By Brian Krans, bkrans@qconline.com

Photo: John Greenwood Consumers who purchase pre-filled propane tanks might not always get the number of pounds that is listed on the tank.

Exchanging a 20-pound propane tank for Fourth of July grilling? If the tank you purchase feels lighter than it ought to, there's a good chance you didn't get the amount you thought you were buying.

Last summer, about the time propane prices soared, two of the top pre-filled grill tank companies — Blue Rhino and AmeriGas — reduced the amount of propane in their tanks from 17 pounds to 15 pounds, says a recent story by the Atlanta Journal-Constitution.

Since then, propane prices have dropped by half, but the companies have not increased the amount of propane in the tank or decreased the prices, according to the Journal-Constitution.

Last month, a class-action lawsuit was filed against Blue Rhino and AmeriGas, accusing them of conspiring to reduce the amount of propane in the tanks and failing to inform customers.

"It seems to me it's a pretty clear ripoff," attorney Steve Berman of Hagens Berman Sobol Shapiro told KVAL Television in Eugene, Ore. "You're promising people a full tank — and they promised that repeatedly — and they're giving them less than a full tank."

On Friday, a 20-pound-capacity propane tank purchased at a Quad-Cities area home improvement store carried a sticker that said it contained 17 pounds of propane. The tank then was taken to Trevor True Value in Moline, where it was found to contain only 14.4 pounds of gas.

"Like anyone would expect, it's very surprising, but that's the way some people choose to do business," said Butch Trevor, owner of the hardware store at 2842 16th St., Moline.

A manager at the store where the tank was purchased deferred comment to AmeriGas. An AmeriGas customer service representative said 20-pound tanks that used to be filled with 17 pounds of gas now typically are filled with 15 pounds but wouldn't say why.

Mr. Trevor said he can fill a customer's 20-pound tank — and fill it all the way.

"They want 20 pounds, they get 20 pounds," he said.

The actual amount in the tank can vary "depending on variations in the wall thickness of the cylinder, the design of the valve on the tank's overfill prevention device, naturally occurring impurities in the propane and the temperature, which can cause propane to expand," reports the Journal-Constitution.

Because of overflow safety devices on containers manufactured after 2002, Mr. Trevor said there's no reason tanks shouldn't be filled to capacity.

The class-action lawsuit seeks to represent anyone who purchased Blue Rhino or AmeriGas propane cylinders for gas grills, outdoor heaters and fireplaces and other uses from Jan. 1, 2008, to the present, according to KVAL.

An AmeriGas vice president quoted in the article said the company's canisters have carried labels since last August stating the tanks' lower amounts.

"Unfortunately, some consumers feel they were not sufficiently informed, and that's a serious concern to us," Katz told the Journal-Constitution in an e-mail.

AmeriGas is offering a rebate to its cylinder exchange customers. For more information, visit www.amerigas.com.