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Where does the water go when using PBW?

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TkmLinus

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As stated in the title, where does the water go when using PBW? Whenever cleaning carboys with PBW I fill to the very top with warm water and set aside, then when I come back and hour later the water level has dropped a couple of inches! Is someone sneaking sips of PBW water when I am not looking? Is the PBW making some of the water disappear? Anyways I have been wondering this for several years now and am interested to know why this happens... Happy Monday!
 

cubalz

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Never had that happen to me when soaking kegs to the top with PBW overnight. Very odd
 
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TkmLinus

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How warm? Probably a good measure of evaporation. As well as cooling shrinkage as it cools.
Poured from tap, so 115-120 F. I only notice this in the glass carboys with a thin neck.
 

Qhrumphf

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If you look at it as percentage of total volume, the very top of a narrow carboy holds very little volume as compared to total volume. The difference between "hot" water and "cold" water is only a couple percent, but when 3% contraction is shown in an area comprising 2% of volume you're gonna seen it clearly. If you're to the mouth of the neck, simple thermal expansion/contraction is likely it. Evaporation can do the same, but that's gonna show more with surface area.
 
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TkmLinus

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If you look at it as percentage of total volume, the very top of a narrow carboy holds very little volume as compared to total volume. The difference between "hot" water and "cold" water is only a couple percent, but when 3% contraction is shown in an area comprising 2% of volume you're gonna seen it clearly. If you're to the mouth of the neck, simple thermal expansion/contraction is likely it. Evaporation can do the same, but that's gonna show more with surface area.
I never knew that water expanded that much. Thanks for the explanation!
 

Qhrumphf

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It very much depends on the temps. 100f to 90f might not be much difference. 150F to 70F is going to be much more pronounced. The difference between boiling and room temp is pretty significant.

That said, if you've got a full carboy, it's also gonna have some pretty significant thermal mass, and shouldn't lose all that much temp even over an hour.
 
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