A suction chamber is designed to provide a trough of low pressure between the sealed area and the intake, so that air that would otherwise be drawn into the sealed area, through fissures and pores surrounding the seal, is drawn instead into the chamber. If the pressure within the sealed-off mine area is less than that outside, it is necessary to reduce the pressure within the chamber. This can be done by using an ejector or fan to draw air through a pipe in the outer wall of the chamber; a second pipe, fitted with a control valve, serves as an intake. Adjustment of the valve gives regulation of the chamber pressure within fine limits.