Which Dive Computers for High Altitude Diving?

Most diving is obviously done at sea level but there are some freshwater divers who go to lakes in the Sierra Nevadas, the Rocky Mountains and other places 2,500 feet and higher. If they're relying on the dive computers they use while diving at sea level, they may not be getting the correct readings for decompression levels.

"Depth estimation will be affected, and at high altitude, a diver will need to dive deeper in the water column to achieve the same depth reading as at sea level," says Martin Sayer, editor of the journal Underwater Technology. "This effect will be amplified in freshwater if the dive computer pressure sensor is calibrated to brackish or full seawater. Altitude has an obvious effect when the diver surfaces and continues to off-gas at much different pressure gradients to those expected at sea level."

Which dive computers perform best at different levels? Peter Buzzacott of the University of Western Australia and Alex Ruehle of the University of Denver took 11 top-selling dive computers on eight freshwater dives, six at low altitude (130 feet above sea level) and two at 10,000 feet above sea level. At high altitude, time was allowed for each dive computer to clear out residual nitrogen before the first dive. Four of the dive computers failed at the onset, leaving two Uwatec Aladin Sport models, two Dive Rite NiTek models, two Suunto Vypers, and a Delta P Technology VR3.

Agreement between each brand model was good when it came to depth, but the range of no-decompression limits at high altitude was pretty wide. The Suunto Vypers were the least conservative at low altitude, but they were consistently the most conservative at high altitude.

The opposite can be said for the VR3. The other models fell in between. But overall, the computers' no-deco limits were less conservative than those in published dive tables -- they ranged from 31 to 42 minutes at a depth of 60 feet, but published tables for 10,000-foot altitude dives recommend no-deco levels of just five minutes at 60 feet. Buzzacott and Ruehle suggest that manufacturers publish anticipated no-deco levels for dives at a standard altitude, say 5,000 feet, in instruction manuals for more valid comparison between dive computers intended for use at altitude. Until then, they recommend that high-altitude divers using computers also check the appropriate decompression tables. Despite modern dive computers providing decompression information up to 20,000 feet in altitude, the higher the altitude you're doing dives at, the more you should consider your dive computer's no-deco levels to be "experimental."

"The effects of high altitude on relative performance of dive decompression computers," Underwater Technology, vol. 28, no. 2, pgs. 51-55.