Comparing Dive Computers

By reading the depth and recalculating every few seconds, dive computers have enabled dive times to be extended well beyond those permitted by tables on most dives, especially on multi-level dive profiles. However, while over the past few years many of the current computers have been re-programmed to increase conservatism, reducing no-stop times and increasing decompression requirements (even to the extent that parameters such as temperature and gas consumption are factored in), there still remains concern about dive computers’ efficacy in minimizing the incidence of decompression sickness (DCS).

Some of these concerns come from statistics, such as DAN’s data indicating that in 2002, 72% of the divers who were treated for DCS had been using a dive computer. DAN’s data from a 1997 study also indicate that in a very high proportion (93.7%) of similar cases, divers reported diving “within the limits” of their computers. DAN acknowledges that the high proportion of divers using computers could certainly impact the proportion of DCS cases arising from within this group. (Look around the dive boat sometime and count the number of divers seen checking a dive table between dives.) However, there’s also significant variation in the conservatism of dive computer algorithms themselves, and diving “within the limits” of a ‘liberal’ computer may well be riskier than diving “within the limits” of a more conservative model.

Current dive computers vary greatly in the bottom times they allow and decompression stops required. Assessing the level of risk actually being assumed starts with assessing how liberal or conservative the computer itself is. That sounds simple enough, but unfortunately there are few studies actually comparing such variances.

In 2004, John Lippmann, Executive Director of DAN, Southeast Asia-Pacific, and Mark Wellard, a research fellow at the Brain Research Institute, Melbourne, Australia, undertook a comparison of the dive profiles for five common dive computers. The study compared the Suunto Solution, Suunto Vytec, Uwatec Aladin Pro, Uwatec Aladin Smart, and Oceanic Versa over several dive series. The Suunto Solution preceded the Suunto Vytec, and the Uwatec Aladin Pro preceded the Aladin Smart. The earlier models were tested because they are still commonly used and can help determine differences in the updated decompression algorithms incorporated into the newer models. All computers were set in the standard mode with no “safety” or altitude time reductions implemented.

This group of computers was subjected to several series of pressure exposures in a small, Perspex compression chamber filled with fresh water. Although some of these exposures were undesirable from the perspective of DCS risk, the profiles were designed to simulate as closely as possible actual depth-time diving profiles that might commonly occur in actual use. Computers were allowed sufficient time profiles.

The no-stop times allowed and the decompression requirements indicated by the computers were then compared with those generated by the Canadian Forces’ (DCIEM) tables. The DCIEM tables are a widely accepted benchmark for determining decompression risk.

Of all the computers tested, the Vytec times more closely paralleled those of the DCIEM table model. The Vytec was consistently more conservative than its predecessor, the Solution.

The Aladin Pro and Aladin Smart models generated similar no- stop times and decompression times on the rectangular profiles tested. However, the Aladin Smart was considerably more conservative on the multilevel profiles than the Aladin Pro and all the other units tested.

The Oceanic Versa was consistently less conservative than the other dive computers and the DCIEM tables except on a series of deep, repetitive “bounce” dives. In this case, it required decompression times well in excess of the other dive computers and the DCIEM table model. The decompression times indicated in these cases appear to be excessive when compared with other decompression tables.

On occasions, the five models of dive computer tested in this study varied widely on their decompression advice, with up to 25 minutes variation on decompression stop time and up to 38 minutes of allowable no-stop time on some profiles.

Lippmann and findings suggest that it would be prudent for divers to research and choose a dive computer that is relatively conservative on the types of profiles they dive most frequently. The complete abstract of this study, along with charts of the specific profiles, can be found in South Pacific Underwater Medicine Society Journal Vol. 34, No. 3. Since this study, more computer models have been tested. Those results have yet to be published.

For more information on SPUMS, see www.spums.org.au