The aneroid barometer shown in figure 9-7 is an instrument that measures air pressure at sea level. It consists of a thin-walled metal box from which most of the air has been pumped and a dial indicating low- and high-pressure measurements. A pointer on the dial is connected to the box by a lever system. If the pressure of the atmosphere increases, it squeezes the sides of the box. This squeeze causes the pointer to move toward the high-pressure end of the dial. If the pressure decreases, the sides of the box expand outward. That causes the pointer 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 level pressure is 29.92 inches and readings below 27 inches or above 31 inches are rarely seen.

Figure 9-8 illustrates a mercurial barometer. It consists of a glass tube on which measurements are indicated; the tube is partially filled with mercury. The upper end, which is closed, contains a vacuum above the mercury. The lower end, which is open, is submerged in a cup of mercury that is open to the atmosphere. The atmosphere presses down on the mercury in the cup and pushes the mercury up in the tube. The greater the air pressure, the higher the rise of mercury within the tube. At sea level, the normal pressure is 14.7 psi, and the height of the mercury in the tube is 30 inches. As the air pressure increases or decreases from day to day, the height of the mercury rises or falls. A mercury barometer aboard ship mounts in gimbals to keep it in a vertical position 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 always indicates absolute pressure, not relative. When the pressure exerted by any gas is less than 14.7 psi, you have what we call a partial vacuum.

runs into the turbine and causes the rotor to turn. After it has passed through the turbine, it still exerts a back pressure against the blades. If the back pressure were not reduced, it would build until it became as great as that of the incoming steam and prevent the turbine from turning at all. Therefore, the exhaust steam is run through pipes surrounded by cold sea water to reduce the back pressure as much as possible. The cold temperature causes the steam in the pipes to condense into water, and the pressure drops well below atmospheric pressure.

The engineer needs to know the pressure in the condensers at all times. To measure this reduced pressure, or partial vacuum, the engineer uses a gauge called a manometer. As shown in figure 9-9, it consists of a U-shaped tube. One end is connected to the low-pressure condenser, and the other end is open to the air. The tube is partially filled with colored water. The normal air pressure against the colored water is greater than the low pressure of the steam from the condenser. Therefore, the colored water is forced part of the way into the left arm of the tube. A scale between the two arms of the U indicates the difference in the height of the two columns of water. This difference tells the engineer the degree of vacuum-or how much below atmospheric pressure the pressure within the condenser is.

You should remember seven points about force and pressure:

A force is a push or a pull exerted on or by an object. You measure force in pounds.

Pressure is the force per unit area exerted on an object or exerted by an object. You measure it in pounds per square inch (psi).

Spring scales and lever balances are familiar instruments you use for measuring forces.

Bourdon gauges, barometers, and manometers are instruments for the measurement of pressure.

The normal pressure of the air is 14.7 psi at sea level.

Pressure is generally relative; that is, it is sometimes greatersometimes lessthan normal air pressure. Pressure that is less than the normal air pressure is called a vacuum.