Last April I had the bucket list opportunity to visit Truk Lagoon with Mike Gerken of Evolution Underwater Imaging aboard MV Truk Odyssey.

The Odyssey is extremely well set up for tech divers, with 21/36 and 10/50 trimix on tap, and 50% and 100% options for decompression. While oxygen and trimix are extra, nitrox 30% is included for free and is a great option for the majority of the popular wrecks. Deco cylinders are the ubiquitous AL40s or AL80s.

When I went, I opted to do the back-to-back Rec-Tec trip, which gives you 4 or 5 dives per day in the first week and two per day in the second week.

Naturally I wanted to do as many dives as possible while maximizing my bottom time, which meant up to 4 decompression dives per day for the first week and two decompression dives per day in the second week.

In the cold waters of British Columbia, doing more than two dives of any substantial duration per day, especially decompression dives, feels like a big effort. Doing four decompression dives seemed to be a lot of effort but turned out to be perfectly doable given the perfect ocean conditions.

Before I get any further, I wanted to mention that Odyssey’s policy on DCS is that it’s totally up to you how to handle yourself. There is some information on board how to deal with mild cases (e.g. skin DCS) with some suggestions on how long to wait before going back in the water. Suitably warned, we all agreed to be conservative in our approach. It’s a long way to travel to have to sit out dives or worse, end up in a chamber.

Our approach was the following:

• No less than 1000 psi rock bottom (rock bottom is an easy calculation as the majority of wrecks are no deeper than 200′) for everyone on the team who was either on AL80 doubles or the same in sidemount. No gas matching worries.
• At either rock bottom or TTS (time to surface) of 30 minutes, or other agreed-upon TTS, the dive is called and the ascent begins
• Team gas switches at 70′ to ensure consistency and cross-check
• Thereafter, each team member runs their own deco profile and gas switching to 100%, following their computer
• Conservatism set to Bühlmann ZHL-16C GF 55/75
• For extra conservatism, I also taught them the Surface GF concept, recommending we stay longer if possible to get SurfGF below 75, and we often exited in the 60s
• I set my Shearwaters to display slightly different information on the main displays. This allowed me to not have to scroll through the information but I still designated one as my primary
• Information displayed included the ceiling (CEIL) which helped me to understand the amount of leeway in my stop depth. For example, the hang bar was at 15′ on the Odyssey, so I could figure out when it was safe to move from the 20′ blue water deco stop to the hang bar, even if the 10′ stop was not yet available. Similarly, I could use it to move up the sides of the hang bar to an intermediate depth, like 12′, to avoid the jam of divers at 15′
• I used air integration on my Perdixes, feeding both computers from one sensor and with backup analogue pressure gauge on my left post regulator

I’m glad to say nobody had any DCS problems. The TTS diving approach is an excellent way to handle situations like this for sites we weren’t familiar with and with variable profiles. There are additional safety elements that should be mentioned that come with the territory:

• We were a team of five including the guide. Two of our team were sidemount, the rest on backmount doubles
• Pre-dive planning, both by the Odyssey’s detailed site briefing, but also within our dive team and dive guide
• The Odyssey typically ties off just off the wreck so is accessible and visible from depth. It slings an emergency air supply at the hang bar. And of course, any support or extra gas is easy to obtain
• The low current and excellent visibility in the lagoon makes it easy to conduct blue water deco in a low-stress manner
• Warm water temperatures and minimal exposure protection make it easy to do things like clip and rotate cylinders, especially for those trained with bulky gloves and dry suit in cold water
• Each of us had two computers. I brought my trusty Shearwater Perdix 2 and Perdix AI and I believe the others were also Shearwater users
• Post-dive discussion. Even with us not knowing each other before the trip, we gelled quickly and talked about improvements
• General low-stress of a liveaboard such as the elevator, assistance with gear, and low physical demands levels post-dive (air conditioned environment, shade, hydration, relaxation) is likely protective as well.

Our dives tended to cluster around the 60 – 75 minute duration as we were bottom-gas limited to the two AL80s and carrying two deco cylinders (though I did stage extra gas a few times). All dives did not require any deeper stops than our switch to 50% at 70 feet.

I did run 21/36 for several dives, as did some team members, but most opted to dive deep air briefly down to the sand at 200′, which I’m glad to say did not cause anybody to combust, trigger the scuba police, or cause convulsions (tongue firmly in cheek). Narcosis is present but subjectively similar to that experienced 30-40′ shallower in cold water.

This is the log display of one of the dives using 21/29 Trimix with both 50% and 100% O2. It is a fairly square profile dive due to the size of the wreck.

References:

https://shearwater.com/blogs/community/evolution-of-dive-planning