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Old 07-17-2009, 04:06 PM   #1
NonServiam
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Default Slowly falling keg-side pressure?

So I just kegged a batch for the first time yesterday, and I'm trying to force carbonate it with the set-and-forget method. I put the keg & tank/regulator in the fridge at about 5:30pm yesterday and set the keg-side pressure to about 12.5 psi, tank and regulator open. I checked on it at midnight, and the tank-side pressure had fallen quite a bit, as expected, but what surprised me is that the keg-side pressure had also fallen to about 10 psi. Now this morning, about 9 hours later, I just checked again, and the keg-side pressure has fallen to 8 psi (the tank-side pressure doesn't look like it's changed).

Why is my keg-side pressure slowly falling? It was my understanding that the low temperature in the fridge would only lower the tank-side pressure, because of the gas/liquid mixture in there, but not the keg-side pressure. Does the low temperature somehow affect the diaphram in the regulator (Midwest supplies' double-dual-gauge), such that I have to open it more to keep the same amount of pressure on the keg? Or do I simply have a leak somewhere? (This being my first time kegging, I scrupulously slathered every seal with soapy water, but didn't find any bubbles; and if I had a leak, I would expect the tank to be empty now, rather than this slow loss of keg-side pressure.)

Thanks for any advice to a newbie.

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Old 07-17-2009, 04:30 PM   #2
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Could it just be the new beer absorbing the CO2? I have seen the pressure drop like than when a new batch is carbing.

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Old 07-17-2009, 04:34 PM   #3
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Could it just be the new beer absorbing the CO2? I have seen the pressure drop like than when a new batch is carbing.
I was thinking about that, but since the regulator is open, wouldn't new gas immediately flow in from the tank, maintaining a constant pressure of 12psi as the beer absorbs co2? I thought that was the whole point of the set-and-forget method.

If you've seen this before, that's reassuring. Did you just crank the pressure back up until it read 12psi again?
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Old 07-17-2009, 04:39 PM   #4
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if i move my system from a warm room to the fridge the output pressure will drop 2-3 psi as it gets colder... I would just crack it back up to 12psi now that the system is cooled, and watch the pressure.

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Old 07-17-2009, 04:44 PM   #5
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If you've seen this before, that's reassuring. Did you just crank the pressure back up until it read 12psi again?
Yeah, I just give the regulator valve a tap to bump it back up. Sometimes, I think it is temperature swings in the garage (my CO2 bottle is outside the kegerator) more than anything.
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Old 07-17-2009, 04:48 PM   #6
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if i move my system from a warm room to the fridge the output pressure will drop 2-3 psi as it gets colder... I would just crack it back up to 12psi now that the system is cooled, and watch the pressure.
OK, cool, so it doesn't seem to be unusual -- I'd just never heard of it in my pre-kegging research. I've already bumped the output back up to 12psi. I'm curious about the mechanism behind this, though. Is it just the thermodynamics of the keg-side gas, or does the regulator's diaphram get stiffer or something in the cold?

Unfortunately, I leave tonight for a 10-day vacation, so I won't be able to watch the pressure. But Ceres willing, there will be sweet, sweet draft beer waiting for me when I return (BM's Centennial Blonde!)
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Old 07-17-2009, 04:57 PM   #7
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OK, cool, so it doesn't seem to be unusual -- I'd just never heard of it in my pre-kegging research. I've already bumped the output back up to 12psi. I'm curious about the mechanism behind this, though. Is it just the thermodynamics of the keg-side gas, or does the regulator's diaphram get stiffer or something in the cold?
I am not sure. I know I have experienced pressure droppoff in my paintball gun too after firing a ton of shots in a row (since this chills the tank/regulator)

If your pressure is stable since you bumped it back to 12psi, you should be fine while on vacation. I find the best way for me to set pressure is to disconenct from the kegs so that you get an accurate reading more quickly, set it and give it a minute to equalize (sometimes it can creep 1-2 psi very slowly on minor ajustments) then pop it back on the keg.
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Old 07-17-2009, 05:03 PM   #8
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I am not sure. I know I have experienced pressure droppoff in my paintball gun too after firing a ton of shots in a row (since this chills the tank/regulator).
That's very interesting.

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If your pressure is stable since you bumped it back to 12psi, you should be fine while on vacation. I find the best way for me to set pressure is to disconenct from the kegs so that you get an accurate reading more quickly, set it and give it a minute to equalize (sometimes it can creep 1-2 psi very slowly on minor ajustments) then pop it back on the keg.
Ah, I see. So you're basically just pressurizing the gas line up to the disconnect, and since that is a much smaller volume, it will settle on whatever the cold-temperature equilibrium is much faster. Cool.
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