Constant calorific value problem

pomocnik89

Hello, I've got this problem:

Imagine I mix 2 gases - air + methane (e.g.) and I can change/control quantity/volume of each gas. So if I change the volume of air how much methane should I add to keep a constant calorific value KJ/kg? So for example if I add 200 litres of air I need to know how much methane to add to have a constant calorific value, and then if I add only 150 then how much gas i need to add etc.

Hope I explained my problem ... Just some simplified formula/equation would be ok. I need it for my Delphi vizualization program, so it hasn't to be "physicaly 100% correct" :-) Thank you very very much for your help !!!