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Old 04-16-2008, 05:31 PM   #1
maltMonkey
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I always do 5 gallon batches, 60 minute boils and boil off 2 gallons in the hour. Beersmith says that is about a 25.5% evaporation rate.

I'm getting ready to do a 9 gallon batch and using the same rate Beersmith says I need to collect 12.6 gallons.

I am not a physics major but this number seems like BS to me:

--First I assume that because I'm starting out with almost twice the preboil amount that the boil will be less vigorous due to limitations on my burner, resulting in less boil off. I wouldn't expect Beersmith to adjust for this, but it's in the back of my head.

--Second, assuming that I had the exact same boil vigor (temperature), why would more liquid evaporate in the same period of time just because there were more liquid available? The same amount of the liquid's surface area is exposed to the air, is it not?

I have nothing backing me up on this, but it seems to me that boil off should be linear, as in "I boil off 2 gallons per hour", not "I lose 25% per hour". By that math I would collect 11 gallons instead of 12.6....

Maybe I'll split the difference and collect 11.8 gallons just so my OG isn't too far off either way....

 
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Old 04-16-2008, 06:49 PM   #2
FlyingHorse
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Quote:
Originally Posted by maltMonkey
I have nothing backing me up on this, but it seems to me that boil off should be linear, as in "I boil off 2 gallons per hour", not "I lose 25% per hour". By that math I would collect 11 gallons instead of 12.6
I agree. I boil off 1 gallon per hour, and it doesn't change if I start with 4 gallons, 6 gallons, or 8 gallons in the kettle.
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Old 04-16-2008, 07:44 PM   #3
maltMonkey
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I guess I'll find out for sure on brew day tomorrow but I'm glad your experience backs up my theory!

 
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Old 04-16-2008, 07:55 PM   #4
hal simmons
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+1

I boil off the same amount per hour no matter what my starting volume is.

 
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Old 04-16-2008, 09:57 PM   #5
ChrisKennedy
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As long as you keep the vigorousness of your boil the same, you should have a constant rate of boiloff, not a percentage loss.

 
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Old 04-17-2008, 05:28 PM   #6
maltMonkey
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Another question--it is raining today and will continue to do so through my brew session tonight (brewing outside in my garage). Increased air humidity decreases boil off, correct? Anyone have any kind of rough numbers on how this will change my evaporation?


 
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Old 04-17-2008, 05:32 PM   #7
Krisium
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I was wondering do you guys compensate the amount of starting liquid with the evaporation rate? For a 5 gallon beer, should you really be starting with an extra gallon? I've only brewed 1 batch and ended up with about 4 gallons when it was supposed to be 5 gallons.

Do you guys recommend starting with a higher volume of water?

 
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Old 04-17-2008, 05:38 PM   #8
maltMonkey
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Quote:
Originally Posted by Krisium
I was wondering do you guys compensate the amount of starting liquid with the evaporation rate? For a 5 gallon beer, should you really be starting with an extra gallon? I've only brewed 1 batch and ended up with about 4 gallons when it was supposed to be 5 gallons.

Do you guys recommend starting with a higher volume of water?
Yes--since you only ended up with 4 gallons, next time add 1 gallon to your preboil volume (assuming you do the same boil length).

 
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