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This article shows how to convert pressure values expressed in millibars (mbar) to atmospheres (atm). It is often necessary to convert the units between mbar and atm when considering atmospheric pressure measurements, since barometers, which are the instruments used to measure pressure, are not standardized with respect to the units of measurement. Other units used to express pressure values are millimeters of mercury and pascals (Pa).
Pressure is a physical parameter that expresses a force applied per unit area. In the case of atmospheric pressure, it is the weight of the air column of the atmosphere on a certain surface; that is, it is the weight of all the molecules that make up the air in the atmosphere that are on a certain surface in the place of interest. The pressure unit atm was originally defined as the atmospheric pressure at sea level, but later it was correlated with an international system unit, the pascal, and its equivalent is 1 atm = 101325 Pa . The pascal is a force of 1 newton (N) applied on a surface of 1 square meter (m 2 ); Pa = N/m 2 .
The unit of pressure bar is defined in the international system of units in relation to the pascal; 1 bar = 100000 Pa . If you combine the definitions of the units atm and bar in relation to the pascal, you get that 1 atm = 1.01325 bar , or 1 bar = 0.987 atm . Atmospheric pressure is usually expressed in a submultiple of the bar; the millibar (mbar). In that case 1 atm = 1013.25 mbar .
Let’s look at some examples of unit conversion between mbar and atm.
Example 1
The atmospheric pressure at the height of flight of a passenger aircraft is approximately 230 mbar. What is the pressure expressed in atmospheres? As we have already seen, 1 atm = 1013.25 mbar. A simple rule of three can be applied. That is, if 1 atm = 1013.25 mbar, X atm = 230 mb, then X = 1×230/1013.25. And the expression of the atmospheric pressure of 230 mbar is then equivalent to 230/1013.25. That is, 230 mbar = 0.227 atm.
Example 2
If the pressure in a pipe measured with a manometer indicates 4500 mbar, what will be the pressure expressed in atm? Again using the equivalence 1 atm = 1013.25 mbar and applying a simple rule of thumb, it is obtained that the pressure of 4500 mbar measured in atm is 4500/1013.25 = 4.44 atm.
Fountain
Dominguez, Hector. Our atmosphere: how to understand climate changes . First edition. LD Books, 2004. ISBN 9707320524.
International Bureau of Weights and Measures. The International System of Units (SI), Eighth Edition, 2006. ISBN 92-822-2213-6.
Klein, Herbert Arthur. The Science of Measurement: a Historical Survey . Mineola, NY. Dover Publications 0-4862-5839-4, 1988.
McNaught, AD, Wilkinson, A., Nic, M., Jirat, J., Kosata, B., Jenkins, A. IUPAC. Compendium of Chemical Terminology . Second Edition, Oxford, Blackwell Scientific Publications, 2014. doi: 10.1351/goldbook.P04819
