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 suggest future improvement. and to indicate needed maintenance or repairs.
A l s o , they provide means to keep records that can prove invaluable in
determining the causes producing abnormal operation of the system.
3 . 1 Pressure Recorders. P r e s s u r e recorders generally use a low-pressure
d i a p h r a g m or a tube of oval cross-section wound in a helical or spiral coil
shape. The diaphragm or coil end is connected to a pen arm, directly or
t h r o u g h link and levers; the other end is connected to the pressure source.
The pen records upon a chart which is driven by spring wound clockwork, a
s y n c h r o n o u s electric motor, or a battery drive. T h e operating principle is
the same as for the Bourdon tube.
3 . 2 Temperature Recorders. A common type of temperature recorder consists of
a thermal system including a bulb, capillary tubing, and a helical coil. The
s y s t e m may be filled with gas, vapor, or liquid. I n operation, temperature
changes at the bulb cause pressure changes in the medium filling the system.
T h e pressure changes actuate the helical coil (measuring element), which
through the pen arm, records the pressure changes in terms of temperature.
T h e pen is attached at the free end of the helical coil through proper linkage
and records on a chart. A rotating assembly, operated by clockwork or by a
s y n c h r o n o u s electric motor, is used to mount the chart. A temperature
r e c o r d i n g is shown in figure 6-5 as part of a flowmeter.
3 . 3 Flow Recorders. R e c o r d i n g flowmeters are often provided with integrating
m e c h a n i s m s that totalize the amount of flow during a given period of time.
T h e principle of operation follows.
( a ) A primary element produces a localized pressure drop in the fluid.
T h e flow rate is always proportional to the square root of the pressure drop.
(b) A secondary element, consisting of a U-tube mercury manometer,
r e c e i v e s the signal from the primary element. I n operation, t h e l o c a l i z e d
p r e s s u r e drop created by the primary element in the pipe is transmitted to the
U - t u b e through the low- and high-pressure connections.
( c ) The differential pressure acting on the mercury causes the float in
t h e float chamber to rise or fall following the variations in the flow through
t h e primary element.
( d ) Motion of the float is transmitted through suitable linkage to the
p e n arm shaft that records rate of flow on the chart.
(e) The chart is mounted on a rotating assembly driven by a clockwork
mechanism or by a self-starting synchronous motor as shown in figure 6-6.
( f ) Check valves in the high- and low-pressure chambers prevent loss of
mercury in case of:
q Overload
conditions
q Flow reversal
q Violent fluctuations in flow
6-7
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