Abstract
Precise measurements of small mass less than 100 mg are usually achieved by means of weights of small mass calibrated against standard weights and a balance of high precision. It is however rather difficult to maintain the weights for long periods securing their accuracy better than 1 μg. For such fine measurements of mass it is desirable to produce a mass standard as a measure of certain physical property of material which relate to the mass with good stability.
A quartz torsion balance of Rodder Model E (Microtech Services Co., California, U. S. A.) which has very high sensitivity and is successfully used in micro- and ultramicro-chemical analyses has been taken up for the foregoing purpose. A satisfactory reproducibility of readout less than 0.1 μg together with a long term stability of the sensitivity supposedly within a range of 0.15%/10 years was ascertained as a result of statistical study of the dynamic properties of the torsion balance. Effects of ambient temperature and loading mass against the sensitivity of the torsion balance were also investigated.
An empirical equation for the sensitivity S, defined. as the ratio of the readout vs. the mass to be measured, of the torsion balance used was obtained as a function of the temperature t and the loading mass M, as follows,
S =2.32504{ 1-α(t-20)} (1-βM) in div. μg-1
where α and β were the temperature and the loading mass coefficient of the sensitivity whose values were empirically evaluated as 1.31 × 10-4 deg-1 and 5.45 × 10-6 mg-1, respectively. A slight decrease in the sensitivity was found either with a rise of the temperature and an increase of the loading mass.
The accuracy of absolute determination of the mass was within 0.01% of the given mass under the torsion range of 1 to 7 mg, while the detectable limit for the mass was 0.1 μg throughout the loading mass of 0 to 100 mg.
By a help of the sensitivity as a stable parameter for the mass measurement, there was found a possibility of using the quartz torsion balance as a working standard within the torsion range of 7 mg.
The precision of the balance was also ascertained as 0.1 μg or less by Borda's tests comparing three homonominal weights up to 100 mg.
