Larry Williamson is making money selling air to the U.S. Navy. But before anyone starts screaming rip-off, it should be noted that his product has irresistible consumer appeal: survival. It's an emergency tank called Spare Air, a device with two minutes' worth or 30 breaths at sea level. To anyone who has run out of oxygen underwater Williamson's invention is a godsend.

That happened to Williamson, now 43, while he was scuba diving 12 years ago for lobsters off California's Santa Catalina Island. A hundred and forty feet below the surface he suddenly realized he was out of air (the reserve valve on his air tank had been turned on accidentally, emptying it). Through his panic he remembers thinking, "Why doesn't someone make one breath of air you could stick in your pocket?" Just before he blacked out, he pulled the CO2 cartridge that inflated his life vest, which brought him to the surface with a headache, but alive.

Williamson worked out the details of his extra-breath invention back in the two-man machine shop he ran in Huntington Beach, Calif. Four years ago, Marine Corps pilots at nearby Tustin air base became interested in it as an emergency aid for helicopter crews who ditched their craft at sea, and the Navy brass began testing it head-to-head against a similar Coast Guard product (an inflated vest from which air can be sucked through a tube). Late in 1986, Spare Air (which the military calls HEED II, for Helicopter Emergency Egress Device, Type Two) was approved, and Williamson got an initial order for 8,200 tanks, at $195 per canister. The prospect of a $1 million gross delights him, but Williamson says his best reward still comes from fellow scuba divers when they stop by to say, "You know, this is the greatest thing in the world. It saved my life."