Learning to determine how much propane is left in a gas tank can come in very handy. For instance, if you are planning to host a barbecue and you aren't sure if the tank has enough propane to cook all the food, it is a good idea to check how much is left to make sure you don't run out of gas before the food is all cooked and ready to be served. An easy way to tell how much propane you have left in the tank is to use hot water.
Hot Water Method
Pour hot water in a bowl, pitcher, bottle or measuring cup.
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Pour the hot water on the side of the propane tank.
Touch the tank where you poured the hot water. The section of the tank that feels warm is empty; the part that is cold still has propane in it. The empty part of the tank feels warmer because the hot water influences the temperature of the metal on the tank. However, the section of the tank that has propane will feel cooler because the propane -- in liquid form in the tank -- has a condensing reaction to hot water. Propane gas becomes colder as a reaction to condensation.
Disconnect the propane tank from the grill.
Weigh the gas tank by placing it on a bathroom scale. Propane tanks weigh 18 pounds when empty and about 35 pounds when full. Therefore, deduct 18 pounds from the total weight to know how many pounds of propane are left in the tank. For instance, if the tank weighs 25 pounds, there is 7 pounds of gas left.
Multiply the number of pounds left in the tank by 30 to get the amount of grilling minutes you have left. For example, multiply the 7 pounds of propane gas by 30 to get 210 minutes. You can divide the amount of minutes by 60 to get the amount of hours left.