Canadian Rainbirds wrote:
Take a kettle with a quart or so of boiling (or near boiling) water and slowly pour it down the side of the tank from the top.
Feel the tank where you have just poured the water. It will cool very quickly where there is liquid and remain hot a bit longer where there is vapour.
I have been using this method for years to the check the level in the 100 pound tank we have at the stick house. It is pretty accurate. Don't burn your hand!
I first thought this was a tall tail, but it works like a charm.
In as much as your problem, there is a safty valve in the tank that will prevent overfill, mine stuck right after I got my new rig and prevented getting a fill up.
Check with your local LP people, if thats your problem it's an easy fix.