Ambient air is composed of various gases. The pressure “created” by each individual gas is called partial pressure.
Under normal ambient conditions, we find 5 ppm of helium or 5 * 10-4%. Please see the below image to understand the composition of ambient air by symbol, % by volume and PPM.




An example of partial pressure in an application

Here is an example of partial pressure where Container A is filled with pure Oxygen at 5 bar, and Container B is filled with pure Nitrogen at 11 bar.

helium example partial pressure


The total pressure in container C is 16 bar, being the sum of the partial pressure of oxygen and the partial pressures of nitrogen combined.

And our conclusion: the different molecular types do not affect each other in container C, but will hit the walls as many times per second and with equal force when they are mixed (like they would if they were the only gas type present in the container).

See what impact flows have on leak detection in our next section here.