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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
Subsection 2 – Scales and Weighing
3.2.15. Field Standard Test Weights – Weight Dollies
Interpretation
Field standard test weights (weight carts) are being used to test vehicle scales. These
Some carts are electrically powered and many of these weight carts are powered by liquid fuel which is consumed during the conduct of the test. NIST Handbook 105-8, Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures 8. Specifications and Tolerances for Field Standard Weight Carts, provides design criteria for greater cart stability. Following recommended methods for calibration and use will also enhance their use as field standards. In 1980, the adopted NCWM Policy stated that theses carts were inappropriate as standard weights. The proliferation of weight cart use has provided data on which to demonstrate that the benefits of weight cart use outweigh the disadvantages associated with their potential instability for use in scale testing. It is impossible to maintain these devices within the tolerance limits (1/10 000) as required by Handbook 105-1 or within one-third of the smallest tolerance applicable as required by Handbook 44 and are therefore inappropriate as standard weights.
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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For technical questions concerning the Metrology Resources for State and Commercial Laboratories, contact us:
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Date created: March 2000
Last updated: October 1, 2002
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