The main offender is the 1 peso coin from the Philippines, which is worth about W18 (around 1.8 U.S. cents) but is being used and accepted because it is essentially the same size and weight as a W100 coin.

Kang Tae-hyang (27) went to the Philippines last winter. "A number of university students who realized that a 1 peso coin works like W100 in vending machines started bringing back a lot of the coins, which they then use themselves or give to friends as presents," he says.

Not only do the machines misread the coins, but if the refund button is pressed, out comes -- not the peso but a real W100 coin. Thus every time you put a 1 peso coin in the machine, you are in effect making a W82 profit.

According to the Bank of Korea, a 1 peso coin is 24 mm across and weighs 5.42 g. The W100 coin, which is made of a nickel alloy, is exactly the same diameter, and there is only a 0.1 mm difference in the thickness. The weight difference is a mere 0.68 g, and the materials are also nearly identical.

"Commonly a vending machine will recognize a coin based on three factors: size, thickness and materials, but when another coin with similar traits is inserted, it may be misread," a researcher at the Korea Minting & Security Printing Corporation says.

In a similar case, it was once possible to pass off the Korean W500 coin as the almost 10 times more valuable 500 yen coin in Japanese vending machines. One staffer with a vending machine firm says Korea may have to do what Japan did when it added zinc to the Y500 coin.

(englishnews@chosun.com )