How much risk was I at for DCS?

[supercritical]

Back in December, I was making my 15th dive in Barracuda Lake, Coron. The divemaster took me down to 26.4 metres depth; twice my certification. At the end of the dive, I determined that my pressure group on the PADI RDP (30m 40 min) was way past the NDL (the nearest thing we did to a decompression stop was 3 min at 5 m.)
Was I at much risk of DCS? His computer said we were in a safe pressure group (he claimed.)


If this is in the wrong forum, please let me know immediately.

 

[DevonDiver]

If the divemaster were truthful, then there was probably a low risk of DCI. His dive computer would have tracked the dive in real-time and credited any multi-level (shallower) phases of the dive for lower nitrogen absorption. In contrast, your tables track the dive as a 'square profile', assuming your entire dive was spent at the maximum depth.... which, I assume, it wasn't. The dive computer would have given much longer bottom-times, compared to the RDP, assuming that you got progressively shallower as the dive progressed.

The reason you're having to ask this question is because you permitted the dive guide to coax you into what we call a 'trust me' dive. You abdicated responsibility for 'planning your dive, diving your plan' to the divemaster and his computer. That, quite naturally, causes you concern and worry. If something unexpected had happened on that dive, preventing the dive guide from 'shepherding' you, then you may have been beyond your own capacity to resolve it... a dangerous circumstance to allow yourself to enter.

What you learned on your Open Water course should always apply:

1. Plan your dive and dive your plan.
2. Dive within the limits of your comfort, training and experience.
3. Do not share dive computers.

 

[Kevrumbo]

At 30m for 40min, I would have an expected gas consumption of 120bar out of 200bar total of a twinset of 11L/bar cylinders filled with EANx 32, with about 7min of Oxygen Deco at 6m.

IOW, this was clearly not a very well planned dive, and way beyond your current level of Scuba training; glad you came through ok!

 

[CamC]

As DevonDiver says, if the profile wasn't too square then you probably weren't at much risk of DCS. But there are other risks at that depth - for example, did your DM mention narcosis?

 

[supercritical]

@DevonDiver: If I understand what you're saying about the divemaster right, I'm afraid that this practice is awfully common where I am: Listen to the briefing, put on your gear, step into the blue, do your dive, step out of the blue, and log your dive according to the divemaster's computer. Sometimes he tells you maximum depth, in my case he didn't. I haven't seen anyone bring a computer (besides the divemaster) on dives yet. Usually these are on shallow dives not exceeding 15 metres depth for not more than an hour and complete with safety stop, so there isn't much trouble. 26 metres on the other hand...
I believe that the reason I didn't ask him to ascend immediately was the 'trust him' attitude, as you said. At the time, I was also foolishly imagining how surprised my instructor would be.

@CamC: He did say he would take me deeper if my breathing and buoyancy was good, but I assumed (wrongly) that he'd read my waiver. He didn't mention narcosis, but when I saw the depth reading on his computer at 23.4 metres, I began recalling what I'd read on Wikipedia on the subject. I don't think we spent very much time at that depth anyway, so DCS risk was low. Some time afterwards however, I had a pain in my right knee whenever I applied force on it.

 

[Peter Guy]

OP -- do you have any information about this dive other than what you provided? For a rough guestimate of your exposure, it is often more important to know your time-weighted average depth (either your bottom timer/computer will give you that or you can do a pretty accurate estimate by averaging your depth at 5 minute intervals). If, for example, your maximum depth was 26 meters but your time-weighted average depth was only 15 meters, your nitrogen loading would be significantly less than shown on the RDP. (If my estimates are accurate, the RDP NDL limit for 26 meters is 20 minutes whereas the NDL limit for 15 meters is 80 minutes.)

Your dive profile is what is most important for DCS purposes, not your maximum depth.

BTW, the above advice may well be worth exactly what you've paid for it!

 

[supercritical]

I'll reply properly tomorrow. For some reason my reply didn't show.
[Consider this deleted. My reply showed.]

 

[Tigerman]

At 30m for 40min, I would have an expected gas consumption of 120bar out of 200bar total of a twinset of 11L/bar cylinders filled with EANx 32, with about 7min of Oxygen Deco at 6m.

IOW, this was clearly not a very well planned dive, and way beyond your current level of Scuba training; glad you came through ok!

IOW 2640 liters, ~66 liters/minute - wich would be at 4ata, so about 16,5 liters/min (0.586 cuft/min SAC) wich is quite normal for an experienced diver. However the only thing deduced from that is that the dive the OP did obviously was not at 30 meters for the entire dive as hed be using, even with your SAC 440 liters more air than is in his tank..
(Assuming you meant your expected use was not including the going down and back up part of the dive)


I have no reason to believe a DM would lie to you about the NDL left on the computer so the risk the OP was exposed to with regards to DCS where probably minimal, but as others has mentioned, theres other issues with a dive like that..

 

[Jim Lapenta]

I would expect a DM who would do the things this one did to lie. And not trust them as far as I could throw the whole damn boat. Forget the DM's computer. If you can't afford one at this time get a dive watch, a depth guage, and use your dive tables. Trust me dives have the very real potential to kill people. I know of one DM led dive that did for sure and have proof that it did.

 

[Tigerman]

I would expect a DM who would do the things this one did to lie. And not trust them as far as I could throw the whole damn boat. Forget the DM's computer. If you can't afford one at this time get a dive watch, a depth guage, and use your dive tables. Trust me dives have the very real potential to kill people. I know of one DM led dive that did for sure and have proof that it did.

I would suspect a DM like that to do many not-so-smart things, but I dont know a lot of people who would blow off their deco obligations "just for fun" and then lie about having them afterwards. Ive understood from others that being bent (I havent tried this and don't intend to) is no fun and blowing deco obligations would put you at a significant risk of it. I dont think even DM's doing stupid things would want to ignore their own health. Although there is of course some darwin award candidates around..