Figure  11-7.–Bellows  gauge. the  pressure  difference,  rather  than  either actual pressures indicated by the pointer. BELLOWS GAUGE of the two A bellows gauge contains an elastic element that is a convoluted unit that expands and contracts axially with changes in pressure. The pressure to be measured can be  applied  to  the  outside  or  inside  of  the  bellows. However,  in  practice,  most  bellows  measuring  devices have the pressure applied to the outside of the bellows (fig.  11-7).  Like  Bourdon-tube  elements,  the  elastic elements in bellows gauges are made of brass, phosphor bronze,  stainless  steel,  beryllium-copper,  or  other  metal that is suitable for the intended purpose of the gauge. Most bellows gauges are spring-loaded; that is, a spring  opposes  the  bellows,  thus  preventing  full expansion of the bellows. Limiting the expansion of the bellows  in  this  way  protects  the  bellows  and  prolongs its   life.   In   a   spring-loaded   bellows   element,   the deflection is the result of the force acting on the bellows and the opposing force of the spring. Although   some   bellows   instruments   can   be designed for measuring pressures up to 800 psig, their primary application aboard ship is in the measurement of low pressures or small pressure differentials. Figure 11-8.—Diaphragm gauge. Many differential pressure gauges are of the bellows type. In some designs, one pressure is applied to the inside of the bellows, and the other pressure is applied to the outside. In other designs, a differential pressure reading is obtained by opposing two bellows in a single case. Bellows  elements  are  used  in  various  applications where  the  pressure-sensitive  device  must  be  powerful enough to operate not only the indicating pointer but also some  type  of  recording  device. DIAPHRAGM GAUGES Diaphragm  gauges  are  very  sensitive  and  give reliable  indication  of  small  differences  in  pressure. Diaphragm  gauges  are  generally  used  to  measure  air pressure in the space between the inner and outer boiler casings. Figure  11-8  shows  the  indicating  mechanism  of  a diaphragm gauge. This mechanism consists of a tough, pliable,  neoprene  rubber  membrane  connected  to  a metal spring that is attached by a simple linkage system to the gauge pointer. One side of the diaphragm is exposed to the pressure being measured, while the other side is exposed to the atmosphere. When pressure is applied to the diaphragm, it moves and, through a linkage system, moves the pointer  to  a  higher  reading  on  the  dial.  When  the 11-4


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