Q.What kinds of things should we be careful of to ensure that measurement is performed with good precision?
You must first ensure that the balance is installed correctly according to the Q&A item above. As general precautions, further attention must be paid to the following points as well, though no generalizations can be made owing to the type of sample or required precision.
|Between 60 to 80% humidity is appropriate. If humidity falls to 50% or below, the sample is more likely to be charged with static electricity, and the attraction and repelling force of static electricity sometimes causes the indication to become unstable. Alternatively, if humidity is high, some samples absorb moisture, which will influence measured values. And, if humidity is too high, condensation might cause measurement error or cause the electrical circuits to malfunction.
|Influence of static electricity:
|If humidity falls to 50% or below, the sample is more likely to be charged with static electricity, and the attraction and repelling force of static electricity sometimes causes the readingto become unstable. Attention is required with plastic, glass and filter paper made of synthetic resins, for example, since they can be easily electrostatically charged. The effect of this can often be avoided by wrapping the sample or container in aluminum foil or other conductive material. Users can also try using apparatus for eliminating static electricity that are commercially available, though they do not well work on some samples. If humidity is too low, placing a cup filled with water in the weighing chamber (in the case of an analytical balance) is effective in raising humidity.
|Influence of moisture
|Attention is required when measuring samples that easily absorb moisture or volatilize/evaporate, especially, when measuring down to 1 mg or less. One measure is to use a covered sample container or a container with a small-diameter opening.
|Influence of magnetism:
|When measuring magnetized substances, the readingsometimes changes according to the position where the sample is placed. An effective measure against this is to place a non-magnetic spacer (e.g. beaker, aluminum container) on the weighing pan of the balance to ensure space between the sample and the pan. On models that allow below-balance weighing, measurement with the sample suspended also is effective.
|Influence of air density:
|When measuring only small amounts of sample in a beaker, an error occurs due to the density difference of air if the temperature in the weighing chamber differs from that of the air in the beaker. Also, in such cases, the indicated value sometimes drifts due to expansion and contraction when air in the beaker acclimatizes to the surrounding temperature.