Diaphragm pressure gauges
These are suitable only for the measurement of relatively low pressures up to about 1 bar and vacuum. They are sensitive devices well suited for the measurement or detection of small differential pressures applied to each side of the diaphragm. A typical gauge is shown in Figure 5. The travel of the diaphragm and the stroke available to operate the pointer depends on the cross section and the diameter. Extreme movement may only be obtained at the expense of loss of linearity.
The stroke of a single diaphragm can be extended considerably by employing a slack diaphragm made from a suitable limp material such as nylon or leather. Such a diaphragm requires biasing with spring pressure which opposes the direction of the applied pressure and incorporates stops to limit the movement. The advantage of a slack diaphragm is the large area exposed to pressure, making it sensitive for the measurement of very low pressures.
Metallic diaphragms offer the possibility of working at higher pressure ratings, although the movement is dependent on the amount of corrugation. Sensitivity can be improved by fabricating a series of diaphragms in the form of bellows, although the contour adopted can have a bearing on the linearity of stoke.
Welded metal diaphragms, based on a bellows shape or series of capsules are mostly used for pressure measurements within the range 25 mm wg to 0.35 bar. In capsule types, the capsules form hermetically sealed chambers which may be partially evacuated to render the element more sensitive as in an aneroid barometer.
Bellows type elements formed from thin wall tubing have a higher pressure rating and may be used in several ways. For pressure measurements within the range 0.35 to 2.5 bar or vacuum pressures down to 1 torr, open ended bellows, enclosed within a chamber are commonly used. Pressure is applied to the chamber, with mechanical movement taken from the free end. The basic design can be modified by the incorporation of spring loading so as to oppose the movement produced by the pressure force. This expands the bellows when the applied pressure is positive and contracts the bellows in the case of a vacuum. See Figure 6.
Sensitivity may be improved by the use of additional bellows to apply spring bias. Also matched bellows may be used each in a separate sealed chamber; the interiors are connected and filled with a liquid with a high bulk modulus, such as oil. Pressure applied to one chamber causes a contrary movement in the other due to the displaced liquid, thus doubling the mechanical movement available from the applied pressure.