newbie here with 1/2 batch & kegging questions

Hmmm, I guess it would be logarithmic, with a lot of CO2 coming out fast at first, then slowing towards the end. I would hate to test it with an entire keg full of beer, but I guess that is the only way...maybe I could make a 3.25% beer to test it.

Adub71 said:

After filling the keg with CO2, once you remove the gas post/dip tube, all your CO2 will escape (it's under pressure and wants to get out), and it will immediately be replaced by air. Other than wasting CO2, you could put out a small fire with the escaping CO2, that's about all it's good for. Sorry man.

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First, if you read my post closely, there is no over-pressure of CO2 when pre-purging the keg, as the Starsan is being pushed out and replaced with CO2.

Then when removing the gas post and short diptube the small opening of the stub will not allow air to infiltrate quickly. Before inserting the skinny tube, you could pre-purge it with CO2 or insert it while streaming CO2 through it slowly. The inside volume of that tube is very small though, and quite insignificant on scale of things here.

doug293cz said:

Removing the post & tube will reduce the CO2 pressure in the keg to atmospheric pressure. However, the CO2 will not be immediately replaced by air. It will take significant time for air to diffuse into, and the CO2 to diffuse out of, the keg thru the small opening. A small amount of air will get into the keg while it is being filled, but this can be removed by a few purge/fill cycles.

Brew on

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Why would (a small amount of) air make it into the keg while filling? CO2 is being pushed out while being replaced by an equal volume of beer.

As illustrated above, purging of half filled kegs has very diminishing returns for the amount of CO2 needed. I think we'd concluded that a keg that's even 80% filled (20% headspace, 100% filled with air) needs around 5 gallons of CO2 to be purged to a residual O2 level of 2% (~10% of air).

IslandLizard said:

Why would (a small amount of) air make it into the keg while filling? CO2 is being pushed out while being replaced by an equal volume of beer.

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Some air will get in before the tube is inserted, and unless the tube is a tight fit, some air will diffuse thru the gap between the tube and post hole. But, this is a mostly academic discussion. In any case, your method of keg filling will allow much less air into a keg than just sticking a tube in with the lid removed. My response was more aimed at the poster who said air would rush into the post hole and displace all the CO2.

Brew on

doug293cz said:

Some air will get in before the tube is inserted, and unless the tube is a tight fit, some air will diffuse thru the gap between the tube and post hole. But, this is a mostly academic discussion. In any case, your method of keg filling will allow much less air into a keg than just sticking a tube in with the lid removed. My response was more aimed at the poster who said air would rush into the post hole and displace all the CO2.

Brew on

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Yeah, I know you were directing it at the other poster, just wondered where you thought a lot of air could get in. That 1/4" OD (1/8" ID) tubing is a pretty good fit for that keg's stub.

The key is to avoid sucking air in through the racking tube (or auto siphon). You need to pinch it at the right moment before getting greedy to get all the beer out of the fermentor. Besides, you need some beer left behind to swirl up and harvest the yeast cake. I tip the bucket when the beer level gets low, say below 4 inches, then start to tilt.

IslandLizard said:

Yeah, I know you were directing it at the other poster, just wondered where you thought a lot of air could get in. That 1/4" OD (1/8" ID) tubing is a pretty good fit for that keg's stub.

The key is to avoid sucking air in through the racking tube (or auto siphon). You need to pinch it at the right moment before getting greedy to get all the beer out of the fermentor. Besides, you need some beer left behind to swirl up and harvest the yeast cake. I tip the bucket when the beer level gets low, say below 4 inches, than start to tilt.

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I said "a small amount of air." That's rather different than "a lot of air."

You don't really have to worry about shutting off the fill flow to avoid getting air into the keg when the fermenter is going dry. Since the end of the tube is submerged during fill, no air will get below the fill line in the keg (as long as you're not sucking a lot of air bubbles with the beer flow.) When the fermenter is empty (or keg is full) just shut off the valve (or pinch tube), disconnect the tube from the fermenter, siphon, or whatever, and carefully withdraw the tube from the keg. The only air introduced will be the the volume displaced by the tube in the headspace. Then replace the keg post and purge (as many times as it takes to make you happy. Refer to my purge table in an earlier post.)

Brew on