Another common method of obtaining increased air flow capacity a given size     air - oil separator is to manufacture the element in so-called “deep filter” construction. This form of separator uses as many as three different grades of borosilicate glass, wrapped onto the support tube in larger than normal amounts. The finished product then achieves increased air flow capacity very similar to that of the pleated air - oil separator, while at the same time maintaining low initial pressure and residual oil content. Field testing of this element has demonstrated an improvement in oil separation especially in high “ challenge rate” applications, in which more oil aerosol is contained in the compressed air than is normal.