If you watch your propane tank gauge, you may have noticed that your propane supplier delivered your fuel, he didn’t fill your propane tank all the way up to the 100% level.
This is known in the industry as “the 80/20 rule,” and it’s done for a very important reason: safety.
To start with, the propane in your tank is stored as a liquid. The liquid then changes to gas before it leaves the tank. That’s why it’s called liquid petroleum gas (LPG).
The difference with propane is that it expands a lot, and quickly –in fact, its volume increases nearly 17 times the volume of water over the same temperature increase.
So once it’s inside your tank, propane will expand. to make sure there’s room for that expansion to happen safely, aboveground tanks will be left at a capacity of about 80% full.
This is why your delivery driver needs to leave extra space in your tank to allow for propane to safely expand. Aboveground propane tanks are typically filled to about 80 percent capacity; underground tanks can be filled slightly higher because they are insulated against the heat. The extra space in the tank provides a cushion against the pressure that builds up in a tank.
As an example, a 500-gallon tank filled to 80% will safely hold 400 gallons of propane.
This so-called “80% rule” is especially important in hot weather—when liquid propane will expand the most. If you notice that the tank gauge reading fluctuates during quick temperature swings (hot days, cool nights), don’t worry, that’s perfectly normal. Also keep in mind that the amount of gas in the tank doesn’t actually change during periods of expansion and contraction—only its density does.
Propane gas expansion is also a reason why you should never paint your outdoor propane tank a dark color, since dark colors absorb more heat.
Not quite sure how to read the gauge on your tank? It’s easy—and important if you’re a customer who calls in their order. Here’s how:
The gauge is on or near the top of your tank, usually located under the dome, and it resembles a meat thermometer.
The reading on the gauge provides the approximate percentage of fuel in the tank. If the gauge reads 30% or less, you should arrange for a delivery from your local propane company.
The numbers you see on the gauge represent the percentage of propane in your tank—not the actual amount of gallons. When your gauge level reaches about 30%, it’s time to call for a delivery. That ensures that there’s enough time for your propane supplier to get to you before you run out.