AS NZS 1301.441:2018 Methods of test for pulp and paper
B.1.3.1 Make sure that the instrument is level on a surface free from vibrations. Make sure that the internal adjustment of the flowmeter has been carried out according to the manufacturer’s instructions.
B.1.3.2 Prior to performing the following calibration procedure, the variable‑area flow tubes and the air leak adjustment valves on the older style instruments shall first be calibrated to Sheffield units using calibration orifices provided by the manufacturer. In operation, the instrument shall always be checked against the manufacturer’s calibration orifices, prior to using the flow tube, to ml/min calibration graph or chart.
To calibrate a variable‑area flow‑measuring device, disconnect the test assembly from the downstream end of the rubber or plastic tubing and connect the soap-bubble meter in its place. Set the valves to deliver air through the flowmeter to be calibrated and then through the soap‑bubble meter. Adjust the needle valve to give a conveniently measurable air flow and ensure that the flow rate remains constant. Rapidly squeeze the rubber bulb at the bottom of the volumeter so that a soap bubble enters the volumeter tube. The volumeter range should be chosen so that the time taken for the bubble to pass from the first to the second graduation is longer than 30 s.
Record the reading, x, on the flowmeter scale and record the time, t, in seconds for the soap bubble to pass through the volume, V.
Repeat the procedure at about six different air flow rates distributed over the upper 80 % of the flowmeter measurement range.
Record the atmospheric pressure, p.
NOTE  A better calibration may be achieved by leaving the measuring head in position and using a calibration plate device such as that used with an electronic flow‑measuring instrument, since this set‑up includes the restriction within the measuring head.