Henry L. Alexander CMfgE CQE Calibration Program Manager Perry Johnson Laboratory Accreditation, Inc NCSL International Workshop and Symposium Communicating with the Business End of Metrology: The Scope of Accreditation for Calibration Laboratories The Scope of Accreditation is the first line of communication between the calibration laboratory and those in Quality and Testing who are consumers of its services. As such it is imperative that the laboratory present in a clear and concise manner the calibration capabilities for which it is accredited. An effectively written scope of accreditation will accomplish this objective by presenting all relevant information in four distinct categories as follows.

2012 NCSL International Workshop and Symposium 1) Identification of the calibration activity for which the laboratories capability is accredited. e.g. Micrometer 2) Specification of the range of magnitude over which the laboratory is accredited to perform the calibration activity. e.g. 1.0 mm to mm 3) Estimation of the CMC (Calibration and Measurement Capability) associated with the accredited calibration activity. e.g. ( L) µm (“L” must be defined, typically in a footnote) 4) Description of the equipment, standards, methods and published references or standards utilized in performing the accredited calibration activity. e.g. Comparison to Grade 00 gage Blocks and ASME B89.1.9

2012 NCSL International Workshop and Symposium Identification of the calibration activity for which the capability of the laboratory is accredited. The laboratory must state this capability with sufficient specificity that a potential customer can easily determine whether or not the laboratory is accredited to perform the specific calibration they require. What is the laboratory stating that they are capable of calibrating? The laboratories answer to this question must be a careful balance between two conflicting objectives The laboratory must state this capability with sufficient generality that they do not unnecessarily limit the instruments or devices which they are capable of calibrating.

2012 NCSL International Workshop and Symposium Identification of the calibration activity for which the laboratories capability is accredited.

2012 NCSL International Workshop and Symposium Identification of the range of magnitude over which the laboratories calibration capability is accredited. Some types of gages or instruments are capable of measuring a continuum of values bounded by an upper and lower limit while others are single discrete or “Fixed Point” values. As an example, a digital multi meter can read any value of voltage between specified minimum and maximum values while a gage block represents a fixed discrete value of length. For a laboratory with knowledge of the physical magnitude of standards available to them and the range or capacity of measuring and test equipment on hand, stating the range of calibration capability should be a straight forward exercise … o r is it?

2012 NCSL International Workshop and Symposium The range is intended (and required by ISO 17011: ) to state the discrete values or range of values from minimum to maximum for which a laboratory is accredited to perform calibration of the stated gages or instruments to which the range applies. In order to arrive at a consensus on an acceptable manner in which to state the range it is first necessary to consider the intended purpose of stating the range The accreditation certificate shall also identify the following: c) for calibration laboratories: 1) the calibrations, including the types of measurements performed, the measurement ranges and the best measurement capability (BMC) or equivalent1); 2 ISO/IEC 17011:2004 Conformity assessment - General requirements for accreditation bodies accrediting conformity assessment bodies The “measurements” referred to in the passage above are the calibrations to be performed therefore the range is the range over which the calibrations apply

2012 NCSL International Workshop and Symposium In simpler terms calibration is “the comparison of a UUT to a standard for which the measurement value and its associated uncertainty of measurement are known”. Next it is necessary to determine what constitutes the performance of a calibration. What is calibration? Per the VIM 1 calibration is defined as follows: 2.39 (6.11) calibration operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication. 1 International vocabulary of metrology — Basic and general concepts and associated terms

2012 NCSL International Workshop and Symposium It becomes obvious that in order for the laboratory to have performed the calibration of a UUT, it must have a standard (the M&TE) for which the magnitude (or range of magnitudes) and associated uncertainty (or uncertainties) of measurement are known and against which the UUT is compared. If the comparison is made by means of an instrument which measures the parameter of interest of the UUT then the upper limit of the laboratories range is based on the capacity of the instrument. A gage block comparator used to calibrate a gage block. In this case the upper limit of the laboratories capability is the largest gage block the Comparator can measure. A DMM used to calibrate a fixed resistor. In this case the upper limit of the laboratories capability is the largest value of resistance the DMM can measure. Examples: Consider the upper limit.

2012 NCSL International Workshop and Symposium If the comparison is made by means of a standard which is compared to or measured by the UUT (for the parameter of interest) then the upper limit of the laboratories range is based on the magnitude of the standard. A gage block used to calibrate a supermicrometer. In this case the upper limit of the laboratories capability is the largest gage block stack the laboratory can assemble from its available calibrated gage blocks. A multi-product calibrator used to calibrate the ability of a DMM to measure AC voltage. In this case the upper limit of the laboratories capability is the largest AC voltage that can be sourced by their calibrated multi-product calibrator. Examples: ASTM class 1 weight is used to calibrate a class F weight of equal nominal value. In this case the upper limit of the laboratories capability is the nominal value of calibrated ASTM class 1 weight available to the laboratory.

2012 NCSL International Workshop and Symposium The upper limit of the range of calibration capability is either the nominal value of the largest calibrated fixed value standard the laboratory has available or for a calibrated instrument or gage capable of measuring over a range of values it is the maximum nominal measurement capacity. If the comparison is made by means of an instrument which measures the parameter of interest of the UUT then the lower limit of the laboratories range is based on the minimum nominal expected size of the UUT or the minimum nominal calibrated capacity of the M&TE whichever is greater. Examples: Determining the lower limit is not so simple. A gage block comparator used to calibrate a gage block. In this case the lower limit of the laboratories capability is the smallest gage block the laboratory expects to be asked to calibrate or that its gage block comparator can measure.

