Section II. MEASUREMENT
Precise measurement in the compounding of pharmaceuticals is an absolute
necessity if the medication is to be beneficial rather than harmful to the patient.
Physicists make the following distinction between mass and weight: Mass is the
quantity of matter; weight is the force exerted upon a quantity of matter by gravity.
Since the force of gravity is not constant at all altitudes and locations, mass is the better
term for specifying a quantity of matter. Nonetheless, we often use the word "weight" in
the pharmacy to refer to mass. The mass of matter is determined by comparing it to a
standard body, using a balance that cancels the effect of gravity.
a. The Balance. The balance (see figure 1-2) is the apparatus used for
determining mass. It is NOT a scale. A scale measures weight by the use of springs
and is influenced by gravity. Weights determined by scales vary from location to
location. For example, with a spring scale, a body weighing 1,000 grams (g) in Panama
will weigh 1,004 g in Alaska. With a balance, however, a body with a mass of 1,000 g in
Panama will have the same reading in Alaska. In its simplest form, the balance consists
of a horizontal beam supported in the center. From each end of this beam, a pan is
suspended to accommodate the substance to be weighed and the counterbalance. A
pointer and a graduated scale are attached to determine when the pans are in balance.
Prescription balances are designed to provide the degree of accuracy required for
prescription compounding and the manufacture of medicinals. The desired capacity
should permit weighing up to 120 g. In actual practice, we do not try to accurately
weigh quantities less than 100 mg on a prescription balance, nor do we load the
delicate mechanism with weights approaching 120 g. The two most commonly
accepted brands of prescription balances are the Torsion Prescription Balance and the
Troemner Prescription Balance. Both are acceptable by pharmacy law as being of
suitable accuracy and are classified as Class A balances, used for weights less than
120 g. (Larger weights are weighed on less sensitive but accurate balances called
Class B balances. Class B balances are optional in the pharmacy.)
b. Testing a Class A Balance. The rest point is the reading of a balance after
fluctuation of the pointer has stopped. To test a balance, find the rest point when the
pans are empty. Then, using proper weighing techniques, set a 10-milligram (mg)
weight on only one of the pans. The new rest point should be at least one division
removed from the original rest point. The second step is to place one 10-mg weight on
the remaining empty pan and find the rest point with a 10-mg weight on each pan. Now
we add an additional 10-mg weight to one of the pans, so that we now have 20-mg on
one pan and 10-mg on the other. The new rest point should be at least one division
removed from the rest point obtained with one 10-mg weight on each pan.