Figure 9-8.-A mercurial barometer.Figure 9-7.-An aneroid barometer.Mercurial BarometerSince air pressure affects weather, you can see whythe use of a barometer is so important to ships. However,not so apparent is the importance of air pressure in theoperation of the ship’s engine. For that purpose airpressure is measured with a gauge called a manometer.Aneroid BarometerThe aneroid barometer shown in figure 9-7 is aninstrument that measures air pressure at sea level. Itconsists of a thin-walled metal box from which most ofthe air has been pumped and a dial indicating low- andhigh-pressure measurements. A pointer on the dial isconnected to the box by a lever system. If the pressureof the atmosphere increases, it squeezes the sides of thebox. This squeeze causes the pointer to move toward thehigh-pressure end of the dial. If the pressure decreases,the sides of the box expand outward. That causes thepointer to move toward the low-pressure end of the dial.Notice that the numbers on the dial are from 27 to 31.This scale of numbers is used because average sea levelpressure is 29.92 inches and readings below 27 inchesor above 31 inches are rarely seen.Figure 9-8 illustrates a mercurial barometer. Itconsists of a glass tube on which measurements areindicated; the tube is partially filled with mercury. Theupper end, which is closed, contains a vacuum above themercury. The lower end, which is open, is submerged ina cup of mercury that is open to the atmosphere. Theatmosphere presses down on the mercury in the cup andpushes the mercury up in the tube. The greater the airpressure, the higher the rise of mercury within the tube.At sea level, the normal pressure is 14.7 psi, and theheight of the mercury in the tube is 30 inches. As the airpressure increases or decreases from day to day, theheight of the mercury rises or falls. A mercury barometeraboard ship mounts in gimbals to keep it in a verticalposition despite the rolling and pitching of the ship.The dial of most gauges indicate relative pressure;that is, it is either greater or less than normal.Remember-the dial of an aneroid barometer alwaysindicates absolute pressure, not relative. When thepressure exerted by any gas is less than 14.7 psi, youhave what we call a partial vacuum.ManometerThe condensers on steam turbines operate at apressure well below 14.7 psi. Steam under high pressure9-6
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