[Weighing precision of filters used in dust measurements: repeatability and reproducibility]

Measurements of inhaled and respirable dust concentrations in the work environment are most frequently performed by employing the method of the air filtration with analytical filters placed in measuring heads, which is followed by the mass determination of the dust precipitated on the filter respective to the volume of the filtrated air. A twofold weighing of filters is thus the basic measurement technique used to determine the dust mass. It should be emphasized that the weighing precision decides about the precision of measured inhaled and respirable dust concentrations.

The weighing precision of FiPro polypropylene filters, commonly used in industrial dusts measurements in Poland, was assessed. Ten filters of phi 25 mm and 37 mm were placed in open containers and desiccators with constant air humidity of 5% in weighing room. After 24 h, filters were weighed five times at one-week intervals. Weighing was performed by two persons on two balances (0.01 mg); 21 accredited laboratories participated in inter-laboratory comparisons; each of them weighed twice 5 filters of phi 25 mm and 37 mm kept in desiccators for 24 and 48 h.

Standard deviation of repeatability was calculated--mass Sr of filters of phi 25 mm equal to 0.016 mg was 1.4 times lower than mass Sr of filters kept in an open container, whereas mass Sr of filters of phi 37 mm from desiccator, also equal to 0.016 mg, was 1.6 times lower than mass Sr of filters from an open container. Mean standard deviations of reproducibility, S(R), calculated on the basis of the results of inter-laboratory studies (after rejecting significantly departing values) were 0.024 mg and 0.022 mg for filters of phi 25 mm after keeping them in desiccator for 24 and 48 h, whereas these values accounted for 0.058 mg and 0.033 for filters of phi 37 mm.

The results of the study in conditions of repeatability confirm high mass stability of FiPro polypropylene filters. It may be elucidated from inter-laboratory comparisons that keeping FiPro filters in an desiccator for at least 48 h before and after collection of air samples will ensure the highest precision of their mass determination, and thus the highest precision of determinations of inhaled and respirable dust concentrations.