Aerosol special oil PAO-4 (aerosol collagen solution, leak detector special oil) PAO (poly - alpha olefin) is made from ethylene through the polymerization reaction to α olefin, and then further polymerized and hydrogenated. It has good viscosity-temperature properties and low-temperature fluidity. PAO-4 will condense into small droplets under certain circumstances, with a counting median diameter of approximately 0.18um . In general, the leak detection of high-efficiency filters usually uses PAO-4 . After a sufficient concentration of aerosol occurs upstream of the filter, a photometer is used to detect the aerosol concentration upstream and downstream of the filter to determine whether the filter is leaking.