But I was within the limits of the dive computer

Dive incident insight: an example of when the dive computer algorithm can let you down.

September 11, 2023

Most divers these days use dive computers and appear to be reasonably confident that by doing so they will avoid decompression illness (DCI) as long as they dive within its limits.

One DAN Member, who called for help, commented: “But I didn’t believe it could be DCI as there were no violations according to my computer”, a relatively common reaction.
Deeper dives, longer dives, short surface intervals and repetitive diving, especially multiple repetitive diving, can all lead to higher inert gas loads, greater bubble formation and consequently less accurate decompression calculations. When these factors are combined, as they often are, we can be inviting problems.

Dive computer algorithms are based on decompression models, which are only generalisations of what might be occurring within a diver’s body. They cannot account for variations in each divers' age, weight, height and physical conditioning, or the types of dives undertaken.

Case example

Location: Truk LagoonProblem: Diver suffered severe cerebral DCI
Dives Undertaken: The diver conducted three dives each day for four consecutive days. The deepest dive was on the last day and was to 54 msw. The average depth of all the dives undertaken was 30 msw. The length of the dives ranged from 45 to 69 minutes, with an overall average of 52 minutes of total underwater time. The length of the daytime surface intervals ranged from 1:20 to 2:35, with an average of 1 hour 53 minutes. As many of you reading this will know, these dives are quite typical of what many divers complete at Truk Lagoon (and some other dive destinations).

Symptoms: Lethargy, headache, the diver was talking in riddles and was disoriented. Neurological tests were conducted, which the diver failed. Fortunately the DAN Diving Emergency Service (DES) Hotline was promptly called. In addition, the diver was quickly put on oxygen, which he breathed throughout the night.
Treatment/Outcome: Following two chamber treatments (Table 6 followed the next day by a Table 5) the diver made a full recovery.

A Word of Caution
Divers need to be aware that deeper diving is associated with a higher risk of DCI and the resulting DCI is more likely to affect the nervous system. Very long dives also carry a higher risk of DCI.

Extended Surface Intervals
Bubbles can often be found in a diver’s venous blood after surfacing from a dive and this is common after recreational dives. These bubbles are usually detectable by an ultrasound probe (Doppler) within the first hour after diving and can continue to appear in peak numbers for several hours. Therefore, in most circumstances, it is a good idea to extend surface intervals as long as possible between dives. Although it is often convenient and relatively common on dive boats to have surface intervals of around one hour or so, it is probably safer to extend these to at least two–three hours, preferably longer, especially after deeper dives.

So, what does this all mean?
In reality, relatively few divers suffer DCI given the enormous amount of diving that takes place around the world, but it is wise to balance the benefits of the extra depth, dive times and dives against the potential problems that can arise, and make an informed choice about the level of risk you are willing to take.