SM0486
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.
133
Previous Page
