CLAS Requirements Document 3, May 2018
- 1.1 Laboratories certified by the CLAS program shall use measurement standards that enable the laboratory to provide traceability to the International System of Units (SI) or to standards acceptable to the CLAS program, as described in CLAS Requirements Document 9, Requirements for measurement traceability. These measurement standards shall have the demonstrated uncertainty and stability for the intended use. This is a requirement to assure that the calibrations carried out by the laboratory meet an acceptable quality level.
- 1.2 The topic of measurement standards is inherently linked to the evaluation of measurement uncertainty. In the evaluation of measurement uncertainty CLAS certified laboratories shall meet the following requirements:
- Laboratories shall evaluate and report uncertainty in a manner consistent with the Guide to the Expression of Uncertainty in Measurement (known as the GUM)
- Laboratories shall evaluate and report uncertainty meeting the requirements of ILAC P14, ILAC Policy for Uncertainty in Calibration.
- 1.3 Quality in this context is related to whether the calibration results meet or does not meet expectations. The maximum allowable expanded uncertainty for measurement standards to provide an acceptable quality level depends on several factors. These factors are all related to the three types of services recognized by CLAS. A laboratory can offer more than one type of service.
2.0 Measurement standards for Type I service
- 2.1 Type I service is intended primarily for the calibration of measurement standards. Laboratories providing Type I calibration services are often referred to as standards or standards calibration laboratories. A laboratory providing Type I calibration services has the appropriate reference standards, working standards, check standards, and calibration systems to assess dynamically and to quantify its measurement uncertainty. The measurement process is monitored continually and results documented. A high level of environmental control and monitoring is in place.
- 2.2 Reference standards used to support Type I services must have full redundancy. That means that a physical quantity represented by a reference standard must be duplicated by another standard held by the laboratory so that the measurement system can be systematically assessed. Usually, the adequacy of the measurement standards will not need to be a separate concern since their uncertainty contributions are fully accounted for in the uncertainty budgets of the calibration or measurement systems.
3.0 Measurement standards for Type II service
- 3.1 Type II service is intended primarily for the calibration and adjustment of test, measurement, and diagnostic equipment for use in such areas as product testing, manufacturing, and servicing. Laboratories providing Type II calibration services are often referred to as test equipment calibration laboratories. Equipment that is calibrated through a Type II service usually constitutes the end of the traceability chain and is generally not used to calibrate other calibration standards.
- 3.2 A laboratory providing Type II calibration services has the appropriate working standards and calibration systems to calibrate to a specification/tolerance, usually a manufacturer's specification/tolerance or a published standard. This type of calibration is usually called compliance testing and is usually carried out in a suitably controlled and monitored environment. See Recommended Practices for Calibration Laboratories.
- 3.3 In order for a laboratory to qualify for Type II service, it must have calibrated working standards with known measurement uncertainty. In some cases, the uncertainty in the reference standard is taken as the uncertainty in the entire measurement system. The laboratory then needs to demonstrate that all other influences on the measurement system are negligible. A list of such influences can include:
- non-representative sampling in time;
- personal biases in reading instruments;
- imperfect approximations or assumptions incorporated into the measurement procedure;
- variations in repeat measurements that cannot necessarily be attributed to the instrument under test;
- influences of the environment; and
- inexact values of constants and other parameters used in calculations.
- 3.4 The calibration certificate for compliance testing must have a decision rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement. The decision rule must be clearly defined, communicated to, and agreed with, the customer.
- 3.5 Examples and techniques showing possible decision rules can be found in ILAC G8, Guidelines on Decision Rules and Statements of Conformity.
- 3.6 It is occasionally possible to demonstrate that a specific system performs better than published specifications that are intended to represent a large group of similar systems when used under a wider range of measurement conditions. The improved performance can be demonstrated through statistical process control, or through actual performance verified against higher echelon standards, or through other suitable means. In this case, the demonstrated uncertainty in the specific measurement system can be used (in lieu of the published specifications) to satisfy the minimum 4:1 TUR criterion, keeping in mind that this claimed improvement would have to be assessed.
4.0 Measurement standards for Type III service
- 4.1 This service is the same as Type II service except that the laboratory is not expected to have the capability to verify thoroughly the calibration status of its measurement standards. Instead, the laboratory assures the quality of the measurement results through such other quality control techniques as interlaboratory comparison, replication, retesting, and correlation with other measurement characteristics. Measurement standards used for Type III services are robust and are generally not subject to large drift or change within the laboratory's scope of measurement.