Chapter 7-PRINCIPLES OF MEASUREMENT 



38.211CA 

 Figure 7-22.— Gear mechanism of duplex 

 Bourdon-tube pressure gage. 



15 



10 



20 



25- 



38.211DX 



Figure 7-23. — Bourdon-tube vacuum gage. 



Differential pressure may also be measured 

 with Bourdon-tube gages. One kind of Bourdon- 

 tube differential pressure gage is shown in 

 figure 7-25. This gage has two Bourdon tubes 



Figure 7-24. — Compound 



38.211EX 

 Bourdon-tube gage. 



but only one pointer. The Bourdon tubes are 

 connected in such a way that it is the pressure 

 difference , rather than either of the two actual 

 pressures, that is indicated by the pointer. 



BELLOWS ELASTIC ELEMENTS. -A bel- 

 lows elastic element is a convoluted unit that 

 expands and contracts axially with changes in 

 pressure. The pressure to be measured can be 

 applied to the outside or to the inside of the 

 bellows; in practice, most bellows-type measur- 

 ing devices have the pressure applied to the 

 outside of the bellows, as shown in figure 7-26. 

 Bellows elastic elements are made of brass, 

 phosphor bronze, stainless steel, beryllium- 

 copper, or other metal suitable for the intended 

 service of the gage. 



Most bellows-type gages are spring-loaded— 

 that is, a spring opposes the bellows and thus 

 prevents full expansion of the bellows. Limiting 

 the expansion of the bellows in this way pro- 

 tects the bellows and prolongs its life. In a 

 spring-loaded bellows-type element, the deflec- 

 tion is the resultant of the force acting on the 

 bellows and the opposing force of the spring. 



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