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Purging kegs...


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#21 gnef

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Posted 28 December 2017 - 07:19 PM

I'm not so dedicated.  I fill my kegs probably about half way with sanitizer and push that out.  the kegs often sit after that so I figure the CO2 mostly settles to the bottom.  then I rack out of an open primary through the out post on the keg and leave the PRV open to let the air and CO2 out of the keg.  when I'm done transferring I try to evacuate the head space of air as much as possible by replacing it with CO2.

I know this is nitpicking, but over time, the CO2 will not settle, it will mix evenly with the other gases. When the gases first come in contact with laminar flow, they can layer, but they will evenly mix given enough time, and if there is turbulent flow, that mixing will happen much sooner.

 

This is also the understanding behind that table posted above and why it takes so many cycles to fully purge out the O2. You are mixing it at a specific pressure, purging out a percentage of it, but there is still O2 left in the headspace, mixed with the CO2 that was added. You continue to do this to remove a higher and higher percentage of O2, but it continues to mix in this fashion. This is also why it takes fewer cycles at higher pressure. If the CO2 settled to the bottom, you should be able to just purge once, and completely remove the O2. I hope that makes sense!



#22 Genesee Ted

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Posted 28 December 2017 - 11:52 PM

Gnef, that’s a good explanation! To wit, as long as you get a good purge and there’s a blanket of CO2 At the bottom the beer replaces the gas anyway. You can always purge it further once full and I bet the numbers would include much less O2 than the full tank purge stats.

#23 gnef

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Posted 29 December 2017 - 12:17 AM

Gnef, that’s a good explanation! To wit, as long as you get a good purge and there’s a blanket of CO2 At the bottom the beer replaces the gas anyway. You can always purge it further once full and I bet the numbers would include much less O2 than the full tank purge stats.

Yes, that should be correct, percentage-wise based on the headspace to the entire volume. If you mostly have CO2 in the keg in the first place, that will definitely reduce Oxygen exposure to the beer.

 

Just as a random thought along these lines - atmospheric oxygen percent is just under 21%. Atmospheric pressure is just under 15 psi. So if you add 15 psi of CO2 into the keg, you still have the same absolute amount of Oxygen in the keg, but it is now only around 10%. When you purge out back to atmospheric pressure, the remaining gaseous mixture should be 10%. If you look at the table posted, this matches with the ppm changes at 15 psi. In fact, taking a closer look at the table, it looks like they are using the atmospheric ppm based on the percentage/partial pressure, and then scaling that for each purge at each psi with what happens with the combined partial pressure of added CO2 and the dilution ratio for when it is purged back to atmospheric pressure.

 

With all that said, it definitely makes more sense to me to fill with sanitizer and then push it out with CO2 to make this process easier and simpler. To reduce CO2 waste, just use a lower pressure and transfer the sanitizer out just a bit slower.



#24 positiveContact

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Posted 29 December 2017 - 08:16 AM

Yes, that should be correct, percentage-wise based on the headspace to the entire volume. If you mostly have CO2 in the keg in the first place, that will definitely reduce Oxygen exposure to the beer.

 

Just as a random thought along these lines - atmospheric oxygen percent is just under 21%. Atmospheric pressure is just under 15 psi. So if you add 15 psi of CO2 into the keg, you still have the same absolute amount of Oxygen in the keg, but it is now only around 10%. When you purge out back to atmospheric pressure, the remaining gaseous mixture should be 10%. If you look at the table posted, this matches with the ppm changes at 15 psi. In fact, taking a closer look at the table, it looks like they are using the atmospheric ppm based on the percentage/partial pressure, and then scaling that for each purge at each psi with what happens with the combined partial pressure of added CO2 and the dilution ratio for when it is purged back to atmospheric pressure.

 

With all that said, it definitely makes more sense to me to fill with sanitizer and then push it out with CO2 to make this process easier and simpler. To reduce CO2 waste, just use a lower pressure and transfer the sanitizer out just a bit slower.

 

good info.  I had not thought about atmospheric pressure.  I'm assuming when I'm looking at the gauge on my CO2 tank that the atmosphere is already subtracted out?  otherwise how can I seal a keg up below 15 PSI effectively?  and why would I be able to vent a keg that has less than 15 PSI?  shouldn't the air just rush in?  



#25 gnef

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Posted 29 December 2017 - 01:57 PM

good info.  I had not thought about atmospheric pressure.  I'm assuming when I'm looking at the gauge on my CO2 tank that the atmosphere is already subtracted out?  otherwise how can I seal a keg up below 15 PSI effectively?  and why would I be able to vent a keg that has less than 15 PSI?  shouldn't the air just rush in?  

That is exactly correct. When our gauges read '0' psi, it is actually calibrated with atmospheric pressure, which changes based on altitude and weather. Most gauges have a stop pin and are 'good enough'. This is one reason why I have been moving towards gauges that can be rezeroed - I've been getting Winters P9S gauges as I can (I have a lot of gauges to replace, so I couldn't do it all in one go). I also have a Wika test gauge that is accurate to .25% that I use to zero each gauge before I use it, I'll also use it to check the gauges at least annually.

 

What I find even more fascinating is that we are under atmospheric pressure all the time, we just don't really notice it, so there is always about 15 pounds of force per square inch across our bodies, and this is actually how we breathe using our diaphragm to create a partial vacuum to draw atmosphere into our lungs.




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