As anyone who has spent enough time using a gas grill will tell you, a propane tank will always run out at the least convenient moment--usually right after the store has closed, or before an expensive steak is finished. The question soon follows: Why does the tank not have a pressure gauge on it? Nor can one found as an option when purchasing the tank or grill, regardless of price. As it so happens, adding a pressure gauge to a propane tank would also be effectively useless.
Propane Isn't Always a Gas
Inside a propane tank, you are not dealing exclusively with a gas. As you add propane gas to an empty tank, at first the pressure will increase. However, once a certain pressure is reached, the gas begins to condense into a liquid form. If you continue to add more propane, the pressure will stay mostly the same while the level of liquid propane at the bottom of the tank increases.
A full propane tank will be have mostly liquid propane stored in it, with a pocket of propane gas at the top near the valve. When you open the valve, that gas will be pushed out by the pressure inside the tank. As the pressure in the tank decreases, some of the liquid propane will change back to a gaseous form until the pressure rises again. This process will continue for as long as the tank valve is opened, with the level of liquid decreasing until all of the propane has changed back to a gas. With no liquid left in the tank, the pressure will drop rapidly until the gas no longer comes out of the tank. This is why it appears that propane tanks become empty with no warning.
While you hear, and smell, propane gas coming out of the tank, inside the tank the pressure will stay the same. Only the level of liquid propane will drop. If the pressure drops, more liquid changes back to a gas, increasing the pressure again. If the pressure increases, more gas turns to liquid, dropping the pressure. If you were to add a pressure gauge to the tank, and the pressure stays mostly the same, any gauge would not be able to tell you how much propane is left in the tank.
Determining Propane Levels
If you cannot judge how much propane is left in a tank by the pressure, the next question is how you can know how full it is. The most common method is by the weight of a tank. As the level of liquid propane increases inside a tank, so does the overall weight of the tank. A common size used for home grills is a 20 pound tank, which means it can hold 20 pounds of propane inside. Though remember to also include the weight of the tank. When full, a 20 pound propane tank will weight approximately 40 pounds.
If lifting a 40 pound tank full of propane seems inconvenient, there is another method used to check how full the tank is. Many home improvement stores sell a long sticker or magnetic strip that attaches to the side of the tank. Pouring near boiling water on the strip causes it to change color only where there is gas inside the tank, but it will not change where there is liquid. This allows you to visually see how much liquid is left in the tank. If you need to exchange tanks when getting it filled, remember to buy a magnetic version and take it off the tank before exchanging.
One thing to remember with a propane tank is that the pressure inside the tank can change, even if a gauge won't tell you how full it is. As the tank cools, the pressure will decrease. As the tank warms, the pressure will increase. If the tank pressure gets high enough, the safety valve will open to let out the extra pressure. This is why you should never leave a propane tank in a hot car, store it indoors, or let it anywhere near a fire.