PRINCIPLES OF NAVAL ENGINEERING 



3. The temperatures 100° C and 212 F 

 are identical. 



In some scientific and engineering work, 

 particularly where heat calculations are 

 involved, an absolute temperature scale is used. 

 The zero point on an absolute temperature scale 

 is the point called absolute zero . Absolute zero 

 is determined theoretically, rather than by 

 actual measurement. Since the pressure of a 

 gas at constant volume is directly proportional 

 to the temperature, it is logical to assume that 

 the pressure of a gas is a valid measure of 

 its temperature. On this assumption, the lowest 

 possible temperature (absolute zero) is defined 

 as the temperature at which the pressure of a 

 gas would be zero. 



Two absolute temperature scales have been 

 in use for many years. The rankine absolute 

 scale is an extension of the Fahrenheit scale; 

 it is sometimes called the Fahrenheit absolute 

 scale. Degrees on the Rankine scale are the 

 same size as degrees on the Fahrenheit scale, 

 but the zero point on the Rankine scale is at 

 -459.67 ° Fahrenheit. In other words, absolute 

 zero is zero on the Rankine scale and -459.67 

 degrees on the Fahrenheit scale. 



A second absolute scale, the kelvin, is more 

 widely used than the Rankine. The Kelvin scale 

 was originally conceived as an extension of the 

 Celsius scale, with degrees of the same size 

 but with the zero point shifted to absolute zero. 

 Absolute zero on the Celsius scale is -273.15° C. 



In 1954, a new international absolute scale 

 was developed. The new scale was based upon 

 one fixed point, rather than two. The one fixed 

 point was the triple point of water — that is, 

 the point at which all three phases of water 

 (solid, liquid, and vapor) can exist together in 

 equilibrium. The triple point of water, which 

 is 0.01° C above the freezing point of water, 

 was chosen because it can be reproduced with 

 much greater accuracy than either the freezing 

 point 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 the 

 symbol ° is used; instead, the unit is called 

 a "kelvin" and the symbol is K rather than 

 ° K. 



In 1960, when the triple point of water was 

 finally adopted as the fundamental reference 

 for this temperature scale, the scale was given 

 the nameof International Practical Temperature 

 Scale. However, you will often see this scale 

 referred to as the Kelvin scale. 



Although the triple point of water is con- 

 sidered the basic or fundamental reference for 

 the International Practical Temperature Scale, 

 five other fixed points are used to help define 

 the scale. These are the freezing point of gold, 

 the freezing point of silver, the boiling point of 

 sulfur, the boiling point of water, and the boiling 

 point of oxygen. 



Figure 7-1 is a comparison of the Kelvin 

 (International-Practical), Celsius, Fahrenheit, 

 and Rankine (Fahrenheit-Absolute) temperature 

 scales. All of the temperature points listed 

 above absolute zero are considered ^s fixed 

 points on the Kelvin scale except for the freez- 

 ing point of water. The other scales, as pre- 

 viously mentioned, are based on the freezing 

 and boiling points of water. 



TEMPERATURE MEASURING DEVICES 



Since temperature is one of the basic en- 

 gineering variables, temperature measurement 

 is essential to the proper operation of a ship- 

 board engineering plant. The temperature of 

 steam, water, fuel oil, lubricating oil, and other 

 vital fluids must be measured at frequent in- 

 tervals and the results of this measurement 

 must in many cases be entered in engineering 

 records and logs. 



Devices used for measuring temperature may 

 be classified in various ways. In this discussion 

 we will consider the two major categories of 

 (1) expansion thermometers, and (2) pyrometers. 



Expansion Thermometers 



Expansion thermometers operate on the 

 principle that the expansion of solids, liquids, 

 and gases has a known relationship to tempera- 

 ture changes. The types of expansion thermome- 

 ters discussed here are (1) liquid-in-glass 

 thermometers, (2) bimetallic expansion ther- 

 mometers, and (3) filled-system expansion 

 thermometers. 



LIQUID - EST - GLASS THERMOMETERS. - 

 Liquid- in- glass thermometers are probably the 

 oldest, the simplest, and the most widely used 

 devices for measuring temperature. A liquid- 

 in-glass thermometer (fig. 7-2) consists of a 

 bulb and a very fine bore capillary tube con- 

 taining mercury, mercury-thallium, alcohol, 

 toluol, or some other liquid which expands 

 uniformly as the temperature rises and con- 

 tracts uniformly as the temperature falls. The 

 selection of liquid is based on the temperature 



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