Pressure units and the barometer

Commonly used pressure units are listed here.

Unit	Symbol	1 atm equivalent
Millimeters of mercury	mm Hg	760 mm Hg
Pounds per square inch	PSI	14.7 PSI
torr	torr	760 torr Hg
Pascal	Pa	101,325 Pa
atmospheres	atm	1 atm
Inches of mercury	in Hg	29.92 inches Hg

Pressure measurements

A list of pressure units is good as a reference but how is pressure measured.

The barometer is the simplest instrument for measuring atmospheric pressure. The earth's atmosphere at sea level has a weight of 14.7 pounds over a square inch of surface. This is the weight of a column of air that extends from sea level at the earth's surface to the edge of the atmosphere. This weight changes as the temperature and composition of the air mass changes. The changes are related to weather changes. A high barometer reading indicates dense air usually associated with stable cold weather. A low barometer reading is usually associated with unstable weather.

A barometer uses a substitute column of mercury fluid in place of the air. A cheaper barometer can be made using a column of water. Original barometers where made using water. This was a nuisance because the length of the water column is about 32 feet. for one atmosphere. Measurements were done using ladders. More convenient barometers can be made using mercury.

Mercury is the most dense fluid available. It has a density of 13.6 g/mL. This is 13.6 times the density for water. A barometer using mercury is more compact. One atmosphere in a mercury barometer is equaled by a column of only 760 mm Hg.

The column of mercury needed is shorter when the weight of the air column decreases. This matches "low" pressure. On the other hand when the weight of the air column increases the length of the mercury column increases. More mercury is needed to equalize the heavier air column. Old time aircraft carried barometers to measure altitude. The higher the altitude the lower the air pressure and the shorter the column of mercury needed to equalize the air pressure. If you go high enough there is no air pressure to speak of the mercury column would be zero. This decrease in pressure with increasing altitude is one reason why commercial aircraft have pressurized cabins. The pressure inside is maintained artificially. The Hollywood adventure movies showing people blown out of airliners are based on some truth. The real fiction appears when actors are shown able to hang on to an aircraft and breathe easily at 30,000 ft.