Originally Posted by broadbill
Also, would tank temperature effect what the tank pressure is? IIRC, the 1000psi reading was when the tank was at room temp, but then it went into the fridge and read the 500 psi a few days later...
This is exactly it, and completely normal.
To expand on this, that little gauge is useless. The way a CO2 tank works is there is mostly liquid CO2 in the tank, with a bit of gaseous CO2. As you suck gaseous CO2 out of the tank, some of the liquid boils and becomes gas to replace it.
The end result of this is: the pressure is always the same in the tank (at a given temperature), whether it is full or almost empty. Only when the last bit of CO2 boils off, (which is right before the tank goes empty), does the reading change.
So it's not like a fuel gauge on a car. Instead, it will read 500 psi until JUST before it runs out, then it will quickly drop to 0.