[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 each of the desired ppm combinations

[Stepan]

In chemistry of gases, ppm usually stands for "ppm by volume". If this is what you need to achieve, than Hydrogen flow needs to be 100/1,000,000; 200/1,000,000 ; and 1,000/1,000,000 of the Nitrogen flow correspondingly.