Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGBOURDONTUBE- SEALED END(FREE TO MOVE)TIPTRAVELjunction) and the reference junctions (cold junctions),the indicating instrument can be markedoff to indicate degrees of temperature eventhough it is actually measuring emf's. The indicatinginstrument is a millivoltmeter or someother electrical device capable of measuringand indicating small direct-current emf's. Thestrips or wires of dissimilar metals are welded,twisted, fused, or otherwise firmly joined together.The extension leads are usually of thesame metals as the thermocouple itself.PRESSURECONNECTIONSTATIONARYSOCKET38.211(147B)Figure 7-9.— C-shaped Bourdon tube.pressure thermometers; the distinction dealswith the major response of the Bourdon tube.PyrometersThe term pyrometer is used to include anumber of temperature measuring deviceswhich, in general, are suitable for use at relativelyhigh temperatures; some pyrometers,however, are also suitable for use at low temperatures.The types of pyrometers we areconcerned with here include thermocouplepyrometers, resistance thermometers, radiationpyrometers, and optical pyrometers.THERMOCOUPLE PYROMETERS.-The operationof a thermocouple pyrometer (sometimescalled a thermoelectric pyrometer ) is based onthe observed fact that an electromotive force(emf)lO is generated when the two junctions oftwo dissimilar metals are at different temperatures.A simple thermocouple is illustrated infigure 7-10. Since the electromotive force generatedis proportional to the temperature differencebetween the measuring junction (hotBasic information on electricity is given in chapter20 of this text.METAL AMETAL 8REFERENCE JUNCTIONS(COLD JUNCTIONS)EXTENSIONLEADSINDICATINGINSTRUMENT(MILLIVOLTMETER,POTENTIOMETER,ETC.)147.55Figure 7-10.— Simple thermocouple.RESISTANCETHERMOMETERS. -Resistancethermometers are based on the principlethat the electrical resistance of a metal changeswith changes in temperature. A resistance thermometeris thus actually an instrument whichmeasures electrical resistance but which iscalibrated in degrees of temperature rather thanin units of electrical resistance.The sensitive element in a resistance thermometeris a winding of small diameter nickel,platinum, or other metallic wire. The resistancewinding is located in the lower end of a bulb(sometimes called a stem ); it is electrically butnot thermally insulated from the stem. The resistancewinding is connected by two, three, orfour leads to the circuit of the indicating instrument.The circuit is a Wheatstone bridge or someother simple circuit which contains known resistanceswith which the resistance of the thermometerwinding is compared.RADIATION AND OPTICAL PYROMETERS.-Radiation and optical pyrometers are used tomeasure very high temperatures. Both types ofpyrometers measure temperature by measuringthe amount of energy radiated by the hot object.The main difference between the two types is intheir range of sensitivity; radiation pyrometersare (theoretically, at least) sensitive to the134