Bellows, Diaphagm, Bourdon tube each are mechanical pressure element which convert mechanical pressure in to force. This element will produce motion proportional to appiled pressure.
Increasing pressure inside a bellows unit causes it to elongate. see bellow in below figure
A diaphragm is nothing more than a thin disk of material which bows outward under the influence of a fluid pressure. Many diaphragms are constructed from metal, which gives them spring-like qualities. Some diaphragms are intentionally constructed out of materials with little strength, such that there is negligible spring effect. These are called slack diaphragms, and they are used in conjunction with external mechanisms that produce the necessary restraining force to prevent damage from applied pressure.
The following photograph shows the mechanism of a small pressure gauge using a brass diaphragm as the sensing element:
As pressure is applied to the rear of the diaphragm, it distends upward (away from the table on which it rests as shown in the photograph), causing a small shaft to twist in response. This twisting motion is transferred to a lever which pulls on a tiny link chain wrapped around the pointer shaft, causing it to rotate and move the pointer needle around the gauge scale. Both the needle and scale on this gauge mechanism have been removed for easier viewing of diaphragm and mechanism.
Bourdon tubes are made of spring-like metal alloys bent into a circular shape. Under the influence of internal pressure, a bourdon tube “tries” to straighten out into its original shape before being bent at the time of manufacture.
Most pressure gauges use a bourdon tube as their pressure-sensing element. Most pressure transmitters use a diaphragm as their pressure-sensing element. Bourdon tubes may be made in spiral or helical forms for greater motion (and therefore greater gauge resolution).
A typical C-shaped bourdon tube pressure gauge mechanism is shown in the following illustration:
A photograph of a C-tube pressure gauge mechanism (taken from the rear of the gauge, behind the pointer and scale) reveals its mechanical workings:
The next photograph shows a spiral bourdon tube, designed to produce a wider range of motion than a C-tube bourdon:
It should be noted that bellows, diaphragms, and bourdon tubes alike may all 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).