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August 2008    Download the Entire Issue (PDF) Available to the Public Vol. 23, No. 8   RSS Feed for Undercurrent Issues
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The Misuse of Dive Computers

from the August, 2008 issue of Undercurrent   Subscribe Now

While dive computers have evolved considerably, their very complexity can lead to increased human errors in interpreting and using them. Here is a recently published case that shows the risk facing divers who don’t master the technology.

Two years before the incident, a 33-year-old female advanced diver learned, while being treated for decompression sickness, that she had a patent foramen ovale (PFO), a small hole in a chamber of the heart that increases the likelihood of DCS. It was surgically closed successfully so she returned to diving and purchased a Suunto Vytec computer, with the view of adjusting its settings to make her decompression management more conservative.

With the Vytec, she completed six dives, two a day, with maximum depths ranging from 52 to 77 feet, total dive times from 50 to 55 minutes, and with surface intervals approximating 2.25 hours. On dive three, she went to 79 feet. The download of that dive from her Suunto Vytec showed that the computer went into decompression mode after 21 minutes; she and her dive buddy had ascended from 79 feet at 42 minutes into the dive, with total dive time of 53 minutes. During ascent, she had two rate warnings -- a violation of depth ceiling, and a recording of the computer being switched into compass mode.

Upon surfacing, she had taken incomplete decompression, with the computer locked in the gauge mode (i.e., it displayed depth/time information only) and displaying an error message “Er.” She and her buddy were confused about this because her buddy’s computer had cleared of any decompression obligation on surfacing. None of the dive party understood the relevance of the “Er” display. She attempted to unlock the computer by hanging it on a shotline during the surface interval. However, the computer remained in gauge mode, so for her subsequent three dives she used a Suunto Gecko computer which had not been used for diving that week.

On the third diving day, two hours after her sixth dive, she reported DCS symptoms and was taken to a hyperbaric chamber. She initially appeared well but later that day she had pain and weakness in her left arm and shoulder, and her walking and balance were unsteady. Despite three further treatments, she lost more muscle control and was transferred to another hospital for more treatments.

The Suunto Vytec can be operated in air, Nitrox or gauge modes. There is a facility to switch gases, optional wireless pressure transmission, extensive memory functions and a built-in dive simulator. The Vytec employs an adjustable, Suunto-modified reduced gradient bubble model (RGBM). It is programmed with eight diver-adjustable settings that can be altered to produce many levels of added conservatism: three for altitude, three for personal conservatism and either the full capacity of the RGBM (RGB 100) or reduced power (RGB50}.

In this case, the download indicated that her computer had been set to an A2 altitude (3200-6400 feet) and remained at the default personal setting of PO. It was, therefore, unsurprising that on the third dive the altered Suunto Vytec had a higher decompression requirement than the dive leader’s unmodified computer. The computer had done what it had been programmed to do and produced a more conservative dive profile. However, this was either ignored, not understood, or the consequences of alterations had been forgotten.

The potential to misunderstand outputs from some dive computers, matched possibly by peer pressure, may be a contributing factor in some decompression incidents. This diver was well aware that her previous episodes of DCS, possibly associated with a PFO, meant that to continue diving she needed to dive more conservatively. This she attempted, but then either ignored or forgot about the changed computer settings. Both she and her dive buddy should have been aware of the adjusted levels on the computer and modified their dive practices accordingly. That she then swapped to another unused dive computer with no residual nitrogen loading and possibly no altered conservatism settings could have contributed to her subsequent injury and could easily have been avoided.

This case was taken from the article The Consequences of Misinterpreting Dive Computers, by Martin DJ Sayer, Colin M Wilson, Gerard Laden and Phillip Lonsdale. It was published in Diving and Hyperbaric Medicine, the Journal of the South Pacific Underwater Medicine Society, March 2008. Undercurrent accepts full responsibility for any errors due to editing.

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