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- Aug 31, 2017
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I have a conundrum regarding how much suck back volume occurs when cold crashing.
Background: I accidentally created a suck back situation when I soft crashed my CF10 fermenter and forgot that I hadn't used my spunding valve on this batch (more precisely I didn't close off the blow off valve, so the spunding valve did nothing). The fermenter was holding a 7 gallon batch, leaving perhaps 8 gallons of headspace (including the domed lid). @doug293cz's analysis suggests this should result in approximately 35 oz of suck back due to gas contraction (head space * 10/293.15) and neglecting CO2 absorption which would increase the suck back over time. However, in actuality the sanitizer only rose 35" up the blow off hose (~40 hours after reducing the temperature). The photo below shows that the sanitizer is an inch or so below the TC connection to the blow off cane. (Note that this picture was taken after closing the valve to the blow off hose, which was done after the beer had cooled to 50F). The hose is 1/2" i.d. vinyl, so 35" is only about 4 oz or less than 1/8th the ideal gas law calculation.
So, this was a puzzle. I woke up this morning with the Ahh! that this calculation assumes atmospheric pressure both before and after the cold crash. But in actuality, the ~4" of sanitizer in flask creates a small positive pressure during fermentation and it should require a larger negative pressure to suck the sanitizer 35" up the hose.
So what should those pressures be? I assumed that since StarSan is mostly water, the pre-crash absolute pressure should be 1 atm + 4"/407 = 1.00982801 atm and the post-crash pressure should be 1 atm - 35/407 = 0.914004914 atm (using an approximation I found on the web to convert inches of H20 to atm). However, when I plug those numbers into my spreadsheet, it gives the nonsensical result that the gas actually expands when it's colder.
PV = nRT = kT
k = PV/T = 1.00982801 * 8 gallons/293.15 = .027558
V = kT/P = .027558 * 283.15 / 0.914004914 = 8.54 gallons
I'm guessing that the problem is that 35" of water in a vinyl hose is not actually equivalent to 35" of a water column, but why? Is it capillary effect? Something else? Some other problem with my analysis? I clearly need some help here.
Background: I accidentally created a suck back situation when I soft crashed my CF10 fermenter and forgot that I hadn't used my spunding valve on this batch (more precisely I didn't close off the blow off valve, so the spunding valve did nothing). The fermenter was holding a 7 gallon batch, leaving perhaps 8 gallons of headspace (including the domed lid). @doug293cz's analysis suggests this should result in approximately 35 oz of suck back due to gas contraction (head space * 10/293.15) and neglecting CO2 absorption which would increase the suck back over time. However, in actuality the sanitizer only rose 35" up the blow off hose (~40 hours after reducing the temperature). The photo below shows that the sanitizer is an inch or so below the TC connection to the blow off cane. (Note that this picture was taken after closing the valve to the blow off hose, which was done after the beer had cooled to 50F). The hose is 1/2" i.d. vinyl, so 35" is only about 4 oz or less than 1/8th the ideal gas law calculation.
So, this was a puzzle. I woke up this morning with the Ahh! that this calculation assumes atmospheric pressure both before and after the cold crash. But in actuality, the ~4" of sanitizer in flask creates a small positive pressure during fermentation and it should require a larger negative pressure to suck the sanitizer 35" up the hose.
So what should those pressures be? I assumed that since StarSan is mostly water, the pre-crash absolute pressure should be 1 atm + 4"/407 = 1.00982801 atm and the post-crash pressure should be 1 atm - 35/407 = 0.914004914 atm (using an approximation I found on the web to convert inches of H20 to atm). However, when I plug those numbers into my spreadsheet, it gives the nonsensical result that the gas actually expands when it's colder.
PV = nRT = kT
k = PV/T = 1.00982801 * 8 gallons/293.15 = .027558
V = kT/P = .027558 * 283.15 / 0.914004914 = 8.54 gallons
I'm guessing that the problem is that 35" of water in a vinyl hose is not actually equivalent to 35" of a water column, but why? Is it capillary effect? Something else? Some other problem with my analysis? I clearly need some help here.