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A number of factors must be taken into consideration in the development and review of the specifications and tolerances for weight carts. All of these factors must be weighed against each other in determining the "best technical solution". These factors include:
• Historical policy and practice;
• Current practice (purchase and use);
• Current calibration requirements; and
• User considerations.
Historical Policy and Practice
The National Conference on Weights and Measures (NCWM) Publication 3, Section 3.2.15 states the following: "Field standard test weights (weight carts) are being used to test vehicle scales. These weight carts are powered by liquid fuel which is consumed during the conduct of the test. It is impossible to maintain these devices within the tolerance limits (1/10 000) as required by Handbook 105-1 or within 1/3 of the smallest tolerance applicable as required by Handbook 44 and are therefore inappropriate as Standard Weights." This policy was proposed by the S&T committee and approved in 1980. Item 309-2, p. 237. From this policy, we can determine that a large number of jurisdictions were, at one time, opposed to the use of weight carts, especially those powered by liquid fuels. We can also determine that the group did not believe that weight carts would maintain necessary tolerances according to NIST Handbook 105-1 nor to NIST Handbook 44. However, whether or not these weight carts can maintain tolerances requires the availability of "as found" calibration data.
The use of weight carts has proliferated in spite of the official policy regarding their use.
Some have indicated that this is due to safety, convenience, and efficiency considerations resulting from having to do additional work with fewer people. NIST Technical Advisors to the NCWM S&T Committee have also suggested that "the National Type Evaluation Program (NTEP) may have played a key role in the acceptance of weight cart use, since in some cases, the use of weight carts is the most practical way of applying the test loads required in NCWM Publication 14 for the evaluation of a vehicle scale with a high capacity and concentrated load capacity (CLC) rating." However, many of the weight carts in current use, were being used prior to the development of the NTEP program. Some have suggested that the increased minimum test load requirements prompted increased usage.
Current Practice
The use of weight carts has significantly increased over the years. Since there has been a policy actually prohibiting the use of weight carts, no standards for the specifications, tolerances, and use have ever been developed. As a result, the size, type of power, wheel types, actual use with standard weights, calibration standards and equipment, calibration procedures, tolerances applied, is extremely non-uniform. For several years, the subject of weight carts and their calibration standards, equipment, and calibration procedures have been discussed at regional metrology meetings. A review of this information shows an obvious lack of uniformity. In many cases, laboratories without suitable standards or test equipment were not consulted regarding their testing capabilities and have been asked to calibrate weight carts after they have already been purchased. A summary from two of the regional metrology groups from 1997 is attached. While the notes are somewhat sketchy, and some states have updated their capabilities since 1997, we can get an idea of the variability present. We can also determine from the notes, that some States have committed to the policy prohibiting their use.
Current Calibration Requirements
A decision regarding applicable tolerances for weight carts will need to be made. Some laboratories have the standards and equipment to appropriately calibrate to Handbook 105-1, Class F test weight tolerances. Other laboratories do not have suitable equipment to test to the level required for Class F and test to the lesser requirements for standards used on scales as identified in Handbook 44.
NIST Handbook 145, which contains laboratory calibration procedures, specifies that the uncertainty must be less than one-tenth of the tolerance (updated draft specifies one –ninth). This is also stated in the NIST Handbook 143, Program Handbook for laboratory recognition. To be declared as "within tolerance", a standard must have a combined and expanded uncertainty of less than one-ninth of the tolerance at the Class F, field standard level. When it is not possible to meet the requirements of NIST Handbook 145, due to the lack of available standards or equipment, metrologists have determined to use the requirements in Handbook 44. This allowance was also incorporated into Handbook 143 and is represented in the updated draft procedures for Handbook 145.
Requirements for uncertainties on field standard calibrations have been specified in all NIST Handbook 105-x series handbooks since 1996. Requirements state that the combined and expanded uncertainties (at k=2, 95%) from all sources must be less than one-third of the tolerances stated in NIST Handbook 44 to be used as a standard. Handbook 44 indicates that the "error" in the standard must be less than one-third of the tolerance for the device under test. To determine error at this level, the uncertainty must be smaller than the allowable error by some margin.
Class F tolerances above 10 pounds are 0.1 lb per 1 000. Using the larger allowances in Handbook 145, the laboratory uncertainty must be less than one-third, or 0.033 lb. The following table provides a sample of the calibration uncertainties.
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Weight (lb)
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Class F Tolerance
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Maximum Uncertainty
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1000
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0.1
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0.033
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2000
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0.2
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0.067
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3000
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0.3
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0.1
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5000
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0.5
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0.17
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To achieve these requirements, a scale with a minimum readability of 0.01 lb is recommended, particularly since at these capacities, the variability in weighing is rarely less than or equal to the readability. In many cases, laboratories are using devices with 0.1 lb readability. It is obvious in these cases, that it is not possible to meet Class F calibration requirements, but it is possible to meet guidelines in Handbook 44.
To determine which tolerances (Class F or Handbook 44) are most appropriate, actual "as found" calibration data and "as used" data is critical. Whichever tolerance is chosen will impact the requirements for minimum load for use. If the weight carts are properly tested to Class F requirements, and can maintain the requirements for Class F tolerances during use, minimum load for use requirements may not be needed.
User Considerations
Comments and suggestions were solicited for nearly a year before this initial draft was developed. A number of suggestions included the idea that tolerances for weight carts should be based on a "loaded" or "actual use" weight for the cart and Class F field standards combined. This requires that a minimum load be specified. An example of this might be that a 2,000 cart would be loaded with 10,000 lb of Class F field standards.
Numerous concerns have been raised regarding the "control" of weight carts. The variability and control of fluid levels and the ease of changing batteries, tires, and repaired parts, cause concern for many. The document will need guidance on what appropriate "controls" should be in place. Data regarding electric versus liquid fuels is also needed to determine if some weight carts will meet tighter requirements than others.
Many jurisdictions have expressed concern that in developing a standard now, it won’t be able to take into account the wide variation of weight carts currently in use. The real concern is one of efficiency rather than only of technical accuracy and stability since many programs have quite a large investment in weight carts. Also, all NIST 105-series handbooks have made grandfathering allowances for standards which may not meet specifications but which maintain tolerances while in use.
Balancing all of the technical requirements and considerations with practical concerns will be quite a challenge.
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