we did an in-house test on this a couple months ago, using a very fancy proofing meter (electronic, not the bobber in a column). We found that 200-proof does indeed absorb some water once it’s opened, but very little and at a very low rate. We left both caps off of a 55gal barrel, new from our supplier, and after a 3 hours of being left open, sampled every hour, it got no lower than 99.5%. And that’s in Humboldt Co. CA, very close to the bay and very humid in general.
I’d love to know if anyone else has run similar tests, and if their results corroborate or disagree with ours
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