MEASURING FLUID PRESSURE

All fluids-both liquids and gasesexert pressure. A fluid at rest exerts equal pressure in all directions. As shown in figure 9-3, water will push through a hole in a submarine, whether it is in the top, the bottom, or in one of the sides.

Many jobs aboard ship will require you to know the pressure exerted by a gas or a liquid. For example, knowing the steam pressure inside a boiler is always important. You can use three different gauges to find the pressure of fluids: Bourdon gauge, Schrader gauge, and diaphragm gauge.

Figure 9-4.-The Bourdon gauge.

Bourdon Gauge

The Bourdon gauge is shown in figure 9-4. It works 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 metal tube, somewhat flattened and bent into the form of a C. Attached to its free end is a lever system that magnifies any motion of the free end of the tube. On the fixed end of the gauge is a fitting you thread into a boiler system. As pressure increases within the boiler, it travels through the tube. Like the snakelike paper whistle, the metal tube begins to straighten as the pressure increases inside of it. As the tube straightens, the pointer moves around a dial that indicates the pressure in psi.

The Bourdon gauge is a highly accurate but rather delicate instrument. You can easily damage it. In addition, it malfunctions if pressure varies rapidly. This problem was overcome by the development of another type of gauge, the Schrader. The Schrader gauge (fig. 9-5) is not as accurate as the Bourdon, but it is sturdy and suitable for ordinary hydraulic pressure measurements. It is especially suitable for fluctuating loads.

In the Schrader gauge, liquid pressure actuates a piston. The pressure moves up a cylinder against the resistance of a spring, carrying a bar or indicator with it over a calibrated scale. The operation of this gauge eliminates the need for cams, gears, levers, and bearings.

Diaphragm Gauge

The diaphragm gauge gives sensitive and reliable indications of small pressure differences. We use the diaphragm gauge to measure the air pressure in the space between inner and outer boiler casings.

In this type of gauge, a diaphragm connects to a pointer through a metal spring and a simple linkage system (fig. 9-6). One side of the diaphragm is exposed to the pressure being measured, while the other side is exposed to the pressure of the atmosphere. Any increase in the pressure line moves the diaphragm upward against the spring, moving the pointer to a higher reading. When the pressure decreases, the spring moves the diaphragm downward, rotating the pointer to a lower reading. Thus, the position of the pointer is balanced between the pressure pushing the diaphragm upward and the spring action pushing down. When the gauge reads 0, the pressure in the line is equal to the outside air pressure.