Pressure is expressed in pounds per unit of area in inches of the static pressure of liquids. The most popular pressure-indicating instruments (gauges) register in pounds per square inch ABOVE the atmospheric pressure (psig) or (psi). The pressure of 0 points per square inch gauge is equal to the atmospheric pressure of approximately 14 .7 pounds per square inch, (15 pounds per square inch is usually used for computation purposes).
Pressures below atmospheric pressure or termed partial vacuums. A perfect vacuum may be described as 0 pounds per square inch absolute (0 psia). Therefore the absolute pressure scale has its zero at a pressure that cannot be further reduced. Pressure may also be indicated in inches of mercury (“Hg) or feet or inches of the water column and may be either above atmospheric pressure are absolute pressure depending on the construction of the gauge. Mercury is usually used for measuring pressure below atmospheric pressure.
The barometer is an example of a mercury gauge. With vacuum is one and of the tube. The atmospheric pressure will support a mercury column 29.92 in. in height at sea level under standard conditions.
For most pressure and volume completions it is necessary to use absolute pressures, (psia).
Example: what is the absolute pressure when a pressure gauge shows a reading of 21 psi?
Solution: absolute pressure equals gauge pressure (psig) plus atmospheric pressure.
Psig + 15 = psia
21 + 15 = 36 psia
It is possible to measure air pressure and the amount of vacuum using a column of water. A column of water would be about 34 feet in height to equal a 29.92-inch column of mercury. This is because mercury is much heavier than water.
In refrigeration and air conditioning work, the service engineer is then testing the pressures of vacuums in the same system. Also under condition conditions of operation, a part of the system may be under considerable pressure and during another part of the operating cycle, it may be under a high vacuum.
Pressure gauges are made which will measure both pressures above atmospheric pressure and pressures below atmospheric. These gauges air called compound gauges. Illustrates a compound gauge used in refrigeration work.
Water columns are often used for measuring small pressures above or below atmospheric pressure such as pressures in air ducts, gas lines, and the like. These pressure measuring devices are called manometers and they are calibrated in inches of water column.
In some large refrigerating machines pressure gauges are calibrated in atmospheres.
One atmosphere corresponds to 15 pounds per sq. in. (psig), to an atmosphere of 30 psig, three atmospheres to 45 psig, and so on.
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Basics of Refrigeration