Pretty obvious… Not So Obvious…
This image represents a Heliox 50, a mixture based on an equal number of elementary particles (atoms or molecules) of Oxygen and Helium.
So, when we talk about gas percentages, we mean the relationship between the amount of different gases expressed in terms of the number of elementary particles, atoms or molecules.
Before going into the heart of the matter, it’s important to clarify a topic apparently obvious.
Let’s imagine preparing a Heliox 50 starting from an empty cylinder.
What does Heliox 50% mean ?
It seems provocative but instead it’s important to stress this point: half Helium and half Oxygen.
Obvious, right ? Only apparently, half and half in what terms?
For example, 100 bar Oxygen and 100 bar Helium are half and half, but this is not a Heliox 50.
If you add 100 bar Oxygen and then Helium to 200 bar in sequence at 20 °C to an empty cylinder, you get a Heliox 57.1
Reversing the order of the gases, Helium first, then Oxygen, you get instead a Heliox 47.6
And even 1,000 grams of Helium and 1,000 grams of Oxygen are half and half, but even this is not a Heliox 50.
The misunderstanding here is even bigger: the relevant difference between the atomic weights of Helium and Oxygen causes a Heliox 11.1 !
The unit of measure used by chemists to measure the amount of gas is the mole, which identifies precisely an exact number of particles.
The mole is widely used in chemistry and has a value of 6.023 * 10 ^ 23 elementary entities of the substance, atoms or molecules in our case. This number expressed in scientific notation is an amount of six hundred thousand billions of billions of entities.
As far as blending by partial pressure is concerned, the procedure does not depend on the capacity of the cylinder.
Therefore, we can then talk in terms of concentration, defined as the number of moles per litre (mol/L) of volume, in order to make the matter under discussion independent from the volume of the cylinder.
In other words, we are talking about how many elementary entities of a certain gas there are per liter of internal volume of the cylinder.