AS 2985:2009 Workplace atmospheres—Method for sampling and gravimetric determination of respirable dust
6.7.2 Microbalance calibration
The following requirements and recommendations apply:
(a) The microbalance shall be calibrated every three years, using the following principles, by either—
(i) a suitable accredited testing authority; or
(ii) competent staff employing test methods as described by Morris and Fen.
(b) Tests for departure from nominal linearity shall be conducted for at least 5 and preferably 10 points from zero up to not more than 50 or 100 mg. If the measured departure from nominal linearity exceeds the expanded uncertainty of the reference weights, the balance should be serviced, or appropriate corrections should be made.
(c) Hysteresis and, where applicable, off-centre pan loading shall be conducted using a weight in the range of 50 to 100 mg. Any hysteresis and off-centre effects greater than the scale of the balance should be rectified by servicing, or by an appropriate change in operating procedures.
(d) Repeatability tests on the microbalance shall be conducted with a weight not smaller than around the working range used for weighing filters, and not greater than 50 mg,every six months, e.g. in accordance with Morris and Fen.
(e) A limit of performance shall be calculated whenever a full calibration of the balance is performed.
7 FLOW RATE DETERMINATION
The sampling pump battery should be re-charged or the dry cells replaced and, if necessary, the pump should be run for up to 15 min in a clean atmosphere immediately before flow rate determination. It is advisable to stabilize the flow rate through a ‘warm-up’ sampler, which is not otherwise used, e.g. in calibration to reduce the risk of contamination. Some automatic flow control pumps require little or no settling-in period.
The pump flow rate should be adjusted accurately to the designated flow rate (see Table 2) using a calibrated flowmeter (see Clause 6.5). Immediately prior to determining the flow rate, disconnect the warm-up sampler and connect the sampling train to be used in the field. The calibration equipment and technique should be of such accuracy that the flow rate can be measured to within ±5%.