Mixing gases to achieve desired ppm

[aliveinmoscow]

I have to mix 2 gases, Hydrogen and Nitrogen to achieve a constant flow of mixed gas with 100, 200... 1000 ppm of H2 in the mixture.

I have mass flow meters to adjust the flow of each individual gas but how do I determine what they should read to achieve the desired ppm

 

[brewnog]

Parts per million of what? Mass? Volume? Molecule count?

 

[aliveinmoscow]

ppm of volume in the container... sorry for being vague. the container has a volume of 2l.

Also, without using the mass flow controllers, if i were to just fill in the container with hydrogen and nitrogen starting from vacuum, what should be the pressure to which i should fill up nitrogen and then add hydrogen to get the desired ppm