Air Pressure

Air pressure is defined as the amount of force aplied to a given area. In the SI-system, pressure is given in Pascal. This is a very small unit when compared to both the psi unit and bar, since Pascal (Pa) describes Newton per square meter (N/m²). The reason for this is that the area that the force in Pascal is distributed over, is so much greater than for both psi and bar.

Area comparison:

psi         = 1 in²              = 6.453 cm²     = 0.6453⋅10⁻³ m² (1 550 times greater area)

bar        = 0.155 in²       = 1 cm²             = 0.1⋅10⁻³ m²      (10 000 times greater area)

Pascal   = 1.55⋅10³ in²   = 10⋅10³ cm²     = 1 m²

Since the area which Pascal operates with is so much greater than both psi and bar, we often use prefixes together with the Pa unit. Prefixes lets us increase, or decrease, the multiplier of the variable without altering the magnitude of the number.

Example 1:

If we have 10 000 Pa, we can simply replace three of the zeros with a kilo prefix. The new way of writing the same number then becomes: 10 kPa = 10 000 Pa.

We can do the same trick with very small numbers as well (though this is not likely with the Pascal unit, since it's already quite small). 

Example 2:

We have a pressure of 0.00065 Pascal. This number can be hard to read, let alone to spell. The way around this problem is simply to use an prefix that alows us to remove some of the decimals. We could for example use the micro (µ) prefix which is equal to 0.000001. The new number, wiht a prefix, would then become = 650 µPa = 0.00065 Pa

The most commone prefix used toghether with Pascal is probably kilo (k = 1000). This prefix seems to fit perfect with other pressure units like e.g. bar. When we state atmospheric pressure in bar we roughly say that it's 1 bar (actually 1 atmosphere is 1.013 125 bar to be precise). Since Pascal is so much smaller then bar we express 1 bar as 100 000 Pascal. Obviously this is a tiresome way of writing pressure and therefore we often replace three of the zeros with a kilo (k) prefix instead. This way we can write one atmosphere as 100 kPa (101.235 kPa to be acurate).