andrew027:

 

Someone gave me one of those magnetic ones once. It seems to be just an LCD thermometer, so when it's cold enough outside the volume of liquid in the cylinder doesn't make much difference. Even when the ambient temperature is warmer it's not particularly accurate.

 

Has anybody had eny experience with the ultrasonic ones, e.g.: Gaslock GasLevel Pen?

 

I can see that based on the mentionings here of ambient vs tank temp.. Might be a case of running hot water down the tank, slap the magnetic on and at lats then get a reasonably accurate line