With the use of blend gas the pressure to the kegs will be approx. 22 PSIG. This is one advantage of using blends. Higher pressure to compensate for temperature fluctuations in systems and for pressure drops in remote systems. 100% co2 cannot accomplish this. CO2 pressure is based on the temperature of the beer in the keg. No more, no less. Not much flexibility.
With 22 PSIG, the goal is to balance the system with enough restriction (measured in PSI) in hardware, gravity, and tubing to acquire 128 oz./min flow speed at the faucet. This is accomplished through building a system with 22 PSI of restriction against the beer while pushing it with 22 PSIG. Call it fluidtronics if you will. Assuming that you are using a Kool Rite tower @ 7.5#s; 1# for the jumper line from the keg to the wall bracket in the cooler; 4' gravity vertical rise @ 2#'s; 15' of 5/16" I.D. Valpar Brewmaster II @ 1.5#s, this equates to 12 PSI of restriction. 12 PSI short of 22. 3/16" choker has 3 PSI per foot. three feet allows for 9 PSI. Actually, three feet four inches would be ideal to install between the tower and the 5/16" Brewmaster II line based on the above assumptions.
As to the 25% CO2/75%N, it is very difficult to place two different gasses in the same bottle and expect to consistently acquire the same mix through out the duration of the cylinder. Even if you could, you would be using a mix designed specifically for nitrogenated beers such as Guinness Stout. Very low carbonation levels with these types of beers. Ales and lagers have double the amount of gas and to maintain this level of gas you would require a mix of approx. 60% CO2/40%N. Mixture designed for stouts would allow ales and lagers to degass with in a short amount of time. Be aware that carbonation in beer has flavor characteristics. You change the recipe of gas in the beer, and you change the flavor. The blender technology is accurate, capable of producing the correct blends and gives you piece of mind so you can sleep at night knowing that the first glass of beer out of the keg will taste the same as the last.
Scott Zuhse, Instructor
Micro Matic Dispense Institute