75-25 mix causing problems?

[HighCountry]

Hey BNers: I need your help getting some of my beers on tap at a local bar/restaurant. They're a large chain that carries a lot of beer on tap. The bar manager is a friend and wants to carry some of my brews. Only problem is that they've had trouble with some of the kegs from one of our local packaging microbreweries. Their managers blame it on the fact that the bar/restaurant uses a 75-25 mix to push beer. The brewery in question that's had some problems is a VERY good shop, and they definitely know what they're doing. So I'm relatively sure that the problem is not on their end. Bottom line is that the bar managers can't tell me anything that will make my kegs compatible with their distribution system.

I'm not very knowledgeable about their system, or why they would use such a mix. I'm assuming it's nitrogen they're using to supplement the CO2, although my friend hasn't mentioned that yet. Any ideas on what kind of mix they're using and why? And why would that cause a problem with CO2 conditioned kegs? I'd love to solve the problem for them so they will like me even better...

In advance, many thanks.

[ajdelange]

Yes, 75/25 mix is 75% nitrogen and 25% CO2. The only reasons to use it are 1) you are serving stout through a faucet with a sparkler plate 2) you have very long lines from the keg to the faucet. For long lines a higher proportion of CO2 (60%) is generally used.

When a mix is used the pressure is set in order to move the beer at a rate that will deliver about 2 Oz/sec. at the faucet and the mix is chosen so that the partial pressure of CO2 sets the carbonation of the beer to the correct level. Thus if the beer is designed for 2 volumes and is intended to be stored at 38 °F pure CO2 would be applied at 6 psig (20.7 psia). If 6 psig won't push the beer fast enough one might try a 60% CO2 mix. 20.7/.6 = 34.5 psia is the pressure for a 60% mix that produces a partial pressure of 20.7 psia CO2 so the gauge on a 60% tank or blender would be set to 34.5 - 14.7 = 19.8 psig. For a 25% mix it would be 20.7/.25 - 14.7 = 68.1 psig and that will drive a lot of line. So I'd suspect that the trouble this account has been experiencing would be that it adjusts the pressure to get the flow right (that's pretty obvious, after all) with the result that the partial pressure of CO2 is too low (because they are using a stout mix) with the result that the beer winds up about as carbonated as stout (1 vol) which would be considered flat for most beers. If 25% CO3 is all they have available (e.g. they have a 25% only blender installed) then the fix would be to run its pressure up to whatever it takes to get the correct volumes in the beer and then add extra line ("choker") coils to reduce the flow to 2 Oz/sec at that pressure. Another solution is to use a blender (or blend) with a higher CO2 content.