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-   -   How much pressure in a newly filled tank? (http://www.homebrewtalk.com/f35/how-much-pressure-newly-filled-tank-386441/)

KEG99 02-01-2013 11:21 AM

How much pressure in a newly filled tank?
I just bought a new 20# cylinder, got it filled, brought it home and it shows 650# overall pressure. I would have thought it would be well over 1000#. Heck, the guage markings suggest refilling at 500#. Did I get shortchanged?

PJM 02-01-2013 11:30 AM

Was it in the fridge when you got that reading? Chilling the tank reduces the pressure. If not, I would think something funny is going on

BigRob 02-01-2013 11:34 AM

The high pressure gauge is only useful when all of the liquid CO2 is gone, and only gas remains. If the tank itself is still cold from filling, or in a cold room (or fridge) the pressure inside the tank is lower.

If you want to make sure you aren't being ripped off, look for the Tare Weight of the cylinder (it should be on the cylinder itself somewhere). Now weigh the whole thing on a scale, and subtract the tare weight, now you can see if they gave you 20# or ripped you off. Make sure when you weigh the cylinder you remove the regulator and such.

KEG99 02-02-2013 12:44 AM

Thanks. I checked the weight. 43.4# Tare weight is 11.2kg (or 25#) So the net is ~18#. Pretty close (if you include the little bit i used last night). I just expected a much higher pressure. Lots to learn.

patthebrewer 02-03-2013 06:32 PM

My completely filled tank in my kegerator, only shows 500psi at 35 degrees. The high pressure gauge is of questionable usage, I think, since it can't tell you how much you have left in your tank. The only thing that can is a scale.

shelly_belly 02-03-2013 10:40 PM

1 Attachment(s)
The pressure depends on the temperature and percentage of fill.

Walking_Target 02-04-2013 04:06 PM

Basically, as above.

CO2 becomes a liquid under two conditions, when it is under enough pressure, or when it gets cold enough.

Once you have a volume of CO2 under pressure, the partial pressure required to keep it liquid fluctuates directly with the temperature. There's a certain threshold where headspace becomes a major role too but realistically, if it's being filled to ~80-90% of rated capacity by *weight* you're pressure will fluctuate directly with the temperature of the tank.

Also, neat fact, if you have a know volume with a known weight of CO2 in a closed system with a pressure gauge, you can accurately measure temperature :D.

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