Page 60 - ebook fluid mechanics_finalize

FLUID MECHANICS

3.3 Bourdon Gauge

The Bourdon pressure gauge (Figure 3.17) uses the principle that a flattened tube
tends to change to a more circular cross-section when pressurized. Although this change

in cross-section may be hardly noticeable, the displacement of the material of the tube is

magnified by forming the tube into a C shape or even a helix, such that the entire tube
tends to straighten out or uncoil, elastically, as it is pressurized.

Figure 3.17 : Bourdon gauge (right), Mechanism of the Bourdon gauge (left)

In practice, a flattened thin-wall, closed-end tube is connected at the hollow end to
a fixed pipe containing the fluid pressure to be measured. As the pressure increases, the

closed end moves in an arc, and this motion is converted into the rotation of a (segment

of a) gear by a connecting link which is usually adjustable. A small diameter pinion gear is

on the pointer shaft, so the motion is magnified further by the gear ratio. The positioning
of the indicator card behind the pointer, the initial pointer shaft position, the linkage

length and initial, all provide means to calibrate the pointer to indicate the desired range

of pressure for variations in the behavior of the Bourdon tube itself.

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