This explains it fairly well and why I junked the pressure gauge.
Many folks have paid for a pressure gauge on their propane tanks in the mistaken belief that it tells you how much propane is left. Me too, I got one before I figured out that it is useless. How come? High school physics class to the rescue!
In the tank, the propane is mostly liquid. At the top of the tank, above the liquid propane, is gaseous propane. When you open the valve, some of the propane gas is released, and some of the liquid vaporizes to maintain what physicists call the vapor pressure – the pressure at which liquid and gaseous propane are in equilibrium. As long as there is some liquid propane in the tank, this pressure remains the same (although it does vary with temperature, being higher when the temperature is warmer). Whether 95% full or 5% full, the pressure is the same. Only when there is no more liquid propane in the tank will the pressure shown on your gauge drop. You can tell how much propane is left by weighing the tank, but that’s a bother. You can also tell by feeling the side of the tank when it is in use (when your grill is on). At some place the tank will be colder below, warmer above. That’s the level of the liquid propane. Bottom line? Keep an extra tank on hand.
