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Home Brew Forums > Home Brewing Beer > General Beer Discussion > Why does my liquid level change after the boil?
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Old 06-17-2014, 05:23 PM   #1
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Default Why does my liquid level change after the boil?

I started brewing a year ago (all grain). After helping a buddy a cple times, I decided that this really isn't any harder than I make it. I am now in 15 batches, and like most people here, its become a hobby.
I have a question.....

I use the same equipement on every brew. I use the same mash and batch sparge method on each batch. I have a rivet in my boil kettle that is a marker for the amount I sparge for the boil. Its about 7.5 gal.s.
In a typical 90 min. Boil the level never comes out the same. Is this due to air temp.?
On most recipes it calls for 6.5 gal.s of wort to get 5 gal.s in the fermenter. I find that this level is way low to end up with an actual 5 gal.s. I guess I am not understanding why the level of beer is always different and why the recipes stick to this general rule????
Any comments/ suggestions appreciated as always

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Old 06-17-2014, 05:26 PM   #2
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I have the same issues. I never know quite how much beer will be going into my fermenter at the end of the day.

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Old 06-17-2014, 05:36 PM   #3
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Humidity in the air is your likely culprit that varies. You can boil off more wort on a slight boil in winter than you can on a vigorous rolling boil in the summer because of differences in humidity. I would put a post boil mark in your kettle to also know where you stand (I mark my mash paddle).

Also as a liquid decreases in temperature (while you are chilling), so does your volume. Ever notice why when you fill a carboy up with oxy and hot water you have to keep adding water to keep it topped off? Its becaues the water is cooling and occupying less volume.

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Old 06-17-2014, 05:37 PM   #4
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E = - rho * L * k * ustar * (q2-q1) / ln(z2/z1)

where E is evaporation, rho is air density, L is the latent heat of vaporization, k is von Karman's constant, ustar is the friction velocity, (q2-q1) is the specific humidity difference between the air and near the water surface, ln is the natural log, z2 is the height of the dew point temperature measurement, and z1 is a height near the water surface.

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