The ability to remove impurities from the air passing through the filtration media is largely dependent on how long the air remains in contact with the media while passing through the filter stack, known as dwell time. A longer dwell time in the filter is an effective way of increasing contact time, and this is proportional to the pressure of the air in the filter housing. By using a back pressure valve the air always takes approximately the same time to pass through the filter and filtration is consistent (assuming a consistent operating speed). The back-pressure valve is usually set to near the working pressure of the compressor to ensure that the air is compressed sufficiently for the filters to work effectively.