Bourdon tube

The measuring element used in certain types of pressure-measuring instruments. A thin-walled metal tube, having an elliptical cross section, is formed into a curve. One end of the tube is sealed and is connected to an arm that moves a pointer. The other end of the tube is open and is secured to the instrument case. The pressure to be measured is connected to the open end of the tube, and this pressure causes the elliptical cross section to try to become round and thus begin to straighten the tube. The amount the tube straightens is proportional to the pressure inside the tube, and as it straightens, it moves a pointer across the calibrated instrument dial.