The full float gauge assembly as pictured in its entirety. The float is visible at the top of the picture. The float rises and falls with the level of liquid propane in the tank.
The face gauge, often called the dial gauge is the part of the gauge assembly that is used to indicate the approximate level of liquid propane inside the tank.
The float gauge is for consumer use only. As indicated in the picture to the left, the float gauge is not to be used for filling as it only gives an approximate tank percentage and cannot be considered 100% reliable. The fixed liquid level gauge is used for filling. Tanks with float gauges measure the volume of the tank as a percentage of the total capacity of the container. If the gauge reads 50% on a 250 gallon propane tank, the tank has approximately, 125 gallons of propane. Many people think this is a pressure gauge or a gallons gauge (although some older tanks do have gallons gauges) but it is a gauge that indicates the volume in the tank as a percentage of the tank's total capacity.
The float gauge in a propane tank consists of moving parts located both inside the tank and outside. At the end of the stem is the float (pictured below) that rises and falls with the level of the propane in the tank. The top of the stem is the pinion gear (pictured left) that turns the gear in the shaft and causes the dial on the external face gauge to turn. As stated above, the float gauge is not considered a fully reliable instrument for measuring tank volume. Float gauges have a number of moving parts that are subject to wear and tear and can also become ineffective in any part of the assembly. If the gauge needs to be replaced, the propane tank must be empty. If the face gauge has a stuck dial, it can be quickly replaced as dial/face gauges use magnets and can be interchanged without the tank needing to be empty.
To convert percentage to gallons, multiply the number displayed on the face gauge (50 means 50% or .50) by the water capacity of the tank. Water capacity can be found on the tank manufacturers nameplate. The chart below indicates percentages converted to gallons on common size propane tanks (at 60°F).