PRECISION GAS FLOW CONTROL BY USE OF COARSE AND FINE CONTROL ELEMENTS

摘要

Precision gas flow control (10') is provided utilizing a first coarse gas flow control element (12) and a second fine gas flow control element (27). The second element is selected and operated to pass only a fraction of the flow passed by the first element. The elements may be connected either in series or in parallel so that at least a portion of the gas flow passes through each flow control element. Control circuitry (33) is also provided which may be responsive at least in part to an indication of desired gas flow through the device for controlling the first element to provide a coarse gas flow adjustment and for controlling the second element to provide a fine gas flow adjustment.