Pressure-indicating devices are most important in the refrigeration field. It is necessary to know the pressures in certain parts of a system to locate trouble spots.
The diaphragm is a flexible sheet of material held firmly around its perimeter so there can be no leakage from one side to the other (see Fig. 4-5). Force applied to one side of the diaphragm will cause it to move or flex. Diaphragms, in some cases, are made of flat sheet of material with a limited range of motion. Other diaphragms are at least one corrugation or fold. This allows more movement at the point where work is produced.
Some types of pressure controllers require more motion for per unit of force applied. To accomplish the desired result, the diaphragm is joined to the housing by a section with several convolutions or folds called bellows. Thus, the diaphragm moves in response to pressure changes. Each holds only a small amount (see Fig. 4-6). The bellow element may be assembled to extend or to compress as pressure is applied. The bellows itself act as a spring to return the diaphragm section to the original position when the pressure differential is reduced to zero. If a higher spring return rate is required, to match or define the measured pressure range, then an appropriate spring is added.
Some types of pressure controllers require more motion for per unit of force applied. To accomplish the desired result, the diaphragm is joined to the housing by a section with several convolutions or folds called bellows. Thus, the diaphragm moves in response to pressure changes. Each holds only a small amount (see Fig. 4-6). The bellow element may be assembled to extend or to compress as pressure is applied. The bellows itself act as a spring to return the diaphragm section to the original position when the pressure differential is reduced to zero. If a higher spring return rate is required, to match or define the measured pressure range, then an appropriate spring is added.
One of the most widely used types of pressure-measuring elements is the Bourdon spring tube, discussed in Chap. 1. It is readily adaptable to many types of instruments (see Fig. 4-7). The Bourdon tube is a flattened tube bent into a spiral or circular form closed at one end. When fluid pressure is applied within the tube, the tube tends to straighten or unwind. This produces motion, which may be applied to position an indicator or actuate a controller.
