Pneumatic gauging circuit

Claim 1

A pneumatic circuit for pneumatic gauging, said pneumatic circuit comprising an inlet to which compressed air is supplied, a first branch passageway connected to said inlet, said first branch passageway having a first outlet, a second branch passageway connected to said inlet, said second branch passageway having a second outlet, a first needle valve adjustable control orifice in said first branch passageway between said inlet and said first outlet defining a first pressure chamber between said first control orifice and said first outlet, a second needle valve adjustable control orifice between said inlet and said second outlet defining a second pressure chamber between said second control orifice and said second outlet, a differential pressure transducer having a first port connected to said first chamber and a second port connected to said second chamber, said differential pressure transducer being adapted to provide an electrical output signal to a measurement display instrument as a function of the pressure differential between said chambers, said first and second adjustable orifices being adjustable for effecting respectively sensitivity and datum scale settings of the measurement display instrument, a first flow orifice connected to said first outlet, and a second flow orifice connected to said second outlet, wherein at least said first flow orifice connected to said first outlet is a variable flow first nozzle whose flow rate is dependent upon the distance separating said first nozzle from a surface causing the pressure of the air circulating in said first chamber to vary as a function of the air flow through said first nozzle and said second flow orifice connected to said second outlet is a calibrated orifice fitting having a fixed area and venting said second pressure chamber to the ambient for effecting measurements by means of said variable flow first nozzle, said calibrated orifice fitting being replaceable by a variable flow second nozzle whose flow rate is dependent upon the distance separating said second nozzle from a surface causing the pressure of the air circulating in said second chamber to vary as a function of the air flow through said second nozzle for effecting differential measurements by means of said variable flow first and second nozzles.