You can swap a propane tank almost anywhere these days. Exchange cages are in front of supermarkets, hardware and convenient stores. But while they are clearly popular, they are also the most expensive way to get a full tank.

In this summer barbecue season, people trying to save money are having their tanks filled the old fashion way - by hand.

"Because it is a lot cheaper," said Serge Libiolle. "I cut my propane costs by half."

Libiolle cooks Belgian waffles at San Diego farmers markets. He stopped at the Expo Propane refueling station on Kettner Boulevard where he filled five tanks. If he swapped the tanks, it would cost him around $20 each to exchange. At the Expo Propane, he paid $1.85 a gallon, so a full tank costs less than $9.

"It's easy," said Libiolle. "You drive in, they fill it up and then you go on."

Elvira Moore spent $17.39 to fill up two tanks. "So I'm basically getting two full tanks for the price of one," she said.

So why are exchange stations so popular? They are convenient and people get a new or reconditioned tank for their old tank. But there are still clear advantages to having your tank filled.

At a refueling station, you only pay for the gas you need, while at an exchange station, you don't get credit if there is propane still in the tank.

"So if you are half full, you only pay for half that cylinder fill," said Steve Moore with Expo Propane. He said tanks they see usually have fuel inside because people did not want to run out.  

"We only charge for what they get," said Moore.

Refueling stations also fill up tanks to 20 pounds, which is often about 4.7 gallons of propane. Some exchanges only fill the tanks to 15 or 18 pounds and not the full amount.

"I do watch the pennies and I kind of hate to waste stuff," Elvira Moore.

One thing to know, however, is that people who fill tanks don't charge the same amount of money for their propane. So the same way you'd shop around for gas to fill your car, you should shop for propane gas.