Principles of naval engineering - Historic Naval Ships Association

Chapter 7-PRINCIPLES OF MEASUREMENTWhere special requirements exist, specialtypes of liquid-in-glass thermometers are used.For example, maximum and minimum indicatingthermometers are used in magazines aboardship, for weather observations, and for variousother applications where it is necessary toknow the highest and the lowest temperaturesthat have occurred during a certain intervalof time. One type of maximum indicating thermometeris shown in figure 7-4, and one typeof minimum indicating thermometer is shownin figure 7-5.The maximum indicatingthermometer shownin figure 7-4 is a mercury thermometer witha special constriction in the bore. When thetemperature rises, the pressure of the expandingmercury in the bulb forces mercury pastthe constriction in the bore. When the temperaturefalls, the mercury does not return to thebulb. Why? Even if the thermometer were in anupright position, the constriction in the borewould prevent the normal return flow of mercuryby gravity in addition, the maximum indicatingthermometer is mounted with the bulb a fewdegrees above the horizontal position, so that themercury column slopes downward from the constriction.Thus the thermometer always indicatesthe highest temperature that has beenreached since the instrument was last set. Expansionand contraction of the mercury in thebore above the constriction does occur withtemperature changes, but it is so slight as toCONSTRICTIONSUPPORTLOWER CAREFULLYTO VERTICALPOSITION TO READFigure 7-4.— Maximum indicating thermometer.5.65AINVERT FORSETTINGCURRENT CURRENT & MIN.TEMPERATURENDEXFigure 7-5.—Minimum indicating thermometer.5.65B131