The bellow type gauges are used for the measurement of absolute pressure. They are more sensitive than bourdon gauges. It may be used for measuring pressures up to 40 mm Hg.
The bellows are made up of an alloy with high strength and ductility. It should have very little hysteresis effect. Commonly brass or phosphor bronze is used for making bellows. For selecting a specific material for bellows, the parameters to be checked are:
(i) Range of pressure
(iii) Fatigue on dynamic operation
(v) Fabrication ease
(vi) Sensitivity to fluctuating pressures
Out of these hysteresis and sensitivity to fluctuating pressures are the most important ones. Hysteresis can be minimized by following a proper manufacturing technique. For strong bellows, the carbon steel is selected as the main element. But the material gets easily corroded.
The bellows are used in two forms. In one arrangement, pressure is applied to one side of the bellows and the resulting deflection is counter balanced by a spring. This arrangement indicates the gauge pressure. In the second arrangement, the differential pressure is also indicated. In this device, one pressure is applied to the inside of one sealed bellow while the other pressure is applied to the inside of another sealed bellow. By suitable linkage and calibration of the scale,
the pressure difference is indicated by a pointer on the scale.
(i) Its cost is moderate.
(ii) It is able to deliver high force.
(iii) It is adaptable for absolute and differential pressures.
(iv) It is good to low-to moderate range.
(i) It needs ambient temperature compensation.
(ii) It is unsuitable for high pressures.
(iii) The availability of construction metals is limited.