Headspace with oxygen is the enemy for longer secondary and tertiary fermentation – beer, wine, mead, etc. I want to introduce a blanket of CO2 into the carboy to occupy the headspace and provide an oxygen barrier usng a CO2 tank. Picture the larger neck hole in the carboy and a siphon hose connected to the CO2 tank. Open the valve and CO2 goes in and in theory the oxygen is pushed out of the empty space in the neck. When the headspace has been purged, install a bung with airlock. Since the CO2 is heavier than the oxygen it should provide a blanket of protection.
So now the question – what pressure and how long should a purge cycle be for a carboy with reasonable (4") headspace – 30 seconds, minute,??
Opinions and recommendations are solicited.
Note that most of the answers provided below did not respond to my original question. Apparently I did not properly explain. The restated question is listed below in this thread.
You would be better off if you first purged the O2 by filling the fermenter with a liquid like sanitizer then using CO2 to push that out. Gases mix, so simply putting in CO2 will leave O2 in there.
I am confused. The carboy is not pressurized – filling it with sanitizer is OK but how CO2 would push it out of the top is a puzzle. To my way of thinking there is no way to push it out of the carboy.
Yep, CO2 blanket (and gas “purge”) is a homebrew myth. Gas laws (Henry’s, Dalton’s) describe why its impossible, at least in this universe. Best bet is like Denny says, liquid purge then a closed transfer. But since any barrier you close the fermenter off with will not be 100% impermeable, there will still be diffusion of oxygen across the barrier into the fermenter over time. But you’ll be way better off than starting out with a mix of gases in the fermenter.
It is possible to devise a way to extend a dip tube to the bottom of the carboy, sealing the top, and using very low (1-2 psig at most) pressure properly purge a carboy. But this can be dangerous, carboys are not pressure vessels! This is one of many excellent reasons not to use glass carboys.
I use one of those orange plastic carboy caps with the two hose fittings sticking out to do this. My stainless racking cane fits in one hole and a barbed CO2 fitting in the other, just enough pressure to get the beer moving, less than 2 psi. Even then, if the tube gets clogged the gas pressure may blow the plastic cap off the carboy. It kinda scares me. I do this when transferring to a keg, not to a secondary. Perhaps if you just transferred to a secondary for a longer fermentation you could do it while there’s still some fermentation going on.
At the risk of being banned because I question the above answers…
Dalton’s and Henry’s laws were formulated in the early 1800s and deal with gas mixing while in solution and the associated pressures. In particular Henery’s Law is gas specific and temperature sensitive.
The other answers discussed pressurization of the entire vessel for the expulsion of the material.
There are ample U Tube videos demonstrating the ability of CO2 to be ‘poured’ and displace oxygen.
This concept has been discussed and recomended by a variety of commercial sources. One that most will recognize is Midwest Supplies.
I apparently did not phrase my question properly so I will try again.
The receiving vessel has head space – assume a necked carboy with 1.5 liter of headspace due to prior racking and the left behind sediment in the source carboy Assume the insertion of a racking cane connected to a CO2 source and positioned above the level of the liquid. Since the CO2 is heaver that the O2 the current notion is that it will sink ‘below’ the O2 and the O2 will ‘escape’ from the neck of the carboy thus providing an oxygen barrier in the headspace…
My original question had to do with what pressure should be used on the CO2 and for how long should the ‘purge’ time be.
The chemistry laws cited by Robert apply, I believe, to gases in equilibrium. The key to achieving a CO2 blanket is to provide non-equilibrium conditions. Anyone who has worked with cryogenic liquids knows that oxygen displacement is a real thing and potentially very hazardous. You can be dead long before equilibrium is reached.
The keys are the flow rate and the density difference. If you blast CO2 from a pressurized cylinder into your head space it will generate a lot of turbulence and the CO2 and O2 will mix and it will not generate a blanket. If you flow the CO2 gently in to the bottom of the head space it will pool without mixing and gradually push out the air. You need to do this fast enough to avoid any diffusion or approach to equilibrium but slow enough to avoid turbulence. it is hard to judge this with an invisible gas.
