each standard used to calibrate a customer’s measuring and test equipment.
• Typically, equipment is calibrated with the laboratory’s working or transfer standards. These are calibrated with primary or reference standards that are calibrated directly by NIST or through an interim lab with measurement traceability to NIST.