Sean makes the statement that if you have a DO meter you become shocked as to how much purging is necessary to achieve low levels. How much is that?
There are procedures to purge vessels of toxic or explosive gases. If you search for “pressure cycle purge equation” you will get hits to some links with information. Without going into all the details and the derivation, you can estimate how many purge cycles you need to reach a level of gas concentration.
N=ln(Clow/Coriginal)/ln(Plow/Phigh)
N=Number of purge cycles
Clow=Lower concentration target
Coriginal=Original concentration, so for air that is 0.21
Plow=Low pressure, 14.7 PSI absolute for our application
Phigh=High pressure for the purging, I use 30 PSI gauge, so 44.7 PSI absolute
For our case.
N=ln(Clow/.21)/ln(14.7/44.7)
selecting a sequence of 0.1, 0.01, 0.001… we can calculate the number of cycles. Round up for your application.
So to get close to 1 ppm (0.000001) you need 11 purge cycles. State of the art canning and bottling lines have about 50 ppb TPO or less.
That will use a boatload of CO2 for an empty keg, so the technique of pushing sanitizer out saves on CO2. If the head space is purged 5 times at 30 PSI, then the headspace will have about 1000 ppm O2. The headspace is small, so there is not a lot of waste for that process.
Keg is full and you want to purge O2 in the headspace.
Set regulator to 30 PSI. Pressurize head space. Pull PRV to vent, letting the gases flow until it is just about to atmospheric. Repeat for X cycles, where X gets you to a low enough concentration.
I should mention that purging techniques are pretty contentious over on ProBrewer. There’s a continent that thinks that it requires less CO2 to fill from the bottom while constantly bleeding the tank from the top via a spunding valve or restricted butterfly.
I’ve only checked product DO levels, but it would certainly be an interesting thing to test with a sniffer.
Is there any info anywhere on what concentration of O2 really matters? I’ve been purging an empty keg 3 or 4 times, and have noticed no oxidation, even for beers that are over a year old in the bottle.
One of the first signs of oxidation is that hop aroma fades. Oxidation takes on many forms, it is not always cardboard. Active yeast can scavenge some O2 too. The pros take lengths to minimize the Total Packaged O2.
Sean might have insights from the pro perspective. Pro Brewers have DO meters for a reason, those aren’t sheep.
Looking at some links, it looks like purging by introducing CO2 one one end of a vessel may be more efficient on CO2.
One could purge an empty keg by attaching to the liquid out, the flow would be bottom up like you describe. How long would one purge for? That is the question? Would be a neat test for someone with a meter.
I don’t think there is an easy answer. Flow is more important than pressure, and the temperature is important, too. If you have a very low flow of very cold CO2 it will hang out in the bottom of the bottle and gradually push all the warm air up and out the top. If the CO2 is warm or the flow is high enough to generate turbulence and mixing then it is very complicated. If you assume perfect mixing then you could calculate the volume of CO2 needed to dilute the O2 to a particular level (you will never reach zero), but that volume is excessive. If you want to dilute from 20% to 20 ppm then you would need to add 10,000 times the volume of the bottle in CO2 to get the dilution.
I just purge from the bottom with cold CO2 at 3-4 psi and count to 25 or until I get bored, then fill the bottle until foam comes out the top and put a cap on it. That seems to do well enough for beer to last a couple of months in the bottle without noticeable staling but I tried a 5 month old bottle last week that was definitely oxidized.
I also currently count to 25 (but at 10 psi) and cap on foam. I bottle to send to competition and the monthly club meeting. I haven’t kept any long term.
I watched the Russian River video again and took a note that they used 19 psi thought they did not talk temp. They swirled it and capped on foam. They got to ~40 ppb TPO
You can fill a bottle with sanitizer and use CO2 to push all of it out (similar to what you would do with a keg), and you’ll be in a better starting place when you begin purging with the counterflow filler. I know that doesn’t answer your question, but it does work well.
I found a kinda answer to my question about how long it takes to purge a 12 oz bottle w/CO2: I found a video from Brian of Low O2 Brewing where he hooked up a DO meter and purge a can (not a bottle) until it registered zero. It took 45 seconds. That’s a long time.
I like the idea of purging by removing sanitizer. May look into that