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Along with temperature, pressure is one of the most important physical quantities in science and engineering, and it can be expressed in a wide variety of units. This makes it necessary to know how to convert from one unit to another easily. In this article, several examples are presented showing how to convert bars to atmospheres, two of the most common and widely used units of pressure.
The atmosphere as a unit of pressure (atm)
As a unit of pressure, one atmosphere, commonly represented as atm, represents the pressure exerted by air in the atmosphere at sea level. This, at least, was the way unity was originally defined. However, due to the fact that atmospheric pressure is not constant at any point on earth, it was redefined in 1954 as a unit of pressure exactly equal to 101 325 Pa (1.01325.10 5 pascals), which is equivalent to the pressure exerted by a column of 760 mm of mercury at 0°C.
It is a unit of absolute pressure which, in terms of its magnitude, is a relatively high unit of pressure. For this reason, the atmosphere is often used to express the value of high pressures, as is the case with compressed gases.
The bar
The bar, on the other hand, is a unit very similar in magnitude to the atmosphere, which is also used to express the value of absolute pressures. It is defined as exactly 100 kilopascals or, what is the same, 10 5 Pa, which makes it a unit of the metric system, although it is not part of the international system of units. By comparing this value with the equivalent pascals of the atmosphere , it is easy to obtain a relationship between the two units:
Due to its easy conversion to pascals (the unit of pressure of the international system) and its multiples, this unit has been gradually gaining ground against the atmosphere. In fact, many standard thermodynamic data that used to be reported at a pressure of 1 atmosphere are today reported at 1 bar instead.
However, many physical and chemical constants, such as the ideal gas constant, are still used in atmospheres, so it is of great importance to be able to convert pressure in bars to atmospheres.
How to convert bar to atm
To convert pressure in bars to pressure in atmospheres (and vice versa), just multiply by the appropriate unit conversion factor. To do this, it is enough to remember that 1 atm = 1.01325 bar. Rearranging this equality, two unit conversion factors can be obtained:
To know which of the two we should use, we just need to remember that the unit we want must always appear in the numerator, and the one we have (that is, the one we want to convert) must always be in the denominator. So the conversion factor that we will use to convert bar to atm will be the one on the left.
Expert tips
- As we can see, the atmosphere is a unit of pressure greater than the bar. Therefore, when we convert a pressure from bar to atm, the result should always be a slightly smaller number. If converting the pressure from bars to atmospheres results in a higher number, then a mistake was certainly made, and the mistake was probably that the conversion factor was used backwards.
- Care must be taken not to confuse the bar with other similar pressure units such as baria which is actually a unit equal to one millionth of a bar, or with barg or bar(g) which is a relative unit of pressure indicating gauge pressure instead of absolute.
Examples of converting bars to atmospheres
Example 1
The pressure outside a passenger plane flying at cruising altitude is about 0.2300 bar. Convert this pressure to atmospheres.
Solution: It is only necessary to multiply by the conversion factor that cancels the units that we have and leaves the units that we want:
Answer: As you can see, a pressure of 0.23 bar is equivalent to 0.2270 atm, a slightly lower value.
Example 2
Convert the normal pressure of the tires of a passenger car, which is 2,206 bar, to atmospheres.
Solution: Again, we multiply the pressure we have by the appropriate correction factor:
Answer: Tire pressure of 2,206 bar is equivalent to 2,177 atm.
Reverse conversion: How to convert atm to bar?
Now that we know how to convert bar to atm, the reverse conversion is just as easy. The only thing that changes is that we use the inverse conversion factor (the one on the right) which has bar in the numerator and atm in the denominator.
Example
It is desired to convert the atmospheric pressure on the surface of the planet Mars which is, on average, 6.433×10 -3 atm to bar.
Solution: In this case, the conversion factor will be (1.101325 bar/1atm), so the conversion will be:
Answer: The pressure on the surface of Mars is 6.518×10 -3 bar.
References
Serway, RA, Beichner, RJ, & Jewett, JW (1999). Physics: For Scientists and Engineers (Saunders Golden Sunburst Series) (5th ed.). Philadelphia, PA: Saunders College Pub.
The mathematical treatment of the measurement results. (n.d.). Retrieved June 29, 2021, from https://espanol.libretexts.org/@go/page/1798
Gas pressure. (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1868
