Thoughts on Bounce Dives

[Rhone Man]

I think i know the reason

You may have misunderstood the thrust of the comment - I meant "why that time, and not any of the other dozens of times when he had dived with a similar ppO2?" There are random variables in the system that we just can't account for.

 

[John C. Ratliff]

I teach adv. nitrox, up to 42m on air as backgas. I am allowed to teach the technical diver course, that is 51m depth on air. But I don't teach that, I teach then normoxic trimix, to 60m/200 ft depth. As I am a human too, I will get narced too on such depths on air. And I am responsable for my students and myself. So that is why I don't teach deeper air than the 42m in the adv. nitrox course.
The psai level 6 course was offered to me, but I refused. My instructor has to dive trimix then, to conduct the dive safe. That is written in standards he said. I don't like to get narced on such depths where I cannot help myself anymore and an instructor needs trimix in case of emergency to help me.

When I was a sportsdiver, I have done bounces to 55m on single tank too. The problem was that I never mentioned narcoses, it did not happen to me I thought. But since I dive trimix I know I was narced too. And that is the danger.
The PO2 on a bounce to 1.6 is not where I am afraid for, it is a bounce, so just a short period on higher PO2. But being narced, that can be a problem. And most sportsdivers never feeled being narced because they don't know what to feel.

As CO2 is a contributing factor for getting narced. The breathing resistance of a regulator on air to 200 ft depth is really higher than diving on a 20/30 trimix. Calculate the eadd (equivalent air density depth), that is 42m on a 20/30 trimix on 200ft depth. That is a big difference from an eadd of 60m when diving air. When using more helium, the eadd will lower more (and the end of course). So trimix has not only the advantage of lowering the end, but the breathing resistance of your regulator too. And when diving trimix in a twinset, and take 1 or 2 decogases, you have time to stay on 200ft depth to see something

Then another problem of getting narced: the faster you go down, the more sensitive most people are for getting narced. This has to do with the partial pressure difference of the nitrogen you are breathing and the nitrogen in your body.

So yes, I dive to 200ft regularly, to 300-400+ ft sometimes (I don't have places to reach that depth in my backyard), but I do that on a twinset with decogases or I take a rebreather. 200ft is a depth where you need extra training for, on a single tank you never have enough gasreserves in a worst case scenario. Doing decodives is not directly dangerous, but you have to know what you are doing.

I'm curious, when you dive to these depths, do you have a recompression chamber immediately available? If not, and something happens, have you set up emergency procedures for quick tzransport to a chamber? If not, what is your backup plan?

SeaRat