How to Calculate a Mercury Barometer Reading Using Air Pressure
Miles of atmosphere above the Earth pushes down with a pressure at sea level of about 15 pounds per square inch. Liquid barometers measure this air pressure by countering it with the weight of a liquid. Mercury is usually the liquid of choice. Its heft means you don't need a column several yards high to counter the pressure of the miles of air above. Calculating from air pressure to an equivalent height of a column of mercury is a simple conversion.
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Instructions
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1
Measure the air pressure. Pascals or pounds per square inch are suitable units. (The Pascal is the SI unit for pressure.)
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2
Convert the air pressure into Pascals.
For example, 1 pound per square inch equals 6,895 Pascals. So, 14.8 PSI becomes 14.8 times 6,895, or 102,046 Pascals.
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3
Divide the air pressure from step 2 by 1,327.5 to get the height (in centimeters) of a column of mercury in a barometer, measured from the height of the mercury-air interface to the height of the top of the mercury.
In this same example, 14.8 PSI becomes 1.02 times 10^5 Pascals, which converts to 76.8 cm of mercury. (The caret ^ refers to exponentiation.) Convert this to inches by multiplying by 0.3937, to get 30.2 inches of mercury.
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Tips & Warnings
The basic strategy of coming up with the conversion factor 1,327.5 is to convert the weight of a centimeter-squared column of mercury into pressure and equate it with the air pressure, measured per centimeter-squared. The factor between the air pressure and the mercury column's height is your conversion factor.
