Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGbe negligible for most purposes because only avery small amount of mercury is contained inthe very narrow bore.The minimum indicating thermometer shownin figure 7-5 is an alcohol-in-glass thermometerwith an unusually large bore. The upper part ofthe bore is filled with air underpressure to helpprevent evaporation of the alcohol. The thermometeris mounted with the bulb a few degreesbelow the horizontalposition. A dumbbell-shapedpiece of black glass (called an index ) is thedevice that makes possible a reading of the minimumtemperature that has occurred since thethermometer was last set. As the temperatureincreases, the alcohol readily flows upward pastthe index without moving it. As the temperaturedecreases, the retreating alcohol column flowspast the index until the top of the column touchesthe upper end of the index. With a further decreasein temperature, the alcohol retreats stillmore and surface tension causes the index tobecarried along down with the column. If the temperatureincreases again, the index is left undisturbedat its lowest point while the alcoholcolumn rises again. Thus the top of the indexalways indicates the lowest temperature that hasoccurred since the thermometer was last set.BIMETALLIC EXPANSION THERMOM-ETER.— Bimetallic expansion thermometersmake use of the fact that different metals havedifferent coefficients of linear expansion. Theessential element in a bimetallic expansionthermometer is a bimetallic strip consisting oftwo layers of different metals fused together.When such a strip is subjected to temperaturechanges, one layer expands or contracts morethan the other, thus tending to change the curvatureof the strip.The basic principle of a bimetallic expansionthermometer isillustrated in figure 7-6. Whenone end of a straight bimetallic strip is fixed inplace, the other end tends to curve away from theside that has the greater coefficient of linearexpansion when the strip is heated.COLDTHIS METAL HAS GREATERCOEFFICIENT OF LINEAREXPANSIONTHIS METAL HAS-SMALLER COEFFICIENTOF LINEAR EXPANSION147.53Figure 7-6.— Effect of unequal expansion ofbimetallic strip.For use in thermometers, the bimetallicstrip is normally wound into a flat spiral (fig.7-7), a single helix, or a multiple helix. Theend of the strip that is not fixed in position isfastened to the end of a pointer which movesover a circular scale. Bimetallic thermometersare easily adapted for use as recording thermometers;a pen is attached to the pointer andis positioned in such a way that it marks on arevolving chart.Bimetallic thermometers used aboard shipare normally used in thermometer wells. Thewells are interchangeable with those used formercury-in-glass thermometers.FILLED-SYSTEMTHERMOMETERS.-Ingeneral, filled-system thermometers are designedfor use in locations where the indicatingpart of the instrument must be placed somedistance away from the point where the temperatureis to be measured.' For this reasonthey are often called distant- reading thermometers.A filled-system thermometer (fig. 7-8) consistsessentially of a hollow metal sensing bulbThe coefficient of linear expansion is defined as thechange In length per unit length per degree change intemperature. As is apparent from this definition, thenumerical value of the coefficient of linear expansionis independent of the units in which the length is expressedbut is not independent of the temperature scalechosen.This is not true of all filled-system thermometers.In a few designs the capillary tubing is extremely shortand in a few it is nonexistent. In general, however,filled-system thermometers are designed to be distant-readingthermometers, and most of them do infact serve this purpose. Some distant-reading thermometersmay have capillaries as long as 125 feet.132