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### 1.4. GAS LAWS AND VAPOR PRESSURE

#### 1.4A. Pressure

There are numerous ways of expressing the pressure exerted by a fluid or system. An absolute pressure of 1.00 atm is equivalent to 760 mm Hg at 0°C, 29.921 in. Hg, 0.760 m Hg, 14.696 lb force per square inch (psia), or 33.90 ft of water at 4°C. Gage pressure is the pressure above the absolute pressure. Hence, a pressure of 21.5 lb per square inch gage (psig) is 21.5 + 14.7 (rounded off), or 36.2 psia. In SI units, 1 psia = 6.89476 × 103 pascal (Pa) = 6.89476 × 103 newtons/m2. Also, 1 atm = 1.01325 × 105 Pa.

In some cases, particularly in evaporation, one may express the pressure as inches of mercury vacuum. This means the pressure as inches of mercury measured “below” the absolute barometric pressure. ...

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