I highly recommend Nigel Hewitt's "Blender or Marcin "Hlp" Kaluza's HLP Blender for gas blending. Both excellent free pieces of software and they're spot on to achieve the desired outcome if you are blending (pure) helium and oxygen on top of a mix.
However, in my case I don't have access to J's of helium so I often end up boosting an existing trimix on top of another mix, especially now I've acquired an electric gas booster. Looking through all of the gas mix calculator spreadsheets I've collected over the years (nearly a dozen of them) none of them will give me the recipe for blending two known trimix plus oxygen plus air to reach a defined target. Yes, you can do it piecewise with the above software but not in a single click.
So this weekend I set out to write Yet Another Gas Mix Calculator. Initially I thought I would just follow the Ideal gas laws but having used the above programs I know this is a poor second to the better approximation of van der Waals (vdW) equation of state (EoS). So mad as I am I decided to tackle vdW's EoS in Excel!!
Thank god for Nigel's treatise on vdW as without that I would have been stumped. I'm actually quite pleased with the end result so I thought I would share with everyone here. Here's an example turning 100 bar of 18/40 into 232 bar of 20/30 using 14/55 and EAN80 - calculated in less than a second!
Download the file here YAGaMiC 2015 v1 (1.7)
Download the file here YAGaMiC 2016 v2 (2.1) with GERG
You will need to have the Excel "Solver" Add-In installed on your computer and to enable macros when the spreadsheet loads. YAGaMiC will load the Solver if it is present so you don't have to faff around in Excel Options.
Most of the tabs are standard stuff that you will find in most blending software (with the twist of having both Ideal and vdW calculations on the same page). The first tab "Blend Calc" is where it fills the need that I have - put in your initial mix and desired target mix, tell it what gases you have available and it will attempt to compute the blending solution - nice, even if I do say so myself.
Some solutions are possible but not useful (e.g. adding a negative quantity of gas) and sometimes Solver cannot converge (in which case you will get an error message) but surprisingly often there's a viable and useful solution. And because it's so quick you can play with the numbers to achieve close to your desired outcome.
Whilst the tabs are 'protected' to stop accidental editing I did not password protect so if you fancy looking at the coding or making further improvements then fill your boots. With Excel VBA being so esoteric it will be a miracle if this runs for anyone else, let alone other platforms like Mac but who knows.
[Polite] feedback welcome and if anyone actually finds this useful I wouldn't mind the odd PM to let me know.