Guest Article
For example, if the indication is 3990 (on a 10-lb division scale) and the amount of error weights required to arrive at the breakpoint for that indication is 4 lb, the actual scale error at that point is minus 8 lb. Keeping track of error weights to determine actual errors can be confusing and some practice is required. Using a diagram with the division broken down into tenths, such as shown on the figure may help. The substitution process can be repeated several times to arrive at a larger “test load.” The Office of Weights and Measures recommends no more than three substitutions due to the uncertainty of the process. Particularly when only direct reading of errors is used (not using error weights and breakpoints), the uncertainty may approach the tolerance applied to the scale.Because of this uncertainty, it is necessary to apply at least 25 percent of capacity in known test weights.If that is not possible, strain load procedures should be used after three substitutions. “Strain Load Testing”. In strain load testing, the scale is tested using known test weights up to the amount of available weight. Then material or product is added to the hopper to some higher reading; in this case the true weight of the material is not known. For example, 5000 lb of known test weight can be used along with the substitution test procedure to test a scale up to 20 000 lb. If the capacity of the hopper is 60 000 lb, however, a strain load test must be used to test the device to a point near or at its capacity. To perform the strain load test in this example, material can be added to the hopper until the reading is 55 000 lb. At this point, error weights are used to determine a starting reference point using the procedures described earlier for error weights. Once the starting reference point is determined, the 5 000 lb of known weight is added to the scale and the scale reading is determined at that point (60 000 lb) using error weights. The difference between the starting reference point and the ending reference point is compared with the 5000 lb of known weight to determine the error at 60 000 lb. In this case, the tolerance is based on the 5000 lb of known weight and not the 60 000 lb value. The major difference and drawback to strain load testing is the fact that there is no way to determine the scale’s performance between the 5000 lb of known weight and the strain load of 55 000 lb. The Test. Now that we have some understanding of substitution and strain load testing we can proceed to review a complete test.
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