Bourdon Gauge

Figure 9-4.-The Bourdon gauge. Bourdon Gauge The  Bourdon  gauge  is  shown  in  figure  9-4.  It works on the same principle as that of the snakelike, paper  party  whistle  you  get  at  a  New  Year  party, which straightens when you blow into it. Within the Bourdon gauge is a thin-walled metal tube, somewhat flattened and bent into the form of a C.  Attached  to  its  free  end  is  a  lever  system  that magnifies any motion of the free end of the tube. On the fixed end of the gauge is a fitting you thread into a  boiler  system.  As  pressure  increases  within  the boiler, it travels through the tube. Like the snakelike paper whistle, the metal tube begins to straighten as the   pressure   increases   inside   of   it.   As   the   tube straightens,  the  pointer  moves  around  a  dial  that indicates the pressure in psi. The  Bourdon  gauge  is  a  highly  accurate  but rather delicate instrument. You can easily damage it. In addition, it malfunctions if pressure varies rapidly. This  problem  was  overcome  by  the  development  of another  type  of  gauge,  the  Schrader.  The  Schrader gauge (fig. 9-5) is not as accurate as the Bourdon, but it   is   sturdy   and   suitable   for   ordinary   hydraulic pressure  measurements.  It  is  especially  suitable  for fluctuating loads. In the Schrader  gauge,  liquid  pressure  actuates a  piston.  The  pressure  moves  up  a  cylinder  against the resistance of a spring, carrying a bar or indicator with it over a calibrated scale.  The  operation  of this gauge  eliminates  the  need  for  cams,  gears,  levers, and bearings. Diaphragm Gauge The    diaphragm    gauge    gives    sensitive    and reliable indications of small pressure differences. We use the diaphragm gauge to measure the air pressure in the space between inner and outer boiler casings. In this type of gauge, a diaphragm connects to a pointer through a metal spring and a simple linkage system   (fig.   9-6).   One   side   of   the   diaphragm   is exposed  to  the  pressure  being  measured,  while  the other   side    is    exposed    to    the    pressure    of    the atmosphere. Any increase in the pressure line moves the  diaphragm  upward  against  the  spring,  moving the  pointer  to  a  higher  reading.  When  the  pressure decreases,     the     spring     moves     the     diaphragm downward,  rotating  the  pointer  to  a  lower  reading. Thus, the position of the pointer is balanced between the pressure pushing the diaphragm upward and the spring action pushing down. When the gauge reads 0, the  pressure  in  the  line  is  equal  to  the  outside  air pressure. MEASURING AIR PRESSURE To  the  average  person,  the  chief  importance  of weather   is   reference   to   it   as   an   introduction   to general conversation. At sea and in the air, advance knowledge of what the weather will do is a matter of great concern 9-4


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