........................................................................................................................... WEIGHED LOAD TEST SUGGESTIONS
Wind is often a problem. If calibration tests are to be stable and repeatable the scale readings can not be
erratic from any cause. Although each plant is different, a 1 TPH rate may be caused by only 1 lb. of wind
pressure on the belt at the position of the weigh idler. Wind screens may help but may not protect against
wind blowing directly down the belt. Top covers or tarps tied over the belt may be helpful. Calibration may
have to be delayed until the wind subsides although most people recognize that this will not eliminate the
errors that will be present on windy days of actual production.
Remember too, that the factors that cause errors when calibrating a belt scale also cause errors when calibrating
feeders to the belt scale.
In Step 3
-All of the material that crosses the belt scale must be delivered to the truck. Ideally there is a useable
divert mechanism at the end of the scale conveyor so that the truck can be loaded directly. All chutes
between the scale and the truck should be clean before the sample is loaded and cleaned into the truck at
the end of the sample.
It is generally not accepted practice to move the scale after calibration. If its absolutely necessary to move
the scale in order to calibrate, it should at least be at the same slope as it will be in its operating position.
Again, this is not an accepted practice and should be avoided whenever possible.
Special precautions must be observed if the material must be run through the drum in order to load the
truck. The most successful procedure is a midstream sample. In doing this each test must be done in
exactly the same manner. The stockpile material must be uniform and as dry as possible. A stopwatch
should be used to time out an exact 15 minute interval after the feeders stop at the end of a sample run.
At exactly 15 minutes after the feeders stop, the drum and material hot elevator should be stopped. The
silo, batcher etc. should be completely emptied and the truck then weighed. The first test at any given
rate should be done precisely but the results ignored. If everything is constant, the material remaining in
the system after this run will be the same amount remaining at the end of the next actual calibration run.
To the extent that this is true, the calibration is therefore accurate and repeatable.
Obtain as large a sample as possible. Accurate belt scale calibration usually requires loads of at least 15
TONS. Asphalt meter calibrations to a 1% limit require loads of 1000 gallons if weighed on a truck scale
with 20 lb. grads. For meter calibrations to 0.25x, loads of 4000 gallons are required. The effect of minor
imperfections in the procedure or equipment can be minimized by the use of larger loads.
page F - 112
SYSTEMS Equipment Corp. 8/31/89 Page (63)