Propane, commonly known as Liquefied Petroleum Gas (LPG), is a hydrocarbon fuel used for heating, cooking, and powering vehicles. Unlike natural gas, which remains a gas under normal conditions, propane is compressed and stored as a liquid in tanks. This change in state allows a large amount of fuel to be contained in a small space. Understanding the weight of this liquid is fundamental for safety, transportation, and commercial transactions. This article details the specific weight of a gallon of liquid propane and the physical principles that influence this measurement.
The Specific Weight of Liquid Propane
The weight of one U.S. gallon of liquid propane is standardized for commercial purposes. At the industry-standard temperature of 60°F (15.6°C), a single gallon of liquid propane weighs approximately 4.2 pounds. Liquid propane is significantly less dense than water, which weighs about 8.34 pounds per gallon. This difference highlights propane’s relatively low density in its liquid state.
Factors That Influence Propane Density
The standard weight of 4.2 pounds per gallon is an average because the density of liquid propane is not constant. Temperature is the most significant factor affecting density, based on the principle that most liquids expand when heated. As the temperature rises, propane expands in volume, becoming less dense and lighter per gallon. Conversely, when the temperature drops, the liquid contracts, becoming denser and heavier. This variability is managed in the industry by using Specific Gravity, which compares the density of propane to the density of water to standardize measurements.
Practical Measurement and Tank Safety
Propane tanks are filled by weighing the total container rather than observing liquid volume. This method ensures accuracy and safety by accounting for the variable density. To calculate the amount added, the filler must know the tank’s empty weight, known as the Tare Weight (TW), which is stamped on the collar. The “80% fill rule” dictates that a tank must never be filled beyond 80% of its total water capacity, leaving the remaining 20% as vapor space to accommodate thermal expansion. Overfilling eliminates this necessary vapor space, and rising temperatures could cause extreme pressure buildup, potentially leading to tank rupture or activation of pressure relief devices.