In the case of air-oil separators of a given size, another common method of obtaining increased air flow is the so-called“Depth filter” structure. This type of separator uses up to three different grades of Borosilicate glass, which are wrapped around the support pipe and are larger than normal. The finished product, while maintaining low initial pressure and low residual oil content, can achieve increased air flow capacity, very similar to the pleated air-oil separator. Field tests of the element have demonstrated improved performance in oil separation, particularly in high “Challenge rate” applications where there is more oil than normal in the Compressed air.