Propane, often referred to as liquefied petroleum gas (LPG), is a hydrocarbon fuel stored as a compressed liquid within a pressurized tank. When released, the liquid instantly vaporizes into a gas, which is the form burned for heat or power. This transition means measuring the fuel involves tracking volume and weight, while the ultimate measure of utility is the energy it contains.
Standard Units for Propane Quantity
The propane industry uses three primary units to quantify the fuel, each serving a distinct purpose for the consumer and supplier. The most common unit for delivery and large residential tanks is the gallon, which measures the volume of the liquid propane. This volumetric measure is convenient for large-scale storage and transport.
For smaller, portable cylinders, such as those used for barbecue grills or recreational vehicles, weight is the standard metric, measured in pounds. One gallon of liquid propane is approximately 4.24 pounds at 60°F. Weight measurement is often more accurate for small tanks because it is independent of the liquid’s temperature-induced expansion.
The ultimate purpose of propane is to deliver energy, quantified using the British Thermal Unit (BTU). One BTU represents the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. A single gallon of liquid propane yields roughly 91,500 BTUs when combusted. This energy-based unit allows customers to compare propane’s efficiency and value against other heating fuels.
Measuring Liquid Propane in Storage Tanks
Determining the amount of liquid propane stored in a physical tank relies on different instruments depending on the tank size. Large stationary tanks, such as those for residential heating, use a fixed liquid level gauge, often called a float gauge. This gauge displays the tank contents as a percentage, providing a convenient, though approximate, reading of the liquid level.
The percentage gauge never displays 100%, even after a full delivery, due to a safety mandate known as the 80% fill rule. Propane is highly susceptible to thermal expansion, and its volume can increase significantly as the temperature rises. The remaining 20% of the tank volume is a required vapor space, which acts as a safety buffer to prevent dangerous pressure buildup from this expansion.
For precise measurement during filling, delivery personnel use a separate tool, sometimes referred to as a bleeder valve. This valve is connected to a dip tube that extends into the tank to the 80% fill level. The driver opens this valve and fills the tank until liquid propane emerges, signaling that the maximum safe capacity has been reached.
For small portable cylinders, the easiest and most accurate way to check the remaining fuel is by weighing the tank. The tank’s empty weight, or tare weight, is stamped on the collar, and subtracting this from the current total weight reveals the precise pounds of propane remaining. An informal method involves pouring hot water down the side of the tank; the liquid propane absorbs the heat, causing a visible condensation line or a noticeable temperature differential to mark the fuel level.
Tracking Propane Consumption
Measuring propane consumption shifts focus from the static volume in the tank to the flow of gas being used. In scenarios where a customer receives propane from a utility-style pipeline system, a volumetric gas meter is used to measure the gas after it has vaporized. These meters typically measure consumption in units like cubic feet or hundred cubic feet (CCF).
When liquid propane is delivered by a truck, the volume transferred is measured by a specialized meter on the delivery vehicle itself. This meter registers the exact number of liquid gallons pumped into the customer’s tank, which is the basis for the billing statement. The gauge on the customer’s tank is secondary to this metered reading.
Propane’s volume changes significantly with temperature, affecting its density. To ensure fair billing based on the fuel’s energy content, delivery truck meters are equipped with automatic temperature compensators. These devices automatically adjust the measured volume to a standard reference temperature, typically 60°F, ensuring the customer is billed accurately regardless of the outside temperature.