The Bourdon Tube is a nonliquid pressure measurement device. It is widely used in applications where inexpensive static pressure measurements are needed.

A typical Bourdon tube contains a curved tube that is open to external pressure input on one end and is coupled mechanically to an indicating needle on the other end, as shown schematically below.

Pic: Typical Bourdon Tube Pressure Gages
Internal linkages have been simplified.
 

The external pressure is guided into the tube and causes it to flex, resulting in a change in curvature of the tube. These curvature changes are linked to the dial indicator for a number readout. Alternatively, a strain gage circuit can be attached on the tube to convert the pressure-induced deflections into electric voltage signals. These signals can then be output electronically, rather than mechanically with the dial indicator.

Pros - Portable, Convenient, No leveling required
Cons - Limited to static or quasi-static measurements, Accuracy may be insufficient for many applications. A mercury barometer can be used to calibrate and check Bourdon Tubes, Most common type of pressure sensor

Increased pressure causes the flattened cross-section to straighten and move the closed end. Closed end is attached to mechanical linkage. Linkage is connected to a pointer or other output device

There are three types of forms:

• C-Tube

• Spiral

• Helical 

Definitions of pressure on the Web:

• the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit); "the compressed gas exerts an increased pressure"

• a force that compels; "the public brought pressure to bear on the government"