Figure 9-4.-The 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
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,
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
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