Page 16 - Instrumentation and Measurement

Figure 2.5 Differential bellows pressure gauges for direct scale reading
2.2.3 Bourdon Tube
The Bourdon tube is an almost rectangular or elliptical cross-section tube made from materials such
as stainless steel or phosphor bronze. With a C-shaped tube (Figure 2.6 (A)), when the pressure inside
the tube increases the closed end of the C opens out, thus the displacement of the closed end becomes
a measure of the pressure.
A C-shaped Bourdon tube can be used to rotate, via gearing, a shaft and cause a pointer to move
across a scale. Such instruments are robust and typically used for pressures in the range 10 kPa to
100 MPa with an accuracy of about ± 1% of full scale.

FIGURE 2.6 Bourdon tube instruments: (A) geared form, (B) potentiometer form.
Another form of Bourdon instrument uses a helical-shaped tube (Figure 2.6 (B)). When the pressure
inside the tube increases, the closed end of the tube rotates and thus the rotation becomes a measure
of the pressure. A helical-shaped Bourdon tube can be used to move the slider of a potentiometer and
so give an electrical output related to the pressure. Helical tubes are more expensive but have greater
sensitivity. Typically, they are used for pressures up to about 50 MPa with an accuracy of about ±
1% of full range.

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