C02/Pressure Drops when temperature drops

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jaybee84

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I am brewing my first batch of mangrove jacks stout (extract pouch) in a stainless steel fermenter. Ive noticed that when the temperature drops sometimes the pressure drops as my airlock liquid indicates. Ive been putting a heater on it but the second i switch the heater off or move it further away the pressure seems to drop instantly. Ive been trying to keep it around 22 degrees C but its quite costly to keep the fan heater on 24/7. Generally it hasnt gone below 18 degrees C even with out the heater. Any suggestions? Im 5 days in.
 

RM-MN

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By 5 days into fermentation the beer should be near completion at which point the temperature doesn't matter much. Turn the heater off and leave the beer for another 2 weeks or so.
 

Golddiggie

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CO2 pressure is effected by temperatures, this is pretty well known. It's part of why when you target a CO2 volume level at one temperature, one PSI is needed. Reduce, or increase, the temperature and the CO2 pressure needed also changes (also increasing or decreasing).

I, also, wouldn't use an airlock to be any kind of pressure indicator. Since gauges will actually TELL you what the PSI level inside the fermenter is. Not to mention, an airlock isn't built to deal with pressure changes like a spunding valve would. The spunding valve I use on my conical fermenters (Spike CF10's) is a one way valve, so CO2 can escape, but nothing will go into the fermenter from it.

Which fermenter are you using? Is it pressure rated (at above 2psi)??
 

gello22

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This is one of those instances when if you can remember back to high school chemistry there was a lesson on ideal gas law. 99% of students sat there thinking when will I ever use this....
 

marc1

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When the temperature drops, the gas inside contracts (pressure drops). This causes suck back - it's pulling airlock fluid and air into the fermenter.
 

Beholder

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That ideal gas law is PV=nRT. For fixed volume of gas, if T drops, then P or V will drop. If the volume is fixed, then pressure drops. If the volume is not fixed, then volume will drop, aka suck back.

I have to remember this when I pump warm wort into my conical and need to get it down to pitching temps… need to pressure it up to 5 psi so that the temperature drop doesn’t suck air back in through the PRV lol.
 

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