switching from a CO2 to Nitro mix

Jikel

I only recently got set up with a beer gas tank and stout faucet and have trouble with an excessive head on the dispensed beer. It may be that when it was originally kegged it was with CO2 only and I believe that this will have put too much into solution. What is my best option: (1) release all pressure on the keg and let it flatten before re-pressurising with nitro blend OR (2) give up on the Nitro blend on this keg and revert to straight CO2?
I think I read that the pressure for Nitro should be between 30 - 40 psi. I am currently running this keg at about 25 psi but continue to have major foam issues (all foam, no beer) on the initial pull. I think it is because it is over carbonated and is dispensed through the stout faucet.
Not sure what is the best way out of this and all advise will be welcomed.

AlexKay

Beer gas and a stout faucet will give you a glass full of foam; that’s kind of the point! It’s dense foam, though, that drops a lot of liquid as it dissipates, with that “cascade” effect as it does that.
I’d definitely bump the pressure to 30 or 35. Sometimes the results actually improve, counterintuitively.
If you do have a lot of dissolved CO2, as you say, you could definitely try venting it to bring the carbonation down.

doug293cz

What pressure and temperature was the beer at when it was on pure CO2 (that will determine the current carbonation level.) With beers on nitro, you usually want the carb level down around 1 - 1.2 volumes. What temp do you plan to keep the beer on mixed gas? I've attached a carbonation calculator (both Excel and LibreOffice versions) that works for mixed gas.
Brew on

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Bobby_M

If you're getting a full glass of lightweight foam, the beer is overcarbonated. For nitro pours, I like the beer to have about 6-7 psi of CO2 while most CO2-only beers would be closer to 10-12psi. Disconnect the gas and vent the headspace to near zero and let it sit a while. Vent again. Essentially you run the beergas pressure, assuming it's 25/75%, at 4x the pressure of the CO2 carb level because they are equal regarding the CO2.

Odie

I had a similar problem. I did a RIS and it was stored under pressure for a year. Not a lot of pressure but still enough to cause foaming issues on my nitro set up.
I basically vented it for a while to bleed carbonation and backed the nitro regulator way down. It now pours better but it took more than half the keg to reach that point...
Just unhook it, vent it a bunch, put a spunding valve on it and let it alone for a while. probably a week or longer. you need to get the CO2 volume down. On nitro at 25-30 psi is only like a normal amount of dissolved CO2 in the beer.

FloppyKnockers

What all these guys said.
I'll add though that when I'm overcarbed and impatient, I'll give the keg a shake, relive the pressure, rinse, and repeat until it has flattened to where I want. Works faster when the keg/beer is room temp.

whattabrau

You can shake the keg, vent it, shake the keg, repeat. In essense you're doing reverse force carbonation.
However, you need to remember that you can't vent it below 0psi gauge pressure, and at 0psi you'll still have *more* CO2 in the headspace than what [in most cases] you'd get from pressurizing the keg with beer gas. So, unless you're interested in fighting it, you might just as well save beer gas for the next keg.
I serve nitro beers on pure nitro. I prefer them after they lose significant residual fermentation CO2. If I don't shake the keg, a sufficient level of carbonation tends to stick around even without injecting any CO2 into the keg. CO2 really eats away the nitro head, to the point where it would have been easier/cheaper to serve it just with very little CO2 pressure, a minimal line, and a rough pour.

