This topic comes up fairly frequently. I would like to throw out two issues
I have with using gas to top up. Comments welcome.
1) One of the questions is how much gas is needed. Someone recently said
that we need to remove all the air. Well that is not possible. We only
need to get it down to the level that would be in the carboy normally. A
normal head space would be 3 to 4 cu.in. of air. If you have one gallon of
head space (231 cu.in.) then you need to dilute the air down to the
equivalent of 3 to 4 cu.in. Even a heavy gas like Argon will mix when it is
added, it will not go in as a blanket on the bottom. If we put in 1 gal of
inert gas we will theoretically have diluted the air to half. Each
additional volume will dilute by a half again. Continuing this, we need to
put in 6 gal's of gas to get the equivalent air down to between 3 and 4
cu.in. Of course some of the gas will escape as you are doing this (even
with argon but a lot with CO2) so you really need to put in a lot more than
6 times the head space. Any comments on how much?
2) Another issue is protecting the wine with an airlock when topping up
with inner gas. An airlock only has a little more that 0.5 cu.in. of liquid
in it. As temperature and to a lesser extent pressure changes the volume of
the contents of your carboy changes and the liquid in the airlock will swing
back and fourth. If the volume in the carboy changes by more than the
liquid volume in the airlock, gas will bubble in or out of the carboy. If
temperature goes up, gas will bubble out of the airlock. If it goes down it
will bubble in and that is not a good thing. The question here is: How
much protection from temperature swings does the airlock give?
Let's say we have a normally topped up 6.5 gal. carboy. It will have 3 or 4
cu.in. of air in the head space and roughly 6.5 gal's of liquid (at 231
cu.in./gal) Air expands by about 0.0015 units/deg. F and water (sort of
like wine I guess) expands at 0.0001 units/deg. F near room temp. Using
these numbers you can calculate the volume swing per deg. change in
temperature for the wine/air system as 0.16 cu.in./deg. If the airlock
will accommodate a 0.6 cu.in. volume change, this suggests that the airlock
will protect against about 4 deg. F. temperature swing.
If you do the same calculation on a 6.5 gal carboy with 1 gal of head space
you will find that you are only protected from about 1.25 degrees of
temperature change. And if you have 2 gal. head space you are protected
against a 0.75 deg. F temperature swing.
Now it is true that a large carboy will protect wine from a fairly large
temperature swing simply because of the time it takes to change the
temperature of the wine, but I am not sure it will keep it below 1 deg.
This suggests that a large inert gas head space could cause the carboy to
breath through the airlock, maybe on a daily basis. That would not be a
Have I missed something? Comments?
The biggest problem with headspace is actually the exchange of gas
through the airlock. gas dissolves into the liquid in the airlock on
the atmospheric side and then is released on the wine side.
that reastion is a concentration controlled process. That is the
oxygen will diffuse across the airlock barrier to equalize the O2 level
in the headspace at ~20%. the airlock will NOT stop that. it only
slows it down. The concentration of oxygen in the headspace WANTS to
go to 20%. So the gas in the headspace will equalize with the
atmosphere eventually. That gas can then dissolve into the wine
causing oxidation. The larger the headspace the more gas you can
accomodate in the carboy and the faster the oxidation will take place.
That is why i put metabisulfite in my airlocks and keep it fresh (once
a month or so). the SO2 gas will react with any O2 that dissolves in
the airlock and take it out of solution (of course by doing that it
just speeds up the rate that the gas dissolves in the solution, becasue
it too wants to equalize wiht the atmosphere).
Ray Calvert wrote "This topic comes up fairly frequently. I would like to
throw out two issues I have with using gas to top up. 1) One of the
questions is how much gas is needed. Someone recently said that we need to
remove all the air. Well that is not possible."
Ray - I believe you can remove essentially all the air from a carboy and
especially from a Corny keg if you happen to use them. This technique is
probably more suited to the kegs. Fill the keg brim full with water. Seal.
Pressurize with CO2 (or the gas of your choice) while allowing the water to
escape through the tap. When all of the water is gone the keg is totally
filled with gas. For a carboy siphon slowly while filling the top space
with gas. I guess you could use the gas to force the water out of a carboy
using a two-hole stopper but I agree with those who caution against
pressurizing glass containers.
That said I would never trust wine to a partially filled carboy or Corny
keg. Learned the hard way years ago. However, several winemakers in our
club do this and still make pretty good wines.
Can't comment on the air lock issue. I only leave about 1/2 inch space
beneath the stopper in carboys and I don't see much movement in the air lock
liquid...I use Vodka.