2012 NCSL International Workshop and Symposium More examples: ASTM class 1 weights used to calibrate a laboratory balance. The smallest class 1 weight in the ASTM classification is 1 mg. Very good sub mg weights are available but they are not included at this time in the ASTM classification. Since (at this time) it is physically impossible for the laboratory to have a calibrated standard (a class 1 weight) less than 1 mg, they cannot calibrate the balance at a value less than 1 mg. A reasonable lower limit to the range of weights over which the laboratory is accredited to calibrate laboratory balances should not be less than 1.0 mg. Although the minimum physical limit to gage block size is 1 molecular layer, in practice manufacturing difficulties coupled with increased thermal and geometric instability dictate a lower limit to gage block size of typically in or 0.25 mm. A reasonable lower limit to the range of sizes over which the laboratory is accredited to calibrate gage blocks should not be less than in

2012 NCSL International Workshop and Symposium If the comparison is made by means of a standard which is compared to or measured by the UUT (for the parameter of interest) then the lower limit of the laboratories range is based on the magnitude of the standard. A gage block used to calibrate a supermicrometer. The supermicrometer is capable of measuring to very near zero. However since calibration requires comparison to a calibrated standard for which the size and uncertainty of measurement are known, the lower limit of the laboratories capability to calibrate a supermicrometer is the determined by the size of the smallest calibrated gage block available to the laboratory. A reasonable lower limit to the range of sizes over which the laboratory is accredited to calibrate a Supermicrometer should not be less than in unless the laboratory possesses a calibrated gage block of smaller size.

2012 NCSL International Workshop and Symposium CMC is defined as “the best uncertainty of ne\measurement attainable by the laboratory when performing a more or less routine calibration of a nearly ideal device under nearly ideal conditions” Estimation of the CMC (Calibration and Measurement Capability) All uncertainty estimates include terms whose value is constant over time as well as terms the value of which vary from calibration to calibration. When estimating the CMC the terms that are constants during uncertainty estimation are retained as constants with the same value. Those terms which are variable in the uncertainty estimate are entered at their optimum values (minimum attainable value) when estimating the CMC. Since the optimum value of any term is not likely to occur and it is even less likely that all terms will attain optimum values at the same time, the CMC can be thought of as an estimated uncertainty in a hypothetical situation that may be approached in reality but is never attained.

2012 NCSL International Workshop and Symposium Estimation of the CMC (Calibration and Measurement Capability) Examples of sources whose value is constant: (not inclusive) Resolution Uncertainty of a standard (obtained from the certificate of a current calibration the results of which have been determined to be traceable Bias (when the nature of the bias is such that it can not be minimized through adjustment or repair). (e.g. the deviation from nominal of a gage block length as reported on its calibration certificate) Examples of sources whose value is variable: (not inclusive) Temperature and temperature related effects Repeatability of the unit under test Relative humidity and humidity related effects

2012 NCSL International Workshop and Symposium Estimation of the CMC (Calibration and Measurement Capability) ISO/IEC 17025:2005 requires that laboratories have a procedure that is used to estimate measurement uncertainty for all calibrations performed ILAC-P14 requires that CMC be estimated in the same way as measurement uncertainty (using the same procedure) Estimated uncertainty of measurement can never be smaller than the stated CMC on the scope for the calibration to which the CMC applies

2012 NCSL International Workshop and Symposium Calibration equipment and reference standards used This column should include the following information: Optional features of the M&TE that might enhance its performance Published standards (ASTM, ASME, ISO, SAE etc.) which are used or referenced in performing the stated calibration. The type of M&TE used to perform the calibration Any addition unique or relavent information that affords additional insight into the calibration.

2012 NCSL International Workshop and Symposium Identification of the calibration activity: State the CMC: Use the same procedure used for estimating uncertainty of measurement Briefly descripe the M&TE and list published standards referenced Summary for Laboratory personnel Be as general as possible (do not restrict calibration opportunities) Be as specific as necessary (specific calibration activity) Specification of the range 0.0 is rarely appropriate for the low end of the range. Terms such as “Up to” imply a lower limit of 0.0 and as a result are rarely acceptable Typically determined by the availability of calibrated standards or calibrated capacity of M&TE for both high and low limits of the range

2012 NCSL International Workshop and Symposium Identification of the calibration activity: CMC: Understand the CMC. The calibration uncertainties will not be less than the CMC. In almost all cases they will be larger. Study the M&TE used and published standards reference. Compare alternate calibration methods and the associated M&TE, CMC etc. Summary for Laboratory Customers Be knowledgeable of your requirements vs. the laboratories capabilities Be as specific as necessary (specific calibration activity) Specification of the range: Understand the range Beaware that the range is bounded by available calibrated standards. Properly interpret stated ranges. Ask questions. Be informed. It defines the laboratories calibration capability not the capacity of your equipment.

2012 NCSL International Workshop and Symposium Perry Johnson Laboratory Accreditation, Inc. 755 W. Big Beaver Rd., Suite 1325 Troy Michigan Telephone: LABS (5227) Fax: Fax Henry L. Alexander CMfgE CQE Calibration Program Manager