Analog Pressure Gauges

There are several common types of mechanical analog pressure gauges including bellows, Bourdon tubes, capsule elements and diaphragm element gauges. Analog pressure gauges should be selected considering the media and ambient operating conditions. Gauge selection should take into consideration the corrosive environment in which it is to operate. The media being measured must be compatible with the wetted parts of the pressure instrument. Improper application can damage the analog pressure gauge, causing failure or personal injury and property damage. Diaphragm seals (also called gauge isolators) can be added to the system to protect the gauge from corrosive attack, and prevent viscous or dirty media from clogging Bourdon tube analog pressure gauges.

Bourdon tubes are circular shaped tubes with an oval cross-section. The pressure of the media acts on the inside of this tube resulting in the oval cross-section becoming almost round. Because of the curvature of the tube ring, the Bourdon tube bends when tension occurs. The end of the tube, which is not fixed, moves thus being a measurement of the pressure. Bourdon tubes with a number of superimposed coils of the same diameter (helical coils) are used for measuring high pressures.

Capsule element analog pressure gauges consist of two circular shaped, convoluted membranes sealed tight around their circumference. The pressure acts on the inside of the capsule and a pointer indicates the generated stroke movement. Pressure gauges with capsule elements are more suitable for gaseous media and relatively low pressures.

Diaphragm element analog pressure gauges combine both a chemical seal and a pressure gauge into one unit. Diaphragm elements are circular shaped, convoluted membranes that are either clamped around the rim between two flanges or welded in place. The measured media exerts a force on the diaphragm. A metal pushrod welded to the top of the diaphragm transmits the deflection of the diaphragm to the linkage. The linkage, in turn, translates the lateral motion of the push rod into a rotational motion of the pointer.