When to refill CO2 tank?

Obviously aside from when it runs out.

I just started brewing and kegging and bought a two keg system with a 5lb tank. It has a Taprite regulator. The reason I ask is because it has that wide range of red on the dial, where it says to refill. It also has the wide range of green "Full" space on the dial. I got it filled and it was about at the low end of the green part of the dial and now after force carbing two kegs (and drinking the entire first keg) it's down to about halfway between the green and red. Seems a little fast, but I did have a leak in the second keg which I discovered after a day and had to send it back. But I guess my question is, do I have to get it refilled right when it hits the top of the red space, or can I wait until it's almost to the bottom of the dial?

Thanks for the help!

If your tank is in a cold environment it'll read lower than it should, like 400psi or so in a fridge and 800psi outside in room temp, roughly.

I just recently used up my first 5lb tank and (with tank at room temp, so 800psi was 'full') once it started dipping into the red I was able to dispense another full 5 gallons of beer and keep two already carbed kegs at 12psi or so. I also purged one recently filled keg.

It ran out very shortly thereafter.

I ran it until it was empty purposefully though, partly because I had to get it recertified (hydro tested) which requires an empty tank, and partly because it was Christmas and New Years and nothing was open to refill!

Psych said:

If your tank is in a cold environment it'll read lower than it should, like 400psi or so in a fridge and 800psi outside in room temp, roughly.

I just recently used up my first 5lb tank and (with tank at room temp, so 800psi was 'full') once it started dipping into the red I was able to dispense another full 5 gallons of beer and keep two already carbed kegs at 12psi or so. I also purged one recently filled keg.

It ran out very shortly thereafter.

I ran it until it was empty purposefully though, partly because I had to get it recertified (hydro tested) which requires an empty tank, and partly because it was Christmas and New Years and nothing was open to refill!

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Good to know, thanks. I didn't realize the change in PSI reading would be so dramatic due to temp.

Yeah I'm sure someone will provide the formula, but gas contracts at colder temps is the essence. My freshly filled tank, which is now in the fridge at 38f or so, is hanging just into the red, around 400-500psi.

CO2 in a liquid/vapor state has a direct pressure/temperature relationship, as can be seen in the chart below. The pressure will stay the same, at a given temperature, until all the liquid has been vaporized. Once only vapor exists, the pressure in the tank will drop rapidly. The only way to know for sure how much liquid CO2 is left in the tank is to weigh the tank, and subtract the tare weight that's stamped on the side of the tank.

Exactly. That will be in the "fill tank" range until about 10 minutes before the tank is empty. It'll go from 400 psi to 0 without stopping in between.

The best way to deal with that "gauge" is to cover it with duct tape.

I am likely going to purchase of a small scale (mechanical) to sit under the CO2 tank as this seems to be the easiest method of gauging the CO2 left in the tank. 7.6 lbs empty, 12.6 lbs full, watch the gauge and as you get close to empty you ask how close do you want to get before refilling.

Pv=nRT, pressure and temp. are directly related. i also exchange before hydro date expires.