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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