Figure 9-4.-The Bourdon gauge.Bourdon GaugeThe Bourdon gauge is shown in figure 9-4. Itworks on the same principle as that of the snakelike,paper party whistle you get at a New Year party,which straightens when you blow into it.Within the Bourdon gauge is a thin-walled metaltube, somewhat flattened and bent into the form of aC. Attached to its free end is a lever system thatmagnifies any motion of the free end of the tube. Onthe fixed end of the gauge is a fitting you thread intoa boiler system. As pressure increases within theboiler, it travels through the tube. Like the snakelikepaper whistle, the metal tube begins to straighten asthe pressure increases inside of it. As the tubestraightens, the pointer moves around a dial thatindicates the pressure in psi.The Bourdon gauge is a highly accurate butrather delicate instrument. You can easily damage it.In addition, it malfunctions if pressure varies rapidly.This problem was overcome by the development ofanother type of gauge, the Schrader. The Schradergauge (fig. 9-5) is not as accurate as the Bourdon, butit is sturdy and suitable for ordinary hydraulicpressure measurements. It is especially suitable forfluctuating loads.In the Schrader gauge, liquid pressure actuatesa piston. The pressure moves up a cylinder againstthe resistance of a spring, carrying a bar or indicatorwith it over a calibrated scale. The operation of thisgauge eliminates the need for cams, gears, levers,and bearings.Diaphragm GaugeThe diaphragm gauge gives sensitive andreliable indications of small pressure differences. Weuse the diaphragm gauge to measure the air pressurein the space between inner and outer boiler casings.In this type of gauge, a diaphragm connects to apointer through a metal spring and a simple linkagesystem (fig. 9-6). One side of the diaphragm isexposed to the pressure being measured, while theother side is exposed to the pressure of theatmosphere. Any increase in the pressure line movesthe diaphragm upward against the spring, movingthe pointer to a higher reading. When the pressuredecreases, the spring moves the diaphragmdownward, rotating the pointer to a lower reading.Thus, the position of the pointer is balanced betweenthe pressure pushing the diaphragm upward and thespring action pushing down. When the gauge reads 0,the pressure in the line is equal to the outside airpressure.MEASURING AIR PRESSURETo the average person, the chief importance ofweather is reference to it as an introduction togeneral conversation. At sea and in the air, advanceknowledge of what the weather will do is a matter ofgreat concern9-4
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