| http://www.w3.org/ns/prov#value | - control pressure chamber, g. a source of vacuum and a source of pressure, and h. power means operable to connect said control chamber to said source of vacuum or to said source of pressure, whereby a vacuum or pressure in said control chamber may be applied to said valve element to open or close said valve depending upon whether there is a vacuum or a pressure applied to said valve element. 7.
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