The strip gauge I had required you to pour a cup of hot water over the tank/gauge.
The temperature variable between the propane gas and the vapor space in the tank created a condensation line indicating the level of propane left in the tank.
The gauge itself just gave a visual of 3/4 or 1/2 or whatever.
This hot water method is probably the most accurate measurement you will find of how much propane is really left in your tank.
Hope this helps.