Guest Article
Once a good zero has been established, small weights can be used to bring the indication into the zone of uncertainty between zero and the first scale division. If six pounds is required from zero to have the indicator alternate between zero and an indication of 10 lb, this becomes the breakpoint for this particular device. Handouts are available from Dick Suiter at: phone (301) 975-4406, fax (301) 926-0647, email rsuiter@nist.gov. “Substitution Test”. In a substitution test, material, product, vehicles, or other objects are substituted for known test weight on the load-receiving element of the scale, then the known test weights are again placed on the load-receiving element.For example, a load of 4 000 lb of known test standards may be applied to the load receiving element. The error is determined as accurately as possible using error weights. Once the error has been determined, the test weights are removed and product or material is placed in the hopper until exactly the same reading (including error weights) is established; again, error weights are used to determine the reading as closely as possible. If the reading of the indicator is exactly the same, including error weights, for both the known test weights and the substituted material, in effect the actual weight of the substituted material is the same as that of the known test standards, irrespective of any scale error at that point. The limitation on accuracy of this technique is the uncertainty of the error weight process and the readability of the scale. It is important to reduce that uncertainty as much as possible. Some of the sources of uncertainty are a change in zero balance, shift in load distribution, load cell creep, and the ability to achieve an exact substitution for known test weights. On a beam scale, as mentioned earlier, it is best to start with some small weights balanced on the scale load-receiving element at zero. This allows the determination of errors in both directions using error weights. At the 4000-lb load with the poise set at the 4000 lb graduation, small weights are added or removed until the beam is in balance. The difference in small weights from the original starting point is the scale error at that point. When the system has a digital indicator a “balance condition” is not available, so an alternative method must be used to establish a reference point. One alternative is to use the breakpoint as a reference point. In the earlier example, the weight required to get to the breakpoint was six pounds. To determine the error at a 4000 lb known test load and an indication of 4000 lb, the breakpoint is found using small weights. Any difference in the amount of weight required to reach the breakpoint and the original six pounds is the scale error at that point If the starting indication is not 4000 lb, the error in the indication must be included.
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