One end of the tube is sealed; the other is connected to the
process. As the pressure in the process increases, tube tends to straighten
out. The resultant motion is transferred through a linkage or rack-and-pinion
mechanism to an indicating pointer.
A bourdon tube is basically a spring that stretches as
pressure is applied. Bourdon tube metals must not be subject to hysteresis i.e.
the metal must not stretch a different amount for increasing or decreasing
pressure, causing the gauge two different reading for one pressure. The choice
of metals depends on corrosion resistance, flexibility, hysteresis
characteristics, pressure range, and cost.
It exists normally in any of the three shapes.
a). “ C”
shape bourdon tube.
It is used up to ranges from 0 to 10,000 psig. The range
depends on material used, flatness of tube and cross sectional area of tube.
b). Spiral
tube
Spiral element is actually a long C- type tube resembling a
flat coil. One end is rigid, and the movement of the free end is linked to a
pointer of indicator.
c). Helical
tube
Helical element is a long C-type spring wound like a
vertical spring. It is more sensitive to small changes.
