added determines the amount of weight the TI is over or under the specified weight. For
example, the test instrument is compared with a ten pound standard. When placed on the
pans the pointer indicates the TI is lighter than the ten pound standard. Then weights are
placed on the same pan as the TI to equalize the balance or bring it to its rest point. The
amount of weight that was added to the TI would indicate how much less the unit under
test weighed compared to ten pounds. If four ounces of weight were added then this
would indicate the TI weighed four ounces less than ten pounds or nine pounds and
twelve ounces.
b. The next method of measuring weight is called the substitution method. While not
being the most accurate it is more accurate than the direct method. This method is
accomplished by placing the TI on one tray or pan and the standard on the other. After
allowing the balance to oscillate at least twice, the procedure is followed to establish the
rest point. Once the rest point has been established and a notation is made, the TI is
removed. Now this is where the word substitution is applied. After the TI has been
removed, it is replaced with standard weights. Enough standard weights are added to
return the balance to the original rest point. When this has been completed the sum of the
standard weights used to replace the TI will represent the total weight of the TI.
c. The last of the three methods used is the transposition method. This method is the most
accurate. It takes a little longer than measuring with the direct or substitution methods,
but one weighing with the transposition method is equal in accuracy to the average of
four substitution method weightings. This method is accomplished by placing the
standard on one pan and the TI on the other pan. Weights, if required, are added to the
lighter side to bring the pointer within the range of the index. Then, as before, we follow
the procedure for establishing the rest point. Once the rest point has been established we
can continue with our measurement. The test instrument and the standard are then
interchanged but the small weights that were added remain on the same pan. Once the
weights have been interchanged then we will again have an unbalance and more weights
will have to be added to the other pan in order to return the balance to the rest point. The
weights that are now added to return the balance to the rest point are totaled and divided
by two to determine the difference in mass between the standard and the TI. Whether the
test instrument was lighter or heavier than the standard determines whether or not this
weight should be subtracted or added to the standard weight to obtain the weight of the
TI.
8. Summary.
This lesson continues the instruction in mass measurements. You should now be able to explain
the three methods of mass measurements, define the three balances used, be able to explain the
process for calibrating weights used at either secondary reference or transfer level to include the
adjustments for any weights found out of tolerance.
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