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c. The numbers directly to the right of the terms indicate the conversion

between adjacent units, while the number to the far right indicates the number of

grains per pound. Notice that the basic difference between the troy and apothecary

systems is the terms employed to subdivide the pound.

The troy and apothecary

systems are used very little and restricted to highly specialized fields.

The

preceding list of values indicates the necessity to identify the system of units

used for each measurement.

d. If 1 grain is equal to 15.432356 grains as stated, we can check the

relationships of the value listed for the different weight measuring systems.

Examples:

(1) In

the

avoirdupois

system,

27.343

grains

x

16

grams

equals

approximately 437 grains per ounce. When you divide 437 grains by 15.432356 (one

gram), you find that 1 avoirdupois ounce is equal to approximately 28.3 grams per

ounce.

Multiplying 28.3 grams per ounce by 16 ounces per pound, you find that

there are approximately 452 grams in 1 pound. (AVOIRDUPOIS)

(2) In the troy system, 24 grains x 20 pennyweights equals approximately

480 grains per ounce. When you divide 480 grains by 15.432356 grains (one gram),

you find that 1 troy ounce is equal to approximately 31.1 grams per ounce.

Multiplying 31.1 grams per ounce by 12 ounces per pound, you find that there are

approximately 373 grams-in one pound. (TROY)

e. For your convenience Table 2 lists conversion factors for most of the

common mass units. You should be able to use the values listed to convert from one

system to another. Let's continue our study of mass measurement theory associated

with analytical balances by examining two considerations which are factors in the

accuracy of measurements made with analytical balances.

These considerations are

buoyancy and sensitivity.

f. Mass Measurement Considerations. Although the material presented in this

section concerns the construction and operation of analytical balances, the

discussion of buoyancy and sensitivity which follows applies to any other mass

measurement device.

g. Buoyancy. The lifting effect which air has on a body is considered when

standard masses are used or calibrated. Any body immersed in a fluid or suspended

in air is buoyed up by a force equal to the weight of the displaced fluid or air.

Because of this buoyant force, exact numerical values of the apparent mass of

standards used with a typical analytical balance are based on specific values of

air density and the density of the standard mass.

h. The density of air depends on temperature, pressure, and humidity.

From

specified values of these factors, standard conditions for air are specified as 1.2

milligrams/cm3.

However, because the density of a given material is a factor in

buoyant force determinations, the comparison of weights of different densities

requires the calculation of a buoyancy correction.

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