Principles of naval engineering - Historic Naval Ships Association

Chapter 7-PRINCIPLES OF MEASUREMENTPRESSURE.BEING-MEASUREDHEIGHTOFMERCURYCOLUMNfl--VACUUMMERCURY7-17, 7-18, and 7-19 has only one Bourdon tubeand measures only one pressure. (The pointermarked RED HAND in figure 7-17 is a manuallypositioned hand that is set to the normal workingpressure of the machinery or equipment onwhich the gage is installed; the hand markedPOINTER is the only hand that moves in responseto pressure changes.)When two Bourdon tubes are mounted in asingle case, with each mechanism acting independentlybut with the two pointers mounted ona common dial, the assembly is called a duplexgage. The dial of a duplex gage is shown infigure 7-20. The two Bourdon tubes and theoperating mechanism are shown in figure 7-21,and the gear mechanism is shown in figure 7-22. Note that each Bourdon tube has its ownpressure connection and its own pointer. Duplexgages are used to give simultaneous indicationof the pressure at two different locations.Bourdon-tube vacuum gages are marked offin inches of mercury, as shown in figure 7-23.147.57Figure 7-15.— U-tube liquid-column element formeasuring absolute pressure.RED HANDPOINTER_ien,61. 4XFigure 7-16. — Two types of manometers.(A) Standard U-tube. (B) Single-tube.B38.211BXFigure 7-17.— Dial of a simplex Bourdon-tubepressure gage.139