CO2 Bottle - How much is left?

To those of you running CO2 bottles, how do you know how much you have left? I used my bottle for the first time this past weekend, I reseated three beads, and aired up a total of 7 tires (long story). I don't want to head out next weekend and run out, but on the other hand I don't want to pay $20 (the cost to fill it) every time I use it either!

Weigh the tank when full, then weigh it sometime when you are completely empty. I'm assuming the difference will be around 10lbs if you have a 10# bottle.

bring the bottle inside and use it a little bit (let some air out). condensation will form on the outside up to the level of the liquid CO2. If no condensation forms, feel the outside of the bottle. It will be cold up to the depth of the liquid.

There is a weight stamped on the outside of the cylinder which will tell the empty weight then find a scale.

I bought a fish scale from Bass Pro. I keep it in my CO2 bag. When it runs empty next time, I'm going to mark the scale with a marker in that spot, then I'll do the same when I fill it. I can't remember how much it weighs full because it's been a while since I last filled it.