PSI (when to replace)

Hi everyone quick question, my 5lb tank when filled was over 800, maybe about 820, now after several weeks it's gotten down abit but still over 800, at what PSI do I have to consider replacing it? When it reaches 0? I have no clue thank you

You will find that the needle will stay where it was when filled for a long time then suddenly start falling after 6 kegs or so. Use it all up until you're at zero then fill.

ok thank you

For reference.... The reason is that the tank is full of a liquid with a gas on top. As you allow gas to escape off of the top the liquid converts back into gas. The ratio of gas to liquid is dictated by the vapor pressure of the liquid and the temperature of the room (I think, chemistry is a little rusty). So as long as there is liquid left in the tank the pressure on the high pressure gauge will read about the same thing (800 or so). Once the liquid is gone and there is only gas left it will DROP fast. Same thing goes for propane... thats why most propane tanks don't have gauges. If you really want to know how much gas is left measure the tanks weight. ... 5# cylinder has 5# of gas... so if you know the tank weighs 10# empty... it would weigh 15#.. and you can figure it out from there. Just depends how motivated you are.