The capsule is formed by a rigid wall (1,8) contg. the inlet and outlet (11,12) and a moving one formed by a diaphragm (13). Diaphragm movement is by an actuating member (17) and it reciprocates between a domed position, in which capsule volume is minimal and a retracted one where it is maximal. The rigid wall has a concave surface, against which the diaphragm bears over its entire surface in the domed position. A pneumatic system generates pressure in the chamber behind the diaphragm with this in the domed position with each stroke, and releases it on the return stroke.
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