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Old 02-04-2009, 05:32 PM   #1
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Default Need a formula

For the engineers and those good with numbers, here's a question:

If I leave 5.5 gallons of beer in a fermenter in an area at 80*f, how long will it take to raise the temp of the beer from 64* to mid-70s?

I have a stout that has been in primary for 2.5 weeks and is stopped at 1.021. The OG was 1.053. I'm thinking I need to drop the SG a little more before I bottle. I'm leaving the fermenter, wraped in a dark towl, on a shady spot on my patio to raise the temp (yes, it gets to 80 degrees in Phoenix in February ). So does this sound like a good move? Will I get off flavors if it warms up too much?

Much appreciation in advance for you help.

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Old 02-04-2009, 08:39 PM   #2
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Perhaps this equation will help:

ts = (Cw * Mw * ΔT) / Q

where:
ts = time in seconds
Cw = specific heat of water (4.187 kJ/kgK) (assume water instead of beer)
Mw = mass of water (1 liter = 1 kg so 5.5 gallons = 20.814 kg)
ΔT = "delta T" or change in temperature
Q = heat added (kJoules)

Q is the only one I can't figure out for your scenario.

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Old 02-04-2009, 08:46 PM   #3
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I don't think warming the beer is going to help much. If this was an extract brew there is a strong possibilty that it is done at 1.021. Typically the only sure way to restart a stuck fermentation is to rack the beer onto a new yeast cake and even that is not guranteed to work. Re-pitching a packet of yeast is suggested often, but most have no luck there either.

Give it a few more days, and if it's done, you will probably be OK to bottle. 1.020 is on the high side, but I've bottled there before and had no issues.

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Old 02-04-2009, 08:53 PM   #4
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Thanks for the help. I guess I should mention it was an all grain recipe. The sample I pulled was tastey (and was 64*), so I guess it's not a huge deal.

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Old 02-04-2009, 09:07 PM   #5
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Not that it matters, but I did some more figurin' and got about 11.5 minutes for 5.5 gallons of water to go from 64°F to 75°F (in an 80°F environment). That's got to be off because it doesn't account for the thermal conductivity of the container.

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Old 02-04-2009, 09:12 PM   #6
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Quote:
Originally Posted by menschmaschine View Post
Not that it matters, but I did some more figurin' and got about 11.5 minutes for 5.5 gallons of water to go from 64°F to 75°F (in an 80°F environment). That's got to be off because it doesn't account for the thermal conductivity of the container.
Is that in an 80°F air environment?

I could see that quick an ascent if the jug were dropped into an 80°F water bath...but maybe not just place outside.

FWIW, I've never found an issue with raising the temp after initial (read vigorous) fermentation has subsided.

If you do move it and raise the temp...try taking your racking cane and stirring up that yeast cake at the bottom to re-suspend it.
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Old 02-04-2009, 09:45 PM   #7
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@BierMuncher--yep, that's air environment. Thanks for the racking cane suggestion.

@menschmaschine--The fermenter is a plastic bucket, so I guess that's keeping the beer inside insulated. The outside temp is 82 now and the sides of the fermenter feel really cool

I'm not sure how warm it's getting yet. Nothing has changed with the airlock. I think I'll let it sit out the rest of the day and bring it in. If nothing changes oh well, rdwhahb. Worse can happen if I keep futzing with it.

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Old 02-04-2009, 10:49 PM   #8
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Quote:
Originally Posted by BierMuncher View Post
Is that in an 80°F air environment?

I could see that quick an ascent if the jug were dropped into an 80°F water bath...but maybe not just place outside.
Yeah, my calculations were based on the impossible scenario for taking a 5.5 gallon "glob" of 64°F water and suspend it in 80°F air. It doesn't factor the "insulating" fermenter. I won't stand by my numbers though. Physics 1 was my only C in college.
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Old 02-06-2009, 03:07 PM   #9
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Well, nothing happend after leaving it outside in the warm air, so I'm taking a more direct approach and wrapping a heating pad around the fermenter. I never realized just how cool my fermenters were getting. The thermometer I was using to monitor the ice bath was way off and water temps that I thought were in the mid 60s were actually in the 50's and below. I guess it took me a while to catch on because I thought the fermenting process was generating a lot of heat. If my house stays in the mid 60s, I think I'll just leave my next batch out of the water bath and sit at the room temp.

Thanks again for your help.

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