Disks-Each tube is provided with a perforated cylindrical disk 15.3 ± 0.15 mm thick and 31.4 ± 0.13 mm in diameter The disk is made of a suitable transparent plastic material having a specific gravity of between 1.18 and 1.20.Seven 3.15 ± 0.1 mm holes extend between the ends of the cylinder one of the holes being through the cylinder axis and the other parallel with it and equally spaced on a 4.2-mm radius from it. All surfaces of the disk are smooth