Olathe, Kansas USA
"Bill, how do you keep the Vodka from evaporating?"
Paul - The vodka doesn't evaporate quickly in my cellar. But, I'm down
there about every day and check on the carboys. I just add a little more
vodka if an air lock seems low.
Olathe, Kansas USA
Good point Droopy. That is a third problem. Diffution due to concentration
differences. I don't know that it is a larger problem than the other two
but it is a different problem.
Good point William. Don't know how many migth use the meathod but it is a
meathod that should work.
Here is a suggestion that I thought of over the weekend. See who can polk
holes in it.
The reason you use an airlock rather than a solid bung is that that two
things could possibly happen. First it could start ferementing again and
that would be bad. But if you have stabilized it you should be able to stop
Second, you need an airlock because of pressure built up due to temperature
changes. Wine is basically an incompresible fluid. Air is very
compressable. Temperature changes can cause 6 gallons of wine to change
volume by several cubic inches. When it does it is going to compress the
air in the head space by an equal amount. If there are 4 cu.in. of air in
the head space and the wine expands by 2 cu.in. (this is very possible),
then if you have an airlock, the pressure is maintained at 1 atm. But if
you have a solid bung, the pressure will go up to 2 atm or it could go
higher. The bung will blow out and your wine will oxidize if you do not
catch it or, worse, the jug will shatter.
But if you have 4 gallons of wine and a gallon (231 cu.in.) of head space
filled with inert gas, then if the wine expands by 2 cu.in. this will yeild
only a 1% increase in pressure not a doubling. Yes the gas will try to
expand and will yield a little pressure increase as well but not too much.
Maybe the thing to do if you have a large head space filled with inert gas
is to do the opposite of what is noramlly recomended when topping up,
inother words, abandoned the airlock and use a solid bung. This would
address all 3 issues raised.
Well a couple of things, you are assuming that a the wine will change
volume to an extent that I do not think it can. I have never seen a
full carboy fluctuate more than 1/4 or so.
The second assummption you make though is that the solid stopper is a
complete seal and impervious to pressure changes, that is that the bung
will resist a pressure change of 1 or more percent.
When in fact, the pressure will bleed out between teh stopper/carboy
interface. Airlocks get around this by allowing the pressure to
fluctuate. If you stop it up, the air will just seep around the sides
of the stopper. If you can creaste a strong enough seal that will be
fine, but you are looking at a bottle cap type seal then.
Better yet, fill it up to the top and put it in a thermostable location
to stop the fluctuaion to begin with, just like you would with a corked
This is not assumption. Coefficients of expantion are valid. Coefficient
of expansion of wine will be a little different than water but not much.
A solid rubber stopper will seal very well and you should have no leakage
around the rubber. Infact the main arguement against useing them when a
carboy is topped up is that the pressure change can crack the carboy or it
will totally blow the bung out. That is what I am trying to get arround.
I agree, that would be better but such environments are rather expensive. I
may build one when I retire but not right now..
Sure coefficients of expansion are valid. But you are assuming a
temperature fluctuaion of at least 10 degrees F (from about 77 to 68)
to get a volume change of several cubic inches. The SG of water only
changes 0.0012% in that range. Which would give you 1.6 cu in
Even in my old place built in the 20s I did not see a temp fluctuation
The 7 volumes of headspace purge sounds about right to be 'safe'.
Under 'ideal conditions' in a wine barrel or carboy or wine bottle, the
minimum of purge volume using Argon would be 3 volumes of headspace
(purge tube outlet just above the liquid level and very slow argon flow
rate, on the order of 50ml per minute [3 cubic inches per minute]).
Under these ideal conditions, the much denser argon displaces the
air/oxygen without too much mixing.
Nitrogen purge gas is much less effective than argon because it mixes
much more into the air inside the container, rather than displacing it
(air and nitrogen gas have almost the same density, cuz air is about 80%
A beer keg or soda syrup container would not purge as well as a wine keg
or carboy, since they have 'stagnant' (dead) spaces. The outwardly
convex (bowed) shape of the wine barrels minimizes dead air spaces. The
bell-shape curve of the top portion of a carboy/wine bottle accomplishes
the same thing.
And the water portion of the vodka evaporates slowly.
As long as there is at least 10 percent alcohol in the airlock mixture,
nothing harmful to us will grow in the airlock.
[86 proof vodka is about 40% alcohol; 100 proof is about 50% alcohol]
Good idea to top up the airlock with vodka once a week. I use pure
glycerine myself, so I never have to top up the airlock (over a year and
no noticeable evaporation).