melted, however, the temperature of the water will be  raised  as  additional  heat  is  supplied.  If  we  add 180  Btu—that  is,  1  Btu  for  each  degree  of temperature  between  32°F  and  212°F—the temperature  of  the  water  will  be  raised  to  the boiling  point.  To  change  the  pound  of  water  at 212°F to a pound of steam at 212°F, we must add 970  Btu  (the  LATENT  HEAT  OF  VAPORIZA- TION). After all the water has been converted to steam,  the  addition  of  more  heat  will  cause  an increase  in  the  temperature  of  the  steam.  If  we add about 44 Btu to the pound of steam that is at  212°F,  we  can  super  heat  it  to  300°F. The  same  relationships  apply  when  heat  is being removed. The removal of 44 Btu from the pound  of  steam  that  is  at  300°F  will  cause  the temperature  to  drop  to  212°F.  As  the  pound  of steam  at  212°F  changes  to  a  pound  of  water  at 212°F, 970 Btu are given off. When a substance is changing from a gas or vapor to a liquid, the heat  that  is  given  off  is  LATENT  HEAT  OF CONDENSATION.   Notice,   however,   that   the latent  heat  of  condensation  is  exactly  the  same  as the  latent  heat  of  vaporization.  The  removal  of another  180  Btu  of  sensible  heat  will  lower  the temperature  of  the  pound  of  pure  water  from 212°F  to  32°F.  As  the  pound  of  water  at  32°F changes  to  a  pound  of  ice  at  32°F,  144  Btu  are given  off  without  any  accompanying  change  in temperature. Further removal of heat causes the temperature  of  the  ice  to  decrease. TEMPERATURE The temperature of an object is a measure of the  heat  level  of  that  object.  This  level  can  be measured with a thermometer. The temperature scales employed to measure temperature  are  the  Fahrenheit  (F)  scale  and the  Celsius  (C)  scale.  In  engineering  and  for practically   all   purposes   in   the   Navy,   the Fahrenheit  scale  is  used.  You  may,  however,  have to convert Celsius readings to the Fahrenheit scale, so both scales are explained here. The Fahrenheit scale has two main reference points—the boiling point of pure water at 212°F and the freezing point of pure water at 32°F. The measure of a degree of Fahrenheit is 1/180 of the total temperature change from 32°F to 212°F. The scale  can  be  extended  in  either  direction—to higher  temperatures  without  any  limits  and  to lower temperatures (by minus degrees) down to the   lowest   temperature   theoretically   possible, absolute  zero.  This  temperature  is  –  460°F,  or 492°F  below  the  freezing  point  of  water. In  the  Celsius  scale,  the  freezing  point  of  pure water is 0°C and the boiling point of pure water is    100°C.    Therefore,    0°C    and    100°C    are equivalent to 32°F and 212°F, respectively. Each degree   of   Celsius   is   larger   than   a   degree   of Fahrenheit.  Only  100°  Celsius  are  between  the freezing  and  boiling  points  of  water,  while  this same  temperature  change  requires  180°  on  the Fahrenheit  scale.  Therefore,  the  degree  of  Celsius is 180/100 or 1.8° Fahrenheit. In the Celsius scale, absolute  zero  is  –  273°C.  To  convert  from  one temperature  scale  to  another,  use  the  following algebraic  equations: From  Fahrenheit  to  Celsius ⁰C  =  5/9  X  (⁰F  –  32) From  Celsius  to  Fahrenheit °F  =  (9/5  x  °C)  +  32 Figure  2-10  shows  the  two  temperature  scales in comparison. It also introduces the simplest of the   temperature   measuring   instruments,   the liquid-in-glass  thermometer.  The  two  thermom- eters shown are exactly alike in size and shape. The  only  difference  is  the  outside  markings  or scales  on  them.  Each  thermometer  is  a  hollow glass  tube  that  is  sealed  at  the  top  and  has  a mercury-filled bulb at the bottom. Mercury, like any liquid, expands when heated and will rise in the hollow tube. View A of figure 2-10 shows the Fahrenheit  thermometer  with  its  bulb  standing  in melting ice (32°F), and view B shows the Celsius thermometer  with  its  bulb  standing  in  boiling water  (100°C). The main point to remember is that the level of  the  mercury  in  a  thermometer  depends  only  on the temperature to which the bulb is exposed. If you  were  to  exchange  the  thermometers,  the mercury in the Celsius thermometer would drop to the level that the mercury now stands in the Fahrenheit thermometer. Likewise, the mercury in  the  Fahrenheit  thermometer  would  rise  to the  level  that  the  mercury  now  stands  in  the 2-14