doug293cz

You can't do gas ratio calculations using gauge pressure. You have to use absolute, and partial pressures. Gauges measure the difference in vessel pressure to atmospheric pressure - which is 14.7 psi (at sea level.) So, when your gauge reads 10 psi, the absolute pressure is 24.7 psi. Carbonation level is determined by the absolute pressure of just the CO2. If the headspace in the keg is pure CO2, then you can just subtract 14.7 from the required absolute CO2 pressure, to get gauge pressure, and put that on your carbonation chart, to make things simple (i.e. no math) for users.
When you have a mix of gases, the absolute pressure of each component (if by itself) is called the partial pressure of that component. Partial pressures are always absolute pressures, never gauge pressures. So, the value that is important for carbonation level when using beer gas (CO2/N2 mix) is the CO2 partial pressure. The partial pressure of any component of a mix is just the volume fraction of that component in the mix times the absolute pressure of all the gas mix. So, if you have 25/75 (CO2/N2) beer gas at 13.3 psi gauge pressure (psig) then the absolute pressure is 14.7 + 13.3 = 28 psia. The CO2 partial pressure is 0.25 * 28 = 7 psi, and the N2 partial pressure is 21 psi.
So, if you have beer at 10.1 psig of pure CO2, that is a CO2 partial pressure of 24.8 psi. To drop the carbonation level in half, you need to cut the CO2 partial pressure in half, which would be 12.4 psi. In a pure CO2 environment, that would translate to a gauge pressure of -2.3 psig (not an easy thing to accomplish.) However in a 25/75 beer gas environment, the 12.4 psi CO2 partial pressure would become 49.6 psia total pressure, or a gauge pressure of 49.6 - 14.7 = 34.9 psig, not the 44.4 psig that would result from the quoted post's (incorrect) method of calculation.
Brew on

VikeMan

^ This is the best explanation of mixed beer gas pressures I've seen in 500 words or less.
ETA: I'm stealing that.

doug293cz

Thanks. I like to think I'm pretty good at explaining things.
Brew on

bracconiere

i'm just glad now i know the difference when i see psig, and psia!

bracconiere

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out of curiosity, is the co2 still liquid in beer gas? and can i calculate something like this in mols, by weight?

bracconiere

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Odie

nope. It's all gaseous. that's why a large "80" tank of beer gas does not go as far as a small "20" tank of CO2.

doug293cz

No. It would be if the tank pressure got high enough, but if that happened it would mess up the gas ratio coming out of the tank.
"High enough" pressure depends on the temperature of the tank. So, if the tank pressure is above ~2000 psi, then don't put the beer gas tank in the keezer.
Brew on

whattabrau

Well whadda you know, never thought about it like that, but it does check out (to an accuracy reasonable for homebrewing, at any rate).
I guess I should actually have known, because I've graphed the relative amounts of CO2 in headspace vs. dissolved with headspace approximated as an ideal gas, and they're almost straight lines, just shifted slightly as a function of the temperature.

doug293cz

Yeah, that's Henry's law. The concentration of dissolved gas at any given temperature is proportional to the gas's partial pressure at the gas liquid interface.
Brew on

whattabrau

Oh, heh, right, that too. As far as I know, the coefficient changes also a function of pressure (not just gas species and temperature and solvent), but it's quite irrelevant for homebrewing, where "decrease the pressure if it's overcarbonated" is the lazy and prercise way to let mother nature handle the calculation for you. ;-)

doug293cz

Not quite. Henry's law says the coefficient is dependent on species, solvent, and temperature, but is NOT dependent on pressure. That's pretty much the definition of Henry's law. Now in practice, real systems often deviate slightly from "ideality," just as with most other laws of chemistry or physics. In fact, CO2 is one of the "worst" offenders in not rigorously following the ideal gas law (PV = nRT.)
Brew on

Bobby_M

Wow, I've been ignorant for a long time on this. Thanks!

Jikel

Thanks to everyone for the input. Very informative. I intend to save all these responses to use on my next beer gas effort. I have been a lifetime consumer of Murphys and Guinness and remember that in Eire there are always multiple glasses of the beer at different stages of 'settling' on the bar. You get the one at the end topped up when you order and they pull a new 'full of foam' glass to add to the start of the series. My glasses were alway too foamy I thought and I could not afford to wait for the beer to appear nor pull 6 glasses to start the sequence. I wonder who gets to finish the beers on the bar at the end of the night! Tough job but someone has to do it ;-)