Chapter 7— PRINCIPLES OF MEASUREMENT 



at one end of a small-bore capillary tube, con- 

 nected at the other end to a Bourdon tube or other 

 device which responds to volume changes or to 

 pressure changes. The system is partially or 

 completely filled with afluid which expands when 

 heated and contracts when cooled. The fluid may 

 be a gas, mercury, an organic liquid, or a com- 

 bination of liquid and vapor. 



The device usually used to indicate tempera- 

 ture changes by its response to volume changes 

 or to pressure changes is called a Bourdon 

 tube. " A Bourdon tube is a curved or twisted 

 tube which is open at one end and sealed at the 

 other. The open end of the tube is fixed in posi- 

 tion and the sealed end is free to move. The 

 tube is more or less elliptical in cross section; 

 it does not form a true circle. The cross section 

 of a noncircular tube which is sealed at one end 

 tends to become more circular when there is an 

 increase in the volume or in the internal pres- 

 sure of the contained fluid, and this tends to 

 straighten the tube. Opposing this action, the 

 spring action of the tube metal tends to coil the 

 tube.^ Since the open end of the Bourdon tube 

 is rigidly fastened, the sealed end moves as 

 the volume or pressure of the contained fluid 

 changes. When a pointer is attached to the sealed 

 end of the tube through appropriate linkages, 

 and when the assembly is placed over an appro- 

 priately calibrated dial, the result is a Bourdon- 

 tube gage that may be used for measuring tem- 

 perature or pressure, depending upon the design 

 of the gage and the calibration of the scale. 



Bourdon tubes are made in several shapes 

 for various applications. The C- shaped Bourdon 

 tube shown in figure 7-9 is perhaps the most 

 commonly used type; spiral and helical Bourdon 

 tubes are used where design requirements in- 

 clude the need for a longer length of Bourdon 

 tube. 



There are two basic types of filled-system 

 thermometers: those in which the Bourdon tube 

 responds primarily to changes in the volume of 



8 



Bourdon tubes are sometimes called Bourdon springs, 

 Bourdon elements , or simply Bourdons . Other devices 

 such as bellows or diaphragms are used in some 

 filled-system thermometers, but they are by no means 

 as common as the Bourdon tube for this application. 

 9 



The precise nature of Bourdon-tube movement with 



pressure and volume changes is extremely complex 

 and not completely describable in purely analytical 

 terms. Bourdon-tube instruments are designed for 

 specific applications on the basis of a series of em- 

 pirical observations and tests. 



147.54 

 Figure 7-7.— Bimetallic thermometer 

 (flat spiral element). 



the filling fluid and those in which the Bourdon 

 tube responds primarily to changes in the pres- 

 sure of the filling fluid. Obviously, there is 

 always some pressure effect in volumetric 

 thermometers and some volumetric effect in 



61.28X 

 Figure 7-8.— Distant- reading Bourdon-tube 

 thermometer. 



133 



