By using a mesh screen, it is possible to produce much smaller hole sizes than with perforated screens. Hole sizes as small as 0.07 mm are achievable. Subsequently, they are used to remove smaller particles which would otherwise pass through a perforated screen. Mesh screens are usually specified in terms of 'mesh'; which represents the number of openings per linear inch of screen, measured from the centre line of the wire. Figure 12.4.6 shows a 3 mesh screen.