Co2 tank pressure

[Ian Grant]

I got my co2 tank at air liquid and never asked how much pressure was in the tank.   I just kegged my first brew today and noticed that the tank reg is only reading 700 psi.  I've done the soap test and there doesn't seem to be any leaks..   I just figured there should be more pressure in the tank.  My guess is that that reg is a dud.

[JohnQ]

700 psi is the low side of Green on my reg, but I've been told by gas vendors that gas pressure is less significant than you might think. I know when mine is full it never reads higher than 900 psi.  I'm sure that the pressure you read on the reg will change with the temp that the bottle is kept at, are you keeping yours inside the fridge, or outside?

The test is really longevity.

I usually get 6+ months to a tank, this tank is 7 months old next week and it's fallen out of the green in the past couple of weeks, the tank before was done on 3 months to the day.  Due to the inconsistency, I've decided I'm going to start weighing the bottle when full, and writing it down and seeing what it weighs again empty to see if there is at least close to a 20 lb drop over the life of the bottle.  Not sure if these are subject to the same 80% rule as propane or not.

JQ

[Ian Grant]

Thanks John..  I think i'll weigh it also when I get a new bottle. I have the tank in the fridge and maybe the cold temp is affecting the pressure or the reg.

[JohnQ]

I have heard that in the fridge makes it read much lower, you probably don't have anything to worry about, the gas will just read lower on the psi scale but will last just as long.  Given mine reads 900 psi when full at room temp, 700 doesn't seem unreasonable.
JQ

[Dean]

you're right ...pressure, temperature and volume are directly related (google Charles' Law). Additionally, when the rapidly fill them at the shop they heat up, causing a high reading ...when you get home it'll have cooled off enough to make a significant difference ...put it in the fridge and the difference becomes even more significant. This is why scuba cylinders are filled in a tank of water.

it's also why you should NEVER leave a filled cylinder in the trunk of your car in the summer ....


incidentally, air liquide is known for short-filling gas cylinders which is why the majority of hobby welders use praxair for our argon mixes.

[Dave Savoie]

Do you keep your CO2 tank in your fridge if so that is the cause of the low preassure

[Ian Grant]

Thanks guys.   I thought there would be more pressure in the tank which cause me to panic a bit..

[Richard]

Bear in mind as well that you're talking about a liquid changing phase to gas, and the gas then causing the pressure. Lower temperature reduces the change from liquid to gas. The pressure is not an indication of the tank's fullness - like JQ said, the only way to tell that is to weigh the thing.

[Ian Grant]

Well I guess I'm glad I got the dual gauge then...   not

[Jmac00]

what's the price of a tank from air liquide if you don't mind me asking?

[Ian Grant]

It's a 3 year lease for $99 and the gas is $47 I believe.

[brew]

Yeah - its interesting as some bottled gases are just compressed (like air in a scuba tank) while some are in liquid form (like propane / butane / CO2). For example, propane boils at around -52 F, CO2 at I think -57 F. You notice that when you shake a propane or CO2 tank they seem to have liquid in them that sloshes around... The idea is that at room temp, the liquid wants to boil and become gas, so in the tank it creates pressure, so long as there is liquid in the tank. That's why the tank pressure always seems to be the same (like 800-1000 psi) - that is until the liquid is gone, then the tank pressure goes down fast. Result is the tank pressure is always the same until its near empty. Keep in mind though, even 700 psi in the tank (with no liquid) is still quite a bit of CO2 - so it can make a good indication its time to get more, so long as you're not doing any heavy duty force carbing (just serving).

Tank pressure goes down significantly with temp - but it will still be constant, and so won't really hurt to keep it in the fridge. Even at 500-600 PSI (like at 36 F or so) it will still give you the same mileage.

Its for this reason CO2 and other liquified bottled gases are measured by weight. We buy CO2 (and propane) by the pound or kilo... A scale is the best way to check how much you have left. Weigh the tank empty, and full - that gives a more precise way to measure how much is left...

Happy carbing!  

[Kyle]

The price of a three year rental on a 20pound tank from Air Liquide (as of 2009) is $99+tax, and the price of a refil using beverage-grade CO2 (as of spring 2011) is $56 including tax and environmental surcharge.

As has already been mentioned, storing the tank in the fridge makes for less pressure, since the gas is more easily compressed at lower temp. Also. If you have the tank outside of the fridge, especially during the summer, it will probably gain 250ponds of pressure, mine does.