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I employed the Kmart collapsible water carrier on a recent batch.

Batch size was 23L in 30L HDPE fermenter. I’d estimate that the bag can be fully inflated 10L volume with approximately .003 to .004 for this batch size, so important not to attach it too early in the ferment, as you risk pressurising the system too much or creating a leak or blowout somewhere.

Something I found useful: if the bag fills before the end of the fermentation, you can close the valve, loosen the lid of the bag slightly and express some CO2, then close lid and reopen valve.

Also I found it sealed well enough to fill w CO2 from the ferment, close valve, disconnect from fermenter and save for use in cold crash later. Of course, I can’t be sure that no air passed into the bag while stored this way, though the size and pressure appeared constant across the ~3 days stored closed and disconnected from the fermenter.

Similar results observed to those mentioned above in terms of the volume of gas taken back into the fermenter during cold crash. I attached the bag at about half full (3-6L estimated) at start of cc and over about 3 days at 1deg the entirety of this was drawn into the fermenter.

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