That is no accident – in fact, we do it to prevent accidents.
You see, the propane inside your propane tank is stored as a liquid, but as a liquid propane can’t ignite; to burn, it must expand into a gas. That expansion happens quickly – about 17 times the rate of a volume of water over the same temperature increase.
Our delivery technicians need to leave extra room in the tank to allow for propane to safely expand as the temperature inside your propane tank rises (while sitting in the sun on a summer day, for example*). Aboveground propane tanks are typically filled to about 80 percent capacity, which leaves 20 percent of the tank empty for expansion (we call this the “80/20 rule”); underground propane tanks be filled slightly higher, because the ground insulates the tank from big temperature swings.
This expansion and contraction of propane is also why you may see some fluctuations in your propane tank’s gauge levels during temperature swings (from a hot day to a cool night, for example). As temperatures rise, propane takes up more room in the tank; as temperatures drop, it takes up less. This is all perfectly normal!
As a propane safety precaution, Thomas Bros. propane delivery teams use a fixed liquid level gauge to prevent overfills – one more way we work to make sure that every propane delivery to your northern North Carolina is a safe one.
*Propane gas expansion is also a reason why you should NEVER paint your outdoor propane tank a dark color, since dark colors absorb heat. More on this in a future blog!