Figure 4-1.Comparison of Fahrenheit and Celsius Thermometers

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heat to warm a room. Another example tells us that IO pounds of water at 80°F will melt more ice in a given length  of  time  than  1  pound  of  water  at  100oF.  The former  has  more  heat,  but  the  latter  has  a  higher temperature.  Temperature  is  the  measurement  of  heat intensity in degrees Fahrenheit or Celsius. Therefore, temperature  measurements  can  be  made  by  using  a glass   thermometer   calibrated   either   in   degrees Fahrenheit or Celsius. The generally accepted way of s t a t i n g   m e a s u r e m e n t s   o f   t e m p e r a t u r e   i n English-speaking countries is in degrees Fahrenheit. The thermometer measures the degree of sensible heat of different bodies. The thermometer can make a comparison  only  between  the  temperature  of  a  body and  some  definitely  known  temperature  such  as  the melting  point  of  ice  or  the  boiling  point  of  water. Figure  4-1  shows  a  comparison  of  the  scales  of Fahrenheit  and  Celsius  thermometers.  It  also  shows the marking of the freezing and boiling points of pure water   at   sea   level.   The   range   of   the   Fahrenheit thermometer   between   the   freezing   point   and   the boiling  point  is  180°  (32°  to  212°  =  180°).  On  the Celsius thermometer, the range is 100° (0° to 100° = 100°) from the freezing point to the boiling point. Figure 4-1.—Comparison of Fahrenheit and Celsius thermometers. To convert Fahrenheit readings to Celsius: (°F - 32°) ÷ 1.8 = °C To   convert   Celsius   readings   to   Fahrenheit: (°C x 1.8) + 32° = °F The  heat  that  can  be  measured  by  a  thermometer and sensed or felt is referred to as "sensible heat." An example  of  sensible  heat  is  presented  by  placing  a small vessel of cold water over a gas flame and putting a  thermometer  in  the  water.  Upon  observation,  you note  that  the  thermometer  indicates  a  rise  in temperature. Also, if you place your finger in the water several  times,  you  will  feel  (or  sense)  the  change  in temperature that has taken place. The  unit  of  measurement  for  a  given  quantity  of heat  is  the  British  thermal  unit,  abbreviated  and commonly  known  as  Btu.  One  Btu  is  the  amount  of heat needed to change the temperature of 1 pound of water 1° Fahrenheit at sea level. If one Btu is added to 1 pound at 50°F, the temperature of that pound of water will be raised to 51°F. All  substances  above  absolute  zero  contain  heat. There is heat even in ice, and its melting point is fixed at 32°F.  Because of a fundamental law of nature, when ice at 32°F melts into water at 32°F, a change of state takes  place.  The  ice  (solid)  has  turned  into  water (liquid).  A  certain  amount  of  heat  is  required  during this change of state. This heat is known as latent heat. Latent heat is the amount of heat required to change the state  of  a  substance  without  a  measurable  change  in temperature. There  are  other  types  of  heat  that  you  will encounter in heating. These are as follows: Specific heat–The ratio between the quantity of heat required to raise 1 pound of any substance 1oF and the amount of heat required to raise the temperature of 1 pound of water 1oF. Superheat–The  amount  of  heat  added  to  a substance above its boiling point. Total heat–Is the sum of sensible heat plus latent heat. We previously mentioned absolute zero. But, what is absolute zero? Scientists have determined that when the  temperature  of  a  substance  has  been  reduced to  -460°F  that  all  the  heat  has  been  removed  from  a substance. At this point all the molecules cease to have motion.   Absolute   zero   is   the   lowest   temperature obtainable. Heat is present in all substances when the temperature is above absolute zero. 4-2

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