What pressure to replace CO2 tank at?

[mnoone]

I know that there isn't exactly a linear relationship between the amount of CO2 in your tank and the pressure in your tank. But is this relationship understood well enough to know when to replace kegs?

My 20 pound tank is sitting at around 500 PSI. Any idea how many kegs it has left in it?

[djc]

The problem is the gage is measuring the gas pressure above the liquid. That pressure is constant right up until there is no more liquid. No more liquid = no more gas right after that. Most people have spare tanks to always have a full one ready to go because there is nothing worse than running out in the middle of a gathering.

[mnoone]

The problem is the gage is measuring the gas pressure above the liquid. That pressure is constant right up until there is no more liquid. No more liquid = no more gas right after that. Most people have spare tanks to always have a full one ready to go because there is nothing worse than running out in the middle of a gathering.

What I had heard was that pressure starts dropping pretty fast once you're near the end of the tank. My pressure has dropped - it was more like 700+PSI a few weeks ago. So I'm guessing I'm near the end - but want to verify.

[djc]

Where was it when you read 700psi? Where is it now? The reading will change quite a bit with ambient temperature, i.e. if you put a room temp bottle into a kegerator it will drop. If the ambient temp has been stable, and the needle moved, you are done.

[mnoone]

Where was it when you read 700psi? Where is it now? The reading will change quite a bit with ambient temperature, i.e. if you put a room temp bottle into a kegerator it will drop. If the ambient temp has been stable, and the needle moved, you are done.

It's been in the same location for the past 6+ months. It is in an office environment so it should have a pretty stable temperature. I'll get a spare tank on hand and expect to replace this one in the near future. From the sounds of it I think I will need to replace it almost immediately? I've been watching it the past couple days and it hasn't dropped any more and there have been probably 5-10 cups poured from the kegerator.

edit: one more thing - when replacing the CO2 tank do I need to disconnect my keg couplers? Is there any chance of residual pressure in the keg pushing beer up the Co2 lines once there is no pressure at the other end of the CO2 lines? I'm guessing no, but just want to double check

[djc]

You should have a duckbill valve on the gas side of the coupler which prevents backflow.