# C02 regulator affected by tank size??

### Help Support Homebrew Talk:

#### kXb

##### Well-Known Member
I have a 5# C02 tank and was told by my lhbs that the pressure gauge doesn't accurately reflect fullness/emptyness of the tank (vs same gauge on larger tank would be more accurate). Has anyone ever heard of that? I suspect I have a leak somewhere and am concerned that if I can't trust the pressure gauge that it will complicate trouble shooting.

#### flars

##### Well-Known Member
I am not an expert, but have read some posts concerning CO2 volume in a tank. Persons posting said the only way to know how much CO2 is in a tank, is by weight. Pressure in the tank, which the gauge reads, is affected by temperature.

#### doug293cz

##### BIABer, Beer Math Nerd, ePanel Designer, Pilot
Staff member
Mod
I am not an expert, but have read some posts concerning CO2 volume in a tank. Persons posting said the only way to know how much CO2 is in a tank, is by weight. Pressure in the tank, which the gauge reads, is affected by temperature.
This is correct. Most of the CO2 in a full tank is liquid, but the CO2 you use is drawn off as a gas. So until the tank is almost empty, the pressure in the tank is determined only by the temperature of the tank. The pressure is the equilibrium CO2 vapor pressure at the particular temperature. When the liquid is gone, then so is most of your CO2, and that is when the high pressure gauge starts to drop (at constant temperature.)

Size of the tank has no effect on the regulator (low pressure stability) or gauge accuracy.

Brew on

#### sandyeggoxj

##### Well-Known Member
Here is a handy little graphic for ya. Say your co2 tank is about 70ºF outside your kegerator. It will stay around 850-900psi on the tank gauge until approximately 30% of the co2 is left. 30% of 5# is 1.5# vs 30% of 20# which is 6#. So you can see that on a small tank when the high pressure needle starts to move you are running pretty low whereas the large tank you still have 6# of co2 left, which is a considerable amount.

This effect is even stronger if you keep the co2 bottle in the kegerator. The needle won't start moving down until you hit about 12-15% left in the tank. Now you are talking less than 1# of co2 in a small tank. In a large tank you still have 2-3# left when it starts dropping.

Doug explained the phase change pretty well.

Does that help at all?

#### doug293cz

##### BIABer, Beer Math Nerd, ePanel Designer, Pilot
Staff member
Mod
Here is a handy little graphic for ya. Say your co2 tank is about 70ºF outside your kegerator. It will stay around 850-900psi on the tank gauge until approximately 30% of the co2 is left. 30% of 5# is 1.5# vs 30% of 20# which is 6#. So you can see that on a small tank when the high pressure needle starts to move you are running pretty low whereas the large tank you still have 6# of co2 left, which is a considerable amount.

This effect is even stronger if you keep the co2 bottle in the kegerator. The needle won't start moving down until you hit about 12-15% left in the tank. Now you are talking less than 1# of co2 in a small tank. In a large tank you still have 2-3# left when it starts dropping.

Doug explained the phase change pretty well.

Does that help at all?
I like the extra notations on the chart!

Brew on

OP