Pressure measurement - Boudan Tube Gauge

Bourdon tube is a type of flexible mechanical pressure measuring device. The pressure changes the shape of the measuring element in proportion to the applied pressure. A metallic flexible pressure measuring element can only be deformed within a limited range due to the considerable material stresses involved. Pressure gages using this principle measure pressures above atmospheric to several thousand psi. They are available in C-shape, Helical and spiral shapes.

C-Bourdon Pressure Gauge

The C-type Bourdon tube is made by folding a tube to form segment of a circle, usually an arc of 250°. The process medium whose pressure is to be measured is connected to the fixed end of the tube, while the other end is sealed. Because of pressure difference between inner tube wall and outer tube wall, the Bourdon tube tends to straighten. The sealed tip end moves in non-linear trajectory. By means of mechanical sector and pinion arrangement, this non-linear motion of tip is amplified and converted to linear motion of pointer.

Commonly used material for making Bourdon Tube

1. Phosphor Bronze
2.  Beryllium copper
3. SS 316M
4. Monel
5. Inconel

Direct measuring C-Bourdon tubes can measure in the span from 0-15 PSI to 0-20000 PSI. Not suitable for very low pressure. The accuracy is ± 1% of full scale.

Spiral Bourdon Pressure Gauge

The free end motion of C-Bourdon tube is insufficient in some cases. A spiral type formation can be used in such cases. When pressure is applied, this flat spiral tends to uncoil and generates greater movement of free end requiring no mechanical amplification. This improves the accuracy and sensitivity of the instrument.

Helical Bourdon Pressure Gauge

This geometry provides highest sensitivity among all Bourdon gauges. There is no need for mechanical amplification. It is also suitable to use in continuously varying pressure system. They are available in Bronze, Beryllium copper, and stainless-steel.

It should be noted that bourdon tubes may be used to measure differential and/or absolute pressure in addition to gauge pressure. All that is needed for these other functionalities is to subject the other side of each pressure-sensing element to either another applied pressure (in the case of differential measurement) or to a vacuum chamber (in the case of absolute pressure measurement).