the case of a non-metric weight, the metric equivalent must be calculated and then
the dials set accordingly. For example, in the case of a 5 lb. weight, the metric
equivalent would be 2267.962 grams. Set the weight control knobs to read 2267.
Table 2 in lesson 1 shows the metric equivalent of the commonly used weights.
(4) Loading the weights to be measured.
Place the standard weight as closely centered on the pan as possible and close the
door. Release the beam arrest knob (6) turning counterclockwise. (Note that the
arrow is no longer visible and the indicator lamp is on when the beam is released).
Observe the indication on the display. It should be pointed out that neither the
indication for the standard nor the weight under test should be at zero so that
negative numbers do not enter into the calculation. It is frequently convenient to
bring the indication to mid-scale if the weight under test is substantially different
from the standard. This can be done most easily by placing a weight on the pan
sufficient to bring the indication to some convenient number on the display. This
weight (usually between 200 mg and 800 mg) must be left on the pan during the
entire measurement. It should be removed when the balance is zeroed. Remove
the standard weight and place the test weight on the pan. Again release the beam
and observe the reading. If both readings are within the on-scale range, weighing
can begin.
(5) Precaution.
When placing weights on the pan either individually or in summations, care
should be taken that the weights are approximately centered so that the hanger is
nearly vertical. Also, good practice would indicate that weight lifters be long
enough to place the weight on the pan without introducing any substantial
quantity of heat from the hands of the operator. As a safety precaution always use
tongs or tweezers when moving weights.
h. Weightings.
(1) The general procedure for precision weighing is a series of double
substitutions. Complete instructions for the technique will be provided by the
appropriate metrology center. In general, a double substitution consists of the
following four weightings performed in order.
1. The standard weight.
2. The test weight alone.
3. The test weight with a sensitivity weight.
4. The standard weight plus the sensitivity weight, used in step 3.
(2) The following format is recommended for taking the data.
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