Seamless tubes that are made roll-formed compressed hydraulically or mechanically as shown in below fig. known as bellows and constructed from metal instead of fabric. By increase in pressure inside to a bellow unit causes it to elongate.
Seamless bellows are made of brass beryllium copper, phosphor bronze, Monel, Inconel, and stainless steel. Bellows can also be manufactured by welding a number of preformed plates together. Welded bellows are made from stainless steel, Ni-Span C, Monel, and Inconel-X.
They are classified by the reference pressure used as absolute, gauge, or differential pressure detectors, and can also be grouped as force or motion balance types depending on the instrument design. A direct local indication is available only with motion balance units, while force balance sensors are more accurate.
Advantages
The main advantages of bellows are their ability to provide longer strokes and to handle higher forces. Therefore, they are well suited as direct input elements into recorders and indicators. Their sensitivity increases with their diameter and is noted for their good linearity
Disadvantages
Their limitations include their sensitivity to ambient temperature effects, work hardening, drift, friction, and elastic hysteresis. In most cases, the elastic action of the bellows is insufficient for accurate measurement, and spring needs to be added to precisely characterize the relationship between force and movement.
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Instrumentation Basics 