DCI and the perils of diving in a mixed EAN/Air Group

[Outbound]

Takeaways - Be very careful about repetitive diving in a mixed air group where you are the only air breather and there is no other check on the performance of your dive computer. Request that the dive guide/leader uses the same gas mix as the most restrictive person in the group (might get some blowback on that). If you have to be the 'tail end charlie' set the P value on your computer to the most conservative value and don't repetitively dive close to limits . If you have more than one computer decide on actions in the event of alarms etc before you get in the water. Get EAN/Nitrox qualified so you always have the option.

Safe Diving. Alan

I'm glad that it was only a minor hit. But I fail to see why a takeaway is that you should be careful about repetitive diving in a mix gas group. I also fail to see why you guide should use air if there is a person using air and the rest are on nitrox.

Apologies if someone else said it first, but the main issue is that you rode your NDL too closely and ended up taking a hit for it. You should have buddied up with someone and explained that you are on air and that you would be the limiting factor in terms of NDL. Then you should have added more conservatism into your dive plan given the fact that you already had two deeper dives that day. I don't know anything about your physical condition, age, etc., but that should be a factor as well in your planning. Since you had a second computer blaring at you to do a deco stop, you most definitely should have done that. But even without that backup computer, when you hit 10 mins of NDL remaining, you could have signaled to your buddy to shallow up and continue diving. You would have been off-gassing and basically doing a on-the-fly deco stop.

I've gone diving on nitrox with people on air, and vice-versa. I've also been on nitrox that was more or less rich than my buddy's. You monitor your own NDL, air time, depth, etc. and signal to your buddy when you have to make alterations in order to accommodate those metrics.

Definitely get nitrox certified, but don't make the mistake in the future of riding your NDL then either. That doesn't make you any safer.

 

[kafkaland]

Note: you have to have your Advanced Nitrox card to get 50%. Standard Nitrox card only gets you up to 40%. So it’s mostly tech trained folks who are going to have this option, unless they have “under the counter” access to O2.

And I know a number of tech divers with access to O2 who will use 40% for that purpose, instead of 50%, because you don’t have to maintain that tank and reg as O2 clean. The difference in deco time between 40% and 50% is almost negligible, but the maintenance is not.

 

[davehicks]

The real issue with this incident is that the OP was the singleton air diver in the group. As a result he pushed his profiles to the edge of his NDL's on each dive, while the nitrox group likely had more buffer on similar profiles. His reluctance to do a longer safety (deco) stop was caused by not wanting to stand out from the crowd.

• The Air vs Nitrox choice is moot if you dive your own profile, don't follow the herd.
• Clearly the more conservative backup computer is a better match for the OP, I'd use that as your primary going forward.
• Pad your safety stops and be the last one out of the water.

 

[fsardone]

@AlanS

If your profile is correct, how can you have so many dives and yet not be Nitrox certified?

@Marie13
I do not count dives but I had been diving since 1980, uncertified up to 1994 the OW and AOW until 2003 probably a 1000+ dives all on air, then I did intro to tech e tech deep air, diving on air until 2013 few hundred dives more, still on air ... so yes you can have several hundreds dives and not used Ean. And then do an/dp .... tmx, oc and ccr.

 

[seeker242]

I was diving virtually to the no-deco limit on every dive .

This doesn't seem like a peril of diving a mixed group, but a peril of maxing no-deco limit on every dive, on a computer with the most aggressive limits. If you did the same thing with nitrox, you could easily end up with the same result.

 

[Saboteur]

Do longer safety stops too. Increase from 3 to 5 and importantly from your stop go to surface as slowly slowly slowly as you can.

I’ve read of the very slow ascent from the safety stop to the surface. How slow is ‘slowly’.

 

[JonG1]

1 minute per m seems to be the accepted norm, post deco so tends to be my default approach (deco/no deco) from 6m up.
In reality you pause at each m interval to catch the time up, I count the seconds to try and keep the run-time but not easy without a stopwatch, which I am going to add to my rig at some point.

 

[doctormike]

This doesn't seem like a peril of diving a mixed group, but a peril of maxing no-deco limit on every dive, on a computer with the most aggressive limits. If you did the same thing with nitrox, you could easily end up with the same result.

Exactly. Well said.

 

[fsardone]

1 minute per m seems to be the accepted norm, post deco so tends to be my default approach (deco/no deco) from 6m up.
In reality you pause at each m interval to catch the time up, I count the seconds to try and keep the run-time but not easy without a stopwatch, which I am going to add to my rig at some point.

Since all dives are decompression dives and diving up to the NDL is considered more dangerous than coming up to staged decompression, in term of supersaturation on surfacing (generally speaking and depending upon GF and decompression schedule), I would definitely recommend to ascend from 6 (or 5) mt to surface at 1 mt/mi. or less. Technique is stop to each mt untill the full minute and then ascend to the next level and stay there until the next full min.
Many dive computer and for sure most of the tech ones can display divetime with seconds. Wait for the xx:00 before ascending to the next mt, then repeat as it was said above.

 

[Mike Walker]

I know I may come across as one of those deco gloom mongers, but maybe sometimes you need to draw a line and perhaps NDL is a good a place as any.

By choosing this as a 'line' with no exceptions it creates a situation where a recreational diver going to 0.001 minutes NDL on their computer is perfectly acceptable but going 1 minute into decompression (and the few minutes that might add to the ascent) is completely verboten. This results in a lot of sheep standing 1" away from the electric fence at the edge of the field because that's where all the tasty grass is. That is - one has created a situation where a lot of people are operating to 100% of their limit even on a 'typical' dive (because they've been told it's a-ok) and have no idea what happens just beyond. What happens to the sheep that one day finds itself on the other side of the electric fence?

By deliberately pushing just a little bit into deco (in the context of a person's particular computer's assessment of the situation) and then safely making ones way back out now the '100% knowledge capacity' threshold is extended. Any dive undertaken within computer NDL limits now becomes a 90% knowledge capacity activity for that diver with a nice smooth grey transition area up to 100% - no longer a 'line'. I would suggest this probably makes each 'typical' dive now inherently safer but, more importantly, if the threshold (which is now at 90% of capacity) is violated by accident that diver is still operating within their knowledge and training. They know they shouldn't be there - but they've been there before and can safely get back.

Lines are only good for the types of people who colour within them.

I should definitely emphasize that my advocating this experimentation is 100% about 'learning you're computer' and not about 'learning deco procedures'. Doing it once - and only once - in friendly conditions (lots of gas, etc.) for a timespan only long enough to make the computer react is really not adding any risk (certainly less than doing it accidentally and unprepared at some point in the future). Ideally this would be integrated into an AOW type course to allow for supervision/guidance - but that might distract from PADI's latest course development efforts on how to best colour match your fins and weight pockets....