MEASURE FOR MEASURE
Pass or Fail
Understanding calibration certificate content and requirements
by Christopher L. Grachanen
For many quality professionals, the only interaction you have with a calibration laboratory—besides procuring a service or coordinating shipping logistics—is evaluating the calibration certificate that accompanies a piece of test equipment after it returns from calibration.
The calibration certificate—commonly referred to as a calibration report—is the documented evidence that a piece of test equipment was, at some point, determined to be within or out of its intended operating specifications.
Calibration validation doesn’t refer to the unit’s functional ability to measure an alternating voltage or produce a sinusoidal waveform. Rather, it’s the unit’s ability to measure or produce a numerical value associated with a phenomenon, such as a measurement parameter, to within specific limits. These limits are inherent in a unit’s published specifications and are known as toleranes during the calibration activity. As a result of a calibration activity, an overall status will typically be provided showing whether a piece of test equipment passed or failed the activity.
Calibration certificates come in three major types, with variations of each type reflecting the calibration provider’s interpretation of the services rendered:
1. Calibration compliance certificate. This certificate states that a piece of test equipment has been found to have fulfilled applicable expectations—a piece of test equipment has met the original equipment manufacturer’s (OEM) published specifications per the OEM’s recommended procedure. This type of calibration certificate is typically provided with a new piece of test equipment unless additional calibration data are requested in a purchase order.
Because a calibration compliance certificate provides only a summary statement of compliance without any measurement data obtained during a calibration activity, it does not fulfill the requirements of many published regulatory guidance documents associated with evaluating and using calibration results.
2. Calibration with data certificate. This certificate includes measurement data and calibration tolerances. It provides measurement data in terms of magnitude and direction so that an impact assessment can be performed in the case that a measurement parameter is found to be out of tolerance. Without magnitude and direction information, the impact on a customer measurement application cannot be ascertained to determine whether corrective action is warranted.
3. Calibration with data and uncertainties. For each measurement parameter, this certificate also provides an accompanying measurement uncertainty. This uncertainty gives the probability (the likelihood) that the obtained measurement value falls within stated tolerances, as inherent of the measurement process. It includes all significant contributors of error associated with the process, such as environmental factors, repeatability and reproducibility.
Of the several major guidance documents dealing with calibration laboratory operations and services, ISO/IEC 17025:2005—General requirements for the competence of testing and calibration laboratories is the global standard for calibration laboratories engaged in providing accredited calibration services. The standard specifies many requirements for quality system content for an accredited calibration laboratory. One of the topics addressed by the standard is calibration certificate content.
According to Clause 5.10.2—Test reports and calibration certificates, each test report or calibration certificate should include at least the following information, unless the laboratory has valid reasons for not doing so:1
- A title (such as "Test Report" or "Calibration Certificate").
- The name and address of the laboratory and the location where the tests and calibrations were carried out, if different from the address of the laboratory.
- Unique identification of the test report or calibration certificate (such as the serial number). Each page should include an identification to ensure the page is recognized as part of the test report or calibration certificate, and the end of the test report or calibration certificate should be clearly identified.
- The name and address of the customer.
- Identification of the method used.
- A description of, the condition of, and unambiguous identification of the items tested or calibrated.
- The date of receipt of the test or calibration items when it is critical to the validity and application of the results, and the dates the test or calibration was performed.
- Reference to the sampling plan and procedures used by the laboratory or other bodies that are relevant to the validity or application of the results.
- The test or calibration results with, when appropriate, the units of measurement.
- The name, functions and signatures or equivalent identification of people authorizing the test report or calibration certificate.
- Where relevant, a statement saying the results relate only to the items tested or calibrated.
The clause also includes two notes:
Note 1: Hard copies of test reports and calibration certificates also should include the page number and total number of pages.
Note 2: It is recommended that laboratories include a statement specifying that the test report or calibration certificate should not be reproduced except in full without written approval of the laboratory.
Additionally, the standard gives the following requirements specifically for calibration certificates in Clause 5.10.4—Calibration certificates:2
Clause 22.214.171.124. In addition to the requirements listed in clause 5.10.2, calibration certificates should include the following, where necessary for the interpretation of calibration results:
- The conditions (such as environmental) under which the calibrations were made that have an influence on the measurement results.
- The uncertainty of measurement and a statement of compliance with an identified metrological specification or clauses.
- Evidence that the measurements are traceable (see Note 2 in section 126.96.36.199.1 of ISO/IEC 17025).3
Clause 188.8.131.52. The calibration certificate should relate only to quantities and the results of functional tests. If a statement of compliance with a specification is made, which clauses of the specification are met or not met should be identified. When a statement of compliance with a specification is made omitting the measurement results and associated uncertainties, the laboratory should record those results and maintain them for possible future reference. When statements of compliance are made, the uncertainty of measurement should be taken into account.
Clause 184.108.40.206. When an instrument for calibration has been adjusted or repaired, the calibration results before and after adjustment or repair, if available, should be reported.
Clause 220.127.116.11. A calibration certificate (or calibration label) should not contain any recommendation on the calibration interval except where it has been agreed upon with the customer. This requirement may be superseded by legal regulations.
It should be noted that each different type of calibration certificate has an associated cost. As you would expect, the compliance calibration certificate is associated with the least expensive calibration service, while the calibration certificate with measurement data and associated uncertainties is related to the most expensive calibration service.
It is prudent and cost effective to request the type of calibration certificate associated with a calibration service that best meets the needs of the customer measurement application while taking into consideration any quality directives that may mandate calibration certificate content. As with procuring any services on behalf of an organization, it is up to the customer to determine what services are needed and that expectations associated with each service are clearly communicated, preferably in writing.
Upon receiving a calibration certificate, it is wise to evaluate it in terms of the aforementioned calibration certificate content. Information that is not understood or is ambiguous should always be brought to the attention of the calibration service provider for clarification.
- International Organization for Standardization and International Electrotechnical Commission, ISO/IEC 17025:2005—General requirements for the competence of testing and calibration laboratories, Clause 5.10.2—Test reports and calibration certificates.
- International Organization for Standardization and International Electrotechnical Commission, ISO/IEC 17025:2005—General requirements for the competence of testing and calibration laboratories, Clause 5.10.4—Calibration certificates.
- International Organization for Standardization and International Electrotechnical Commission, ISO/IEC 17025:2005—General requirements for the competence of testing and calibration laboratories, Clause 5.6—Measurement traceability, section 18.104.22.168.1, note 2.
Christopher L. Grachanen is a master engineer and operations manager at Hewlett-Packard Co. in Houston. He earned an MBA from Regis University in Denver. Grachanen is a co-author of The Metrology Handbook (ASQ Quality Press, 2012), an ASQ fellow, an ASQ-certified calibration technician and the treasurer of the Measurement Quality Division.