Principles of naval engineering - Historic Naval Ships Association

-PRINCIPLES OF NAVAL ENGINEERING3. The temperatures 100° C and 212 Fare identical.In some scientific and engineering work,particularly where heat calculations areinvolved, an absolute temperature scale is used.The zero point on an absolute temperature scaleis the point called absolute zero . Absolute zerois determined theoretically, rather than byactual measurement. Since the pressure of agas at constant volume is directly proportionalto the temperature, it is logical to assume thatthe pressure of a gas is a valid measure ofits temperature. On this assumption, the lowestpossible temperature (absolute zero) is definedas the temperature at which the pressure of agas would be zero.Two absolute temperature scales have beenin use for many years. The rankine absolutescale is an extension of the Fahrenheit scale;it is sometimes called the Fahrenheit absolutescale. Degrees on the Rankine scale are thesame size as degrees on the Fahrenheit scale,but the zero point on the Rankine scale is at-459.67 ° Fahrenheit. In other words, absolutezero is zero on the Rankine scale and -459.67degrees on the Fahrenheit scale.A second absolute scale, the kelvin, is morewidely used than the Rankine. The Kelvin scalewas originally conceived as an extension of theCelsius scale, with degrees of the same sizebut with the zero point shifted to absolute zero.Absolute zero on the Celsius scale is -273.15° C.In 1954, a new international absolute scalewas developed. The new scale was based uponone fixed point, rather than two. The one fixedpoint was the triple point of water— that is,the point at which all three phases of water(solid, liquid, and vapor) can exist together inequilibrium. The triple point of water, whichis 0.01° C above the freezing point of water,was chosen because it can be reproduced withmuch greater accuracy than either the freezingpoint or the boiling point. On this new scale,the triple point was given the value 273.16 K.Note that neither the word 'degrees" nor thesymbol ° is used; instead, the unit is calleda "kelvin" and the symbol is K rather than°K.In 1960, when the triple point of water wasfinally adopted as the fundamental referencefor this temperature scale, the scale was giventhe nameof International Practical TemperatureScale. However, you will often see this scalereferred to as the Kelvin scale.Although the triple point of water is consideredthe basic or fundamental reference forthe International Practical Temperature Scale,five other fixed points are used to help definethe scale. These are the freezing point of gold,the freezing point of silver, the boiling point ofsulfur, the boiling point of water, and the boilingpoint of oxygen.Figure 7-1 is a comparison of the Kelvin(International-Practical), Celsius, Fahrenheit,and Rankine (Fahrenheit-Absolute) temperaturescales. All of the temperature points listedabove absolute zero are considered ^s fixedpoints on the Kelvin scale except for the freezingpoint of water. The other scales, as previouslymentioned, are based on the freezingand boiling points of water.TEMPERATURE MEASURING DEVICESSince temperature is one of the basic engineeringvariables, temperature measurementis essential to the proper operation of a shipboardengineering plant. The temperature ofsteam, water, fuel oil, lubricating oil, and othervital fluids must be measured at frequent intervalsand the results of this measurementmust in many cases be entered in engineeringrecords and logs.Devices used for measuring temperature maybe classified in various ways. In this discussionwe will consider the two major categories of(1) expansion thermometers, and (2) pyrometers.Expansion ThermometersExpansion thermometers operate on theprinciple that the expansion of solids, liquids,and gases has a known relationship to temperaturechanges. The types of expansion thermometersdiscussed here are (1) liquid-in-glassthermometers, (2) bimetallic expansion thermometers,and (3) filled-system expansionthermometers.LIQUID - EST - GLASS THERMOMETERS.Liquid- in- glass thermometers are probably theoldest, the simplest, and the most widely useddevices for measuring temperature. A liquidin-glassthermometer (fig. 7-2) consists of abulb and a very fine bore capillary tube containingmercury, mercury-thallium, alcohol,toluol, or some other liquid which expandsuniformly as the temperature rises and contractsuniformly as the temperature falls. Theselection of liquid is based on the temperature128