Hydrostatic manometers become impractical if pressures higher than 3 atmospheres have to be measured because, even if mercury is used, the column length exceeds 2 meters. On the other hand, hydrostatic manometers are fragile, are not portable and are therefore only suitable for laboratory applications. In an industrial environment, manometers are therefore used in which the difference in pressure causes the deformation of an elastic element.
In the Bourdon gage (Figure 1.6), the sensor is a tube with a flattened section closed at one end and bent as an arc of a circle. The pressure to be measured, applied inside the tube, tends to straighten it, the closed end is free to move and its deflection is communicated through a rack – and-pinion or helical-cam mechanism to a pointer that moves on a graduated scale which is obtained through factory calibration. The range of this type of gage extends from about 1/30 to hundreds of atmospheres.
In the aneroid (or diaphragm) manometer (Figure 1.7) the sensor is a capsule covered with a thin metal diaphragm with concentric corrugations that make it easily deformable. These gages are reliable for accurate indication of differentials as low as 2 mm of water. Such a mechanism is commonly employed in recording pressure indicators and in barometers.
Both the tubular elements and the diaphragm are temperature sensitive and therefore require further calibration in the laboratory when used at temperatures different from the standard.