is there any general guidance for the volume of gas being sucked back when cold crashing?
E.g. if I have a 6.5g carboy and 5.5g of beer, and I cold crash from 68F to 35F, what volume of gas should I expect to suck-back?
if this is pretty constant and predictable, I'd like to create a large enough "gas reservoir" in my blow-off system to prevent both sanitizer and O2 from pulling back into my carboy.
FWIW, I'm thinking of just creating enough of a "gas reservoir" by lengthening my blow-off tube to provide enough volume.
Think outside the balloon.
I bought a couple of undrilled stoppers and carbonation caps, like these:
https://www.morebeer.com/products/carbonation-line-cleaning-ball-lock-cap-stainless.html
I drilled the undrilled stoppers with small holes... so that the carbonation cap barb would be a tight fit, and hold a seal.
After fermenting with the normal stopper/airlock... I sanitized and replaced the stopper with the new stopper and carbonation cap before cold crashing. When the fermenter shrinks due to cold crashing, I pop a ball lock gas connector connected to CO2, and "inflate" the fermenter back to normal size with with my CO2 tank. Presto, no air introduced, and pretty darn simple.
After a time or two... I improved the process a bit by switching out the stoppers earlier. I let fermentation happen with a normal stopper/airlock for a few days... then once I am confident the most vigorous fermentation is over (and krausen isn't getting up into the airlock), I replace the normal stopper/airlock with a stopper with carbonation cap... with a ball lock connector attached to an open ended tube ending dropped into a vessel of sanitizer. Essentially, a 'normal' blowoff-tube setup... but using the stopper/carbonation cap and gas connector. When fermentation is completely over, I just remove the gas connector blowoff tube... (leaving the stopper/carbonation cap on the fermenter) cold crash, let it shrink, and attach a CO2 tank and refill the depleted headspace (and unshrink the fermenter). This method makes it so that the one time I remove the stopper early-mid fermentation... there is still active fermentation occurring, reducing the thought that oxygen will sit in the headspace.
This works great for plastic fermenters, which simply "shrink" due to cold crashing. The stopper has always held a seal, as is apparent by the 'shrunk' fermenter. I've also done similar things with SS fermenters.
If you're cautious... you can just add a burst of CO2 a couple of times as the temperature is dropping, rather than all at once at the end of cold crashing.
And if you don't keg... and don't have a CO2 tank on hand... the above post was unhelpful. (And you should consider kegging!)