Design News for Mechanical and Design Engineers

Newport, RI--When you need one hand for the guide line and the other to hold a submerged wreck, there's nothing left to operate your dive computer. This dilemma faces recreational divers who need to "beat the clock" and avoid decompression on their return to the surface.

Recreational divers avoid conditions that force them to decompress on the way up. Technical divers build decompression into their dive schedule so they can venture deeper or dive for longer periods of time. A combination of the depth and length of a dive causes nitrogen gas to build in body tissue. When this happens, divers must "decompress" on the ascent, pausing at various depths as nitrogen gradually comes out of their system. A straight ascent causes "the bends."

Manual calculations of dive time become cumbersome because divers spend time at several depths. Each level requires a separate calculation, and, once in the water, divers must constantly check their watches to stay within the allotted time. Because manual calculations only estimate actual dive conditions, divers leave themselves a wide safety margin, allowing less time below the surface, Lavoie points out.

The dive computer, which senses depth and time elapsed, indicates the dive time left at any given moment. Should the diver descend, the countdown accelerates. As the diver ascends, the rate slows down. "Basically, when divers use a computer, they get more bottom time," Lavoie notes.

Dive computers come in console- or wrist-mounted models. Typically, a flashlight or clip-on light helps illuminate the display, but these can be difficult to contend with. "For instance, I dive off of Castle Hill in Newport to a depth of 200 ft. It's pitch black down there. If I'm exploring a wreck and there is a strong current, I have to hold onto the ship. It's very difficult to handle a flashlight under these conditions," Lavoie stresses.

Self-lighting console. Recognizing this problem, Suunto Corp. (Espoo, Finland) built electroluminescent (EL) lamps into its premier line of dive computers, including the Eon Lux console-mounted unit, and the Spyder(reg) wrist-mounted model. Lavoie uses the console-mounted computer, noting it is "inexpensive and durable."

With gentle pressure on the right side of the computer console, the display is backlit with a turquoise-colored lamp. Light quality resembles that of a plug-in luminescent lamp similar to a home night light. "Early models that do not use EL lamps have a phosphorescent layer behind the console. The layer is activated with a flashlight and it backlights over a short period of time so you can read the console," explains Erik Lindman, who serves as a product development engineer at Suunto.

Suunto's Eon Lux was the first dive computer on the market with an EL lamp, Lindman claims. "It took a long time before we had a competitor with this feature," he adds. Suunto worked with several lamp manufacturers to develop the unit, but selected Durel Corp. (Chandler, AZ), a joint venture between 3M and Rogers Corp. to supply the lamps. "Durel proved to be the most reliable and knowledgeable lamp supplier," Lindman relates.

Design challenges. The companies worked on two design problems: providing bright, even lighting over the entire face of the computer, and drawing small amounts of energy from the battery.

"With the DUREL(reg) 3 electroluminescent lamp, we could achieve a high-quality backlight. It evenly lights the whole area and is reliable, since the display contrast remains high when the lamp is on," Lindman notes. The lamp also has low power requirements, "helping to greatly extend the battery life," he adds.