MIL-HDBK-1839A
b. The contractor shall assess specifications of the calibration equipment and standards used in support of other TMDE. The calibration equipment and standards identified shall satisfy all parameters of each measurement and comply with the requirements set forth in paragraph 5.4.3 of this standard.
c. Where several items of calibration equipment and standards are used in combination to support other TMDE, the overall test configuration specification and uncertainty shall be documented."
Interpretive guidance: Subparagraph a. addresses the selection of calibration standards including standards in support of standards (if necessary). The selection of optimum calibration equipment/standards should consider the operational requirements, which will be met, but need not be exceeded, and the availability of existing support (i.e. procedures, standards, and intervals in place). Documenting traceability from the prime system to the national standard, may require more than one level of calibration standards in which case the CMRS may have four or more categories of equipment. These categories include system/subsystem, TMDE (test equipment), TMDE (standards), and TMDE (standards to support standards), etc.
"5.4.3 Test Uncertainty Ratio. The recommended TMDE shall be capable of measuring or generating to a higher accuracy than the measurement parameters being supported. Unless otherwise specified, a minimum TUR of 4 to 1 is desired. The actual TUR shall be documented."
Interpretive guidance: TUR is a recognized mechanism for establishing the criteria between equipment listed in proximity categories in the CMRS. Actually, use of the TUR is an alternative to performing a more difficult uncertainty analysis as described in ANSI/NCSL Z540-2-1997. The TUR does not relate directly to uncertainty analysis. However, it can be linked to the probability of making erroneous test decisions. Another important point, design activities often select SE that is more accurate than required for the application due to availability, future planning, cost considerations, etc. That is perfectly acceptable but in the application, as defined by the CMRS, the specifications should be derated to the 4 to 1 TUR so as not to create unnecessarily accurate and costly support requirements for the SE. Creation of unnecessarily over/under specified problems are minimized when the design activity emphasizes concurrent engineering processes where the logistics support planning and design engineering are time related and maintain a close liaison. The importance of not over specifying a piece of support equipment cannot be over emphasized as substantial unnecessary costs can be incurred by designs using overly stringent specifications.
"a. If a TUR of 4 to 1, or the specified TUR, cannot be achieved, the design activity shall analyze the measurement requirements and provide documented justification for the lesser TUR."
Interpretive guidance: It is highly recommended that if the CMRS preparing activity contact their designated service CMRS review agent to discuss any shortcomings in meeting specified TUR prior to submittal.
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