Void counting in assessing membrane filter samples of asbestos fibre

Fibre counting is an onerous and time consuming task. The blank field method has been proposed as a means of easing that task. In this, successive microscope graticule areas are scanned and note is made of those void of fibres. At some defined stopping point the fibre density is estimated from a function of the total number of voids and total areas scanned. This approach was used in two planned experiments to investigate the bias, the variability, and the time savings of the method compared with the standard technique of fibre counting (ARC, 1971). The results indicated that, though the blank method led to substantial time savings, these were offset by an equivalent increase in variability. Only for a narrow range of fibre density was the blank method superior to the standard technique. Nor did the method lead to better inter-counter agreement. An experiment which involved using the standardized method to count just 30 graticule areas rather than 100 led to equivalent time savings to the blank field method but without an increase in variability