possibly the ambient of the processing chamber even though the reaction plate 154 acts as a barrier between the two the rotation mechanism the compression and biasing mechanism and the cooling supply lines for the gate valve 140 may be adapted from the gate valves of fig1 8 it is possible to place the o ring in a groove formed in the upper flange 14 on the cool side of the valve to engage and seal a planar sealing plate 152 however in the open condition of the valve the o ring is then exposed to the hot crucible and to the growing boule passing by it but in cooler applications such a placement would be effective the valve of the invention is not limited to czochralski growth systems and may be used in other applications further although the above description emphasizes the reduced pressures or vacuum of the two chambers connected by the valve 10 the valve 10 may be applied to systems in which one or both of the chambers is subjected to significant positive pressures in this case for the gate valve of fig5 the strength of the spring 106 and the pressure of the pneumatic source may need to be increased to seal against the positive pressure in front of the valve plate 56 it is appreciated that the expandable gate of the various embodiments can be easily adapted for use in a shuttle valve in which an expandable gate moves linearly in a direction transverse to the passageway between a retracted position and a blocking position and is expanded in place at the blocking position that is the arm.