Some classes of weights have provisions for adding or
removing small amounts of weight for adjustment purposes. Class S and S-1 weights
have removable knobs which cover a small chamber within the weight. Small filings
or shot added to or removed from the chamber are used to trim the weights to
maintain tolerance such as those in the preceding list.
Weights of one class
generally are supplied in sets such as those shown in Figure 2.
Weight sets are
delivered in various denomination sequences; however, the most common acceptable
sequences are: 5-3-2-1, 5-2-2-1, or 8-4-2-1.
The sequences listed are in the
descending order. In some instances you may see them identified in the ascending
h. The usual 1-2-2-5 weight arrangement and the recently established 1-2-3-6
sequences are related to a binary-coded-decimal (BCD) sequence. In some instances
a true binary sequence has replaced the BCD sequence because the use of four
weights in 16 possible combinations expands their range by a factor of 1.6 without
additional decoding problems. The consideration of BCD versus binary coded systems
is a matter of decreasing the search-steps (trial and error weight matching) to a
minimum, and at the same time reducing the maximum number of weights required.
Weight measurements are subjected to errors caused by
humidity, absorption, desorption, wear, influence of electrostatic charges, and
temperature effects. These effects tend to cause temporary or permanent changes in
the value of the standard weights, thereby producing weight measurement errors. In
an effort to reduce the errors caused by the changes in materials, large mass
standards are usually restricted to the following material:
Platinum-iridium (90% - 10%)
Stainless steel (25% chromium, 20% nickel)
Brass-plated with rhodium, platinum, or chromium.