【Pa (Pascal)】Easy-to-understand explanation of pressure unit conversion and concrete images



Even though it is said to be 1 Pa, it is difficult to imagine how big it is.

I would like to think about the specific force of air pressure, water pressure, stress, etc.

I will introduce it along with a simple image for understanding.

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Pa Pascal unit




The unit of Pa can be expressed as below.


You can see that pressure is the load on an object divided by its surface area.

For example, if a load of 10[N] (approximately 1kg) is applied to 1[m2], the pressure will be 10[Pa].

Also, I think you can understand from experience that the smaller the area, the greater the pressure.

Being stepped on by a stiletto heel hurts more than being stepped on by a shoe.



Conversion to kgf/cm2 and bar


An old unit is kgf/cm2 (engineering atmospheric pressure).

Also, depending on the industry, bar may be often used.



Intuitively, it may be easier to remember if you think that the digits of [bar] and [kgf/cm2] are the same.



Specific images of atmospheric pressure, water pressure, and stress


Image of atmospheric pressure


Atmospheric pressure is 101325 [Pa].

This means that approximately 10 tons of force is applied to 1 m2.

The atmosphere also has weight, and its density is approximately 1 [kg/m3]. (Density varies depending on temperature and altitude.)



I think it would be better to think of the weight of the accumulated weight of air as being applied to a plane.

The reason why the human body does not collapse even though atmospheric pressure is so heavy is because it is balanced because it is pushed by the same pressure from inside the body.

For example, the reason why a plastic bottle collapses as soon as you create a vacuum is because the atmosphere is gone and there is no longer any pushing force from the inside.



Image of water pressure


The pressure of tap water is said to be approximately 0.25 [MPa] to 0.3 [MPa] (2.5 [bar] to 3 [bar]).

The pressure you can feel when the faucet is fully open is approximately 0.25 [MPa] to 0.3 [MPa]. You feel a lot of pressure.





Stress MPa


The unit of Pa is also used for strength calculations in materials mechanics.

Each material has its own characteristics, and we design to provide a safety factor for yield stress.

For example, if the yield stress of SS400 is 240MPa, and the safety factor is 3, the allowable stress value is 80MPa. Intuitively, the force is equivalent to pulling a 100mm2 SS400 rod with 8000N (roughly 800kg).






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