Chapter 7-PRINCIPLES OF MEASUREMENT 



PRESSURE. 

 BEING- 

 MEASURED 



HEIGHT 

 OF ► 



MERCURY 

 COLUMN 



fl- 



-VACUUM 



MERCURY 



7-17, 7-18, and 7-19 has only one Bourdon tube 

 and measures only one pressure. (The pointer 

 marked RED HAND in figure 7-17 is a manually 

 positioned hand that is set to the normal working 

 pressure of the machinery or equipment on 

 which the gage is installed; the hand marked 

 POINTER is the only hand that moves in re- 

 sponse to pressure changes.) 



When two Bourdon tubes are mounted in a 

 single case, with each mechanism acting inde- 

 pendently but with the two pointers mounted on 

 a common dial, the assembly is called a duplex 

 gage. The dial of a duplex gage is shown in 

 figure 7-20. The two Bourdon tubes and the 

 operating mechanism are shown in figure 7-21, 

 and the gear mechanism is shown in figure 7- 

 22. Note that each Bourdon tube has its own 

 pressure connection and its own pointer. Duplex 

 gages are used to give simultaneous indication 

 of the pressure at two different locations. 



Bourdon-tube vacuum gages are marked off 

 in inches of mercury, as shown in figure 7-23. 



147.57 

 Figure 7-15.— U-tube liquid-column element for 

 measuring absolute pressure. 



_i en, 



RED HAND 



POINTER 



B 



61. 4X 

 Figure 7-16. — Two types of manometers. 

 (A) Standard U-tube. (B) Single-tube. 



38.211BX 

 Figure 7-17.— Dial of a simplex Bourdon-tube 

 pressure gage. 



139 



