extending a recreational dive into short deco?

[tursiops]

So you are saying the difference between OK and not OK and the OP's scenarios is whether or not the whole thing is planned. Is that correct?

Do you agree?

 

[rjack321]

Once you are deco trained you'll realize that just staying a bit longer is a bad habit you can't be getting into. You bring enough gas to do your planned dives. On a rec dive you wouldn't have deco gas for instance and your reserves would be fairly small since you have no required stops. Now at the very end of that plan, with some modest narcosis, you decide you 1) know you don't have a deco gas 2) know your new reserves to do the ascent with backgas deco 3) overstay and turn your recreational dive into a deco dive. Where's the plan and was anyone there to check your math?

 

[clownfishsydney]

With the people I normally dive with, this is an accepted situation. We do not necessarily discuss, but it is something we do if the conditions are very good. If we are diving with someone who may not have dived much with us, then we will advise them of this beforehand. It only ever happens at a couple of spots in any case. This is off my own boat, so no problems with contravening boat's rules!

 

[boulderjohn]

Do you agree?

It appears that way to me.

 

[kensuf]

I kinda see more along the lines of where you are going with this. Lets say you are trained for deco and you got doubles and all that good stuff. You have planned to go to the boiler room of a ship and you have YOUR plan. Your friends are more rec and they are just going to the bridge of the ship and they will stay within their NDL. They have their plan. You descend with them and go with them to the bridge to spend time with them and "hang out for a few minutes with them". You then signal or whatever and let them know you are going to head on to the boiler room and you guys wave and will see each other back on the boat. All of THAT is perfectly fine. You both have your own plans, you both did spend time together for a while on the dive and you both did your own thing.

The important thing is all of this needs to be communicated before anyone hits the water. They need to know that at one point in the dive you will wave "buh bye!" and go do your own thing, otherwise you're going to confuse the hell out of them.

 

[kafkaland]

Not too long ago I was on the other end of a scenario like that, and didn't appreciate it much, to say the least:

New buddy, Great Lakes wreck dive to about 120 ft. He had doubles, I was in sidemount. Neither of us had deco training. The plan was to stay within NDL, but do an extended five minute safety stop because we knew it would be close and the water was cold. The dive itself went smoothly, I had about 2 minutes of NDL time left when we started to ascend. When we reached safety stop depth, the big surprise: buddy signaled ten minutes of deco. He might have dipped a little deeper than I had, one percent leaner nitrox blend, a different algorithm, etc. We had plenty of gas, even if one of us lost everything. So doing the extra five minutes is no big deal? Well, at that point my dry suit was leaking and during those long, cold extra five minutes I was cursing my buddy for the deviation from our plan, and an otherwise very nice dive had turned into something less.

 

[doctormike]

He might have dipped a little deeper than I had, one percent leaner nitrox blend, a different algorithm, etc. We had plenty of gas, even if one of us lost everything. So doing the extra five minutes is no big deal? Well, at that point my dry suit was leaking and during those long, cold extra five minutes I was cursing my buddy for the deviation from our plan, and an otherwise very nice dive had turned into something less.

There is something that I don't understand about this. Did your buddy go past his NDL on his computer? If you ascended at the same time, the things that would have given him a deco obligation when you were within NDL (mix, profile or algorithm) would be accounted for in his computer.

So the only way that could have happened would be that he ignored your ascent plan, which was to ascend when NDL reached zero, right? Even if he was diving air when you were on the ideal mix for the depth, and he had a conservative computer when you had a liberal one, the ascent should have happened based on whoever hit zero NDL first.

 

[kafkaland]

There is something that I don't understand about this. Did your buddy go past his NDL on his computer? If you ascended at the same time, the things that would have given him a deco obligation when you were within NDL (mix, profile or algorithm) would be accounted for in his computer.

So the only way that could have happened would be that he ignored your ascent plan, which was to ascend when NDL reached zero, right? Even if he was diving air when you were on the ideal mix for the depth, and he had a conservative computer when you had a liberal one, the ascent should have happened based on whoever hit zero NDL first.

Agreed. I asked him about this after the dive and never got a clear answer. I suspect he wasn't watching his NDL closely and just following me, or somehow didn't want to "cut my dive short" by being the first to call for the ascent. Or he thought me staying longer than his NDL meant I had implicitly changed the dive plan, and he was fine with it. Maybe narcosis clouded his judgment? It was deep and cold enough for that.

 

[AJ]

May I assume from this you had not agreed on max. depth and bottom time before the dive started? In case of two different computers there's no way of telling which one will signal first or even if there is a true deco obligation.

And no, I don't trust computers for this kind of decision making I prefer to calculate on forehand and stick to the plan not the computer.

 

[doctormike]

May I assume from this you had not agreed on max. depth and bottom time before the dive started? In case of two different computers there's no way of telling which one will signal first or even if there is a true deco obligation.

And no, I don't trust computers for this kind of decision making I prefer to calculate on forehand and stick to the plan not the computer.

I don't have a problem with the dive plan "ascend when the first computer hits NDL=0"... what is wrong with that for a recreational dive? You are right that there is no way of telling ahead of time which one will signal first, but as long as you are communicating with your buddy, ascending when the first one hits NDL seems appropriate, and closer to the real time dive parameters than the plan was. I also would assume that the divers are watching their SPGs and that they ascend with enough gas to make a safe ascent, which kafkaland had. It's pretty rare to dive a true square profile, so the computer always is going to have more accurate data about what you actually did. If you just agree on max depth, then you are probably missing some dive time that is within the parameters of acceptable risk.

As I said upthread, for a deco dive, I calculate a plan and have it on a slate in case both computers fail. And when I started tech diving, I would do the slate ascent profile even if the computers cleared long before I was done with the slate. Now I fly the computers with a conservative algorithm, and I'm happy with that.

If you don't trust your computer, why would you trust your bottom timer? What about CCR diving? That is inherently technology dependent - do you consider that inappropriate? At some point, you need to accept the fact that you are doing an activity with some inherent risk that you can never completely eliminate, but you can use gear that has been shown over millions of dives to be reasonably safe in the hands of a well trained diver who understands how it works.