Cooling the CO2 increases its density and enhances the effect. Cooling it enough that it condenses water vapor into fog makes it visible so you can more easily see what is happening. If you drop chunks of dry ice into your carboy you will generate very cold CO2 at the bottom that will push out all the air and make fog that rises out the top and then sinks down around the edges and gradually dissipates. This allows you to see where the CO2 is. Look at this video: https://www.youtube.com/watch?v=6kxc5yXN80Q . How much O2 do you think is left in that bucket? Not much at all.
So, to answer your original question, the CO2 pressure should be very low (to avoid turbulence), and there is no real way to estimate the amount of time required. If you had a flow meter you might have a chance of knowing by transferring enough CO2 to fill the head space 3 times over, or something like that.
Thank you Richard – your analysis matches my visceral notions.
As you point out the gasses are invisible and laminar flow wants to be the norm to avoid turbulence.
I have selected 5 PSI as the starting point, and a racking cane to help delivery by slowing moving it within the receiving carboy and a duration of 30 seconds. Those are my initial guess. I was hoping someone who has done this before and had some experience could provide their notions.
I may not be understanding what you are trying to do. If you are conducting secondary in the carboy the beer has fermented in, fermentation has already left the headspace filled with CO2. If you are racking to a second carboy, then it is not the headspace after racking that you must worry about, but the entire vessel before filling. Otherwise the beer will be in contact with oxygen during the racking process. This is where a liquid purge of the receiving vessel is the only, or at least most, practical solution. Otherwise you will need to use an uneconomical amount of CO2 to be sure of purging the vessel, whereas, if the vessel is completely purged before filling, the beer will be pushing out CO2 such that oxygen will not enter (you will not create the level of turbulence Richard points out is a concern) and you will be left with just CO2 in the headspace. This is what we refer to as a “closed transfer,” provided you also provide a supply of CO2 to push out, or at least replace the volume of, the beer in the source vessel, so air is not drawn into contact with the beer there.
Everyone is overthinking and making this question too complicated.
There are no kegs involved, just old fashioned fermentation pails and secondary carboys with the use of racking canes and old fashioned bottles. Back in the old days we racked to a secondary and then to bottling bucket and then into bottles. Along the way there was oxygen contact to be sure but things worked.
The question is for an environment like that – fermentation pails and secondary carboys. The only question was how much pressure and the time involved to introduce CO2 into the empty headspace of a secondary or tertiary carboy to avoid degradation during additional ageing.
I purge the headspace of my secondary carboys with 1 or 2 psi for about 5 seconds. Usually there’s only a couple inches of headspace because I fill up all the way into the neck. With more headspace, I might purge for 10-15 seconds. I do purge the empty receiving carboy first. I think this is an important point to make, as the efficacy of purging post-filling depends somewhat on how much O2 was in the empty carboy in the first place.
20 seconds at 10 psi. I settled on these values somewhat arbitrarily, but they work very well for me. I blast the CO2 into the carboy through a racking cane, so the CO2 fills the carboy from the bottom, pushing air up and out. I then immediately stopper the carboy until it’s ready for filling.
You definitely need to purge the whole receiving vessel first, not just the headspace afterwards. The same principles apply for this larger volume. I disagree with using a pressure as high as 20 psi. i would use a very low pressure, 1 psi or so, to let the gas flow gently into the bottom. Here is a trick that may help: cool the CO2 tank as much as you can, being mindful of its ratings. Just before purging, heat all the air in the carboy with a hair dryer. That will increase the density difference between the two. Then let the CO2 into the bottom of the carboy slowly, while putting your hand at the top on the outgoing gas. If you feel that change from warm to cool, then you are done. Of course, dropping in some dry ice would be more definitive and look a lot cooler.
^^^^^
Ok, you guys may know glass freaks me out. But this talk of intentionally subjecting a glass carboy to such temperature extremes really really freaks me out.