National Conference on Weights and Measures
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Source Southern Weights and Measures Association (SWMA) (This item was referred to the Specifications and Tolerances Committee from the Laws and Regulation Committee because it is more appropriate that the S&T; Committee address device field standard use.) Recommendation Modify NCWM Publication 3 Policy, Interpretations, and Guidelines Section 3, Subsection 2, 3..2.15. as follows: Section 3 – Specifications, Tolerances, and Device Inspection Discussion Weight carts are currently being used by both public and private agencies to test large capacity scales. Weight carts provide a safe and efficient means to position larger field standard weights when testing scales. Use of weight carts during the test of large capacity scales reduces the wear on mass standards and more closely simulates the weight load on a scale during actual use. Agencies using weight carts are in conflict with NCWM 3 Section 3.2.15. Field Standard Test Weights – Weight Dollies which states that weight carts are inappropriate to test scales. In 1980, the NCWM adopted Section 3.2.15. to address of the use of weight carts powered by liquid fuel. The NCWM noted that these weight carts consumed fuel during the test and, in most instances, it was impossible for the weight cart to maintain the tolerance for standards described in NIST Handbook 105-1, Specifications and Tolerances for Field Standard Weights (NIST Class F) and NIST Handbook 44 , Fundamental Considerations, Testing Apparatus, Tolerances for Standards. The Handbook 105-1 tolerance is one part in 10 000 for weights 1 kg and larger. In 1980, the Handbook 44 stated that the standard error should not be greater than 25 percent of the smallest tolerance applied when the standard is used. Currently, there is no technical document that specifically addresses weight cart standards. When weight carts are properly designed, calibrated, maintained and used they can be an appropriate standard, although they are not considered a Class F standard. In 1998, the Metrology Subcommittee recognized the widespread use of weight cart standards and the need for specifications, tolerances, calibration and user requirements, and controls for their use. The NIST Working Group on Weight Carts was established in 1998 and is working to develop new Handbook 105-8, Specifications and Tolerances for Field Standard Weights (Weight Carts), a Recommended Standard Operations Procedure for Calibration of Weight Carts, and an Examination Procedure Outline for weight cart users. The Working Group acknowledges that weight carts are unlike most standards because they have mechanical parts and fluids which contribute to error. Accuracy of the scale test procedures is dependent on the field official’s attention to detail and maintenance of the weight cart standard. |
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