Maintenance of Laboratory Instruments , For ensuring the good working of a measuring instrument, not only its calibration but also its repeatability should also remain unaltered or within a specified range.
The standard deviation square root of variance is a good measure of repeatability of an instrument. Hence continuous monitoring of its variance is to be carried out.
Variances, preferably of same number of observations, for ease of calculations, at prescribed regular intervals of time, are found out. The progressive mean of the variances, i.e. the pooled variance, is calculated. For a good working instrument Fisher’s F test is applied. The ratio of the new variance with its pooled variance should not exceed the value of F tabulated for appropriate degrees of freedom at
a prescribed level of significance (say 5%). Degree of freedom of new variance would be one less than the number of observations for which the variance has been calculated; i.e. for pooled variance degree of freedom is
Further a graph of all previous values of standard deviations should also be drawn if it shows an upward trend, and if the new value exceeds a certain specified value then instrument may either be discarded or downgraded for less precise work.
Source of Maintenance of Laboratory Instruments: measurement uncertainty
