PRINCIPLES OF NAVAL ENGINEERING 



- SEALED END 

 (FREE TO MOVE) 



BOURDON 

 TUBE 



TIP 

 TRAVEL 



STATIONARY 

 SOCKET 



PRESSURE 

 CONNECTION 



38.211(147B) 

 Figure 7-9.— C-shaped Bourdon tube. 



pressure thermometers; the distinction deals 

 with the major response of the Bourdon tube. 



Pyrometers 



The term pyrometer is used to include a 

 number of temperature measuring devices 

 which, in general, are suitable for use at rela- 

 tively high temperatures; some pyrometers, 

 however, are also suitable for use at low tem- 

 peratures. The types of pyrometers we are 

 concerned with here include thermocouple 

 pyrometers, resistance thermometers, radia- 

 tion pyrometers, and optical pyrometers. 



THERMOCOUPLE PYROMETERS.-The op- 

 eration of a thermocouple pyrometer (sometimes 

 called a thermoelectric pyrometer ) is based on 

 the observed fact that an electromotive force 

 (emf)lO is generated when the two junctions of 

 two dissimilar metals are at different tempera- 

 tures. A simple thermocouple is illustrated in 

 figure 7-10. Since the electromotive force gen- 

 erated is proportional to the temperature dif- 

 ference between the measuring junction (hot 



junction) and the reference junctions (cold junc- 

 tions), the indicating instrument can be marked 

 off to indicate degrees of temperature even 

 though it is actually measuring emf's. The indi- 

 cating instrument is a millivoltmeter or some 

 other electrical device capable of measuring 

 and indicating small direct-current emf's. The 

 strips or wires of dissimilar metals are welded, 

 twisted, fused, or otherwise firmly joined to- 

 gether. The extension leads are usually of the 

 same metals as the thermocouple itself. 



METAL A 



REFERENCE JUNCTIONS 

 (COLD JUNCTIONS) 



METAL 8 



INDICATING 

 INSTRUMENT 

 (MILLIVOLTMETER, 

 POTENTIOMETER, 

 ETC.) 



EXTENSION 

 LEADS 



147.55 



Basic information on electricity is given in chapter 

 20 of this text. 



Figure 7-10.— Simple thermocouple. 



RESISTANCE THERMOMETERS. -Resist- 

 ance thermometers are based on the principle 

 that the electrical resistance of a metal changes 

 with changes in temperature. A resistance ther- 

 mometer is thus actually an instrument which 

 measures electrical resistance but which is 

 calibrated in degrees of temperature rather than 

 in units of electrical resistance. 



The sensitive element in a resistance ther- 

 mometer is a winding of small diameter nickel, 

 platinum, or other metallic wire. The resistance 

 winding is located in the lower end of a bulb 

 (sometimes called a stem ); it is electrically but 

 not thermally insulated from the stem. The re- 

 sistance winding is connected by two, three, or 

 four leads to the circuit of the indicating instru- 

 ment. The circuit is a Wheatstone bridge or some 

 other simple circuit which contains known re- 

 sistances with which the resistance of the ther- 

 mometer winding is compared. 



RADIATION AND OPTICAL PYROMETERS.- 

 Radiation and optical pyrometers are used to 

 measure very high temperatures. Both types of 

 pyrometers measure temperature by measuring 

 the amount of energy radiated by the hot object. 

 The main difference between the two types is in 

 their range of sensitivity; radiation pyrometers 

 are (theoretically, at least) sensitive to the 



134 



