Pressure, like temperature, is one of the basic engineering measurements and one that must be frequently monitored aboard ship. As with temperature readings, pressure readings provide you with an indication of the operating condition of equipment. PRESSURE is defined as the force per unit area.

The simplest pressure units are the ones that indicate how much force is applied to an area of a certain size. These units include pounds per square inch, pounds per square foot, ounces per square inch, newtons per square millimeter, and dynes per square centimeter, depending upon the system you use.

You also use another kind of pressure unit that involves length. These units include inches of water (in. H,O), inches of mercury (in.Hg), and inches of some other liquid of a known density. Actually, these units do not involve length as a fundamental dimension. Rather, length is taken as a measure of force or weight. For example, a reading of 1 in.H,O means that the exerted pressure is able to support a column of water 1 inch high, or that a column of water in a U-tube would be displaced 1 inch by the pressure being measured. Similarly, a reading of 12 in. Hg means that the measured pressure is sufficient to support a column of mercury 12 inches high. What is really being expressed (even though it is not mentioned in the pressure unit) is that a certain quantity of material (water, mercury, and so on) of known density exerts a certain definite force upon a specified area. Pressure is still force per unit area, even if the pressure unit refers to inches of some liquid.

In interpreting pressure measurements, a great deal of confusion arises because the zero point on most pressure gauges represents atmospheric pressure rather than zero absolute pressure. Thus, it is often necessary to specify the kind of pressure being measured under any given conditions. To clarify the numerous meanings of

Figure 2-11.-Relationships among gauge pressure, atmospheric pressure, vacuum, and absolute pressure.

the word pressure, the relationships among gauge, atmospheric, vacuum, and absolute pressures are shown in figure 2-11

GAUGE PRESSURE is the pressure actually shown on the dial of a gauge that registers pressure relative to atmospheric pressure. An ordinary pressure gauge reading of zero does not mean there is no pressure in the absolute sense; rather, it means there is no pressure in excess of atmospheric pressure.

ATMOSPHERIC PRESSURE is the pressure exerted by the weight of the atmosphere. At sea level, the average pressure of the atmosphere is sufficient to hold a column of mercury at the height of 76 centimeters or 29.92 inches. Since a column of mercury 1 inch high exerts a pressure of 0.49 pound per square inch (psi) at its base, a column of mercury 29.92 inches high exerts a pressure that is equal to 29.92 x 0.49 or about 14.7 psi. Since we are dealing now in absolute pressure, we say that the average atmospheric pressure at sea level is 14.7 pounds per square inch absolute (psia). It is zero on the ordinary pressure gauge.

Notice, however, that the figure of 14.7 psia represents the average atmospheric pressure at sea level; it does not always represent the actual pressure being exerted by the atmosphere at the moment a gauge is being read. Since fluctuations from this standard are shown on a barometer (an instrument used to measure atmospheric pressure), the term barometric pressure is used to

Figure 2-12.-typical barometer.

describe the atmospheric pressure that exists at any given moment. Figure 2-12 shows the operating principle of a typical barometer.

BAROMETRIC PRESSURE is the term used to describe the actual atmospheric pressure that exists at any given moment. Barometric pressure may be measured by a simple mercury column or by a specially designed instrument called an aneroid barometer.

A space in which the pressure is less than atmospheric pressure is said to be under partial vacuum. The vacuum is expressed in terms of the difference between the absolute pressure in the space and the pressure of the atmosphere. Most commonly, vacuum is expressed in inches of mercury, with the vacuum gauge scale marked from 0 to 30 in.Hg. When a vacuum gauge reads zero, the pressure in the space is the same as atmospheric pressure-or, in other words, there is no vacuum. A vacuum gauge reading of 29.92 in. Hg would indicate a perfect (or nearly perfect) vacuum. In actual practice a perfect vacuum is impossible to obtain even under laboratory conditions. A reading between 0 and 29.92 in.Hg is a partial vacuum.

ABSOLUTE PRESSURE is atmospheric pressure plus gauge pressure, or absolute pressure minus vacuum. For example, a gauge pressure of 300 pounds per square inch gauge (psig) equals an absolute pressure of 314.7 psia (300 + 14.7). Or, for example, consider a space in which the measured vacuum is 10 in. Hg; the absolute pressure in this space is figured by subtracting the measured vacuum (10 in.Hg) from the nearly perfect vacuum (29.92 in.Hg). The absolute pressure then will be 19.92 or about 20 in.Hg absolute. Note that the amount of pressure in a space under vacuum can only be expressed in terms of absolute pressure.

You may have noticed that sometimes we use the letters psig to indicate gauge pressure and other times we merely use psi. By common convention, gauge pressure is always assumed when pressure is given in pounds per square inch, pounds per square foot, or similar units. The g (for gauge) is added only when there is some possibility of confusion. Absolute pressure, on the other hand, is always expressed as pounds per square inch absolute (psia), pounds per square foot absolute (psfa), and so forth. It is always necessary to establish clearly just what kind of pressure we are talking about, unless this is very clear from the nature of the discussion.

To this point, we have considered only the most basic and most common units of measurement. Remember that hundreds of other units can be derived from these units; remember also that specialized fields require specialized units of measurement. Additional units of measurement are introduced in appropriate places throughout the remainder of this training manual. When you have more complicated units of measurement, you may find it helpful to review the basic information given here first.