Filter Flasks are thick-walled and the 1, 2, and 4L sizes have side tubulation for 3/8in (9.5mm) ID vacuum tubing. The 2 and 4L sizes are often used as pycnometers, and 1L flasks are often used in line to form a vapor trap for vacuum pump protection. GW-76 meets ASTM D 2041 and AASHTO T 209 specifications as a vacuum container for weighing in air only. Neoprene one-hole stopper with 3/8in (9.5mm) OD. 3/8in Reinforced Vinyl tubing will not collapse under vacuum.