Increasing deco profile conservatism

[Spazzy]

Hi,

Just wondering how people increase the conservatism of their deco profiles?

The reason I ask is that I was diving in the South China Sea on the Repulse and HMS Prince of Wales earlier this year. We were doing a few days of open circuit normoxic trimix dives in the 60m range. As we were a long way from a chamber we collectively decided to increase conservatism by adding an extra 10 or 20 minutes onto our last stop.

Thinking about it seems there would be several other ways to add conservatism. We were planning on V-planner so we could have upped the conservatism there. And I was diving VR3s so I could have set the gradient factor to provide for a more conservative profile. Or I guess we could have arbitrarily added time to the last 2, 3 or 4 stops.....

So I'd be grateful if people would share how they add conservatism and provide a rationale for why that is the best approach please - I know there is no right and wrong in all this and would be grateful if people would tell me 'why' so I can think things through and decide what I will do next time I'm diving a long way from a chamber.

And any good references etc. would be much appreciated.

Thanks!

 

[Kevrumbo]

For my dives on POW/Repulse 2006, I used Ratio Deco which I consider conservative enough to begin with; HMS Repulse for a bottom time of 20min at 51m ave depth --roughly 38min of deco including deep stops; and for HMS Prince of Wales for 15min at 63m ave depth --approx 44min of deco with deep stops. Ratio Deco as implemented at the time was similar to GF 30/85 and a V-planner conservatism of +2, but with different profile curve shaping (i.e. S-curves). Bottom mix was 21/35 and 18/45 respectively, with Eanx50 and Oxygen deco.

If I wanted to increase conservatism because of physical exertion (South China Sea current at depth & deco stops was fierce, wasn't it?:buggy: ), I'd add more time at the 21m/Eanx50 deco stop to a max of 6min in this instance, to make sure my fast tissues (neural tissue types) were clean.

Another option worth considering for conservatism --especially for the deeper Battleship Prince of Wales-- is instead of using Eanx50 at 21m, use trimix 50/25/25 for a lower fraction of inspired Nitrogen (i.e. 25% instead of the 50% in Eanx50), to help steepen the N2 tissue off-gassing gradient. Then switch to O2 at 6m to finally clean out both Helium and Nitrogen inerts for the prescribed time before beginning ascent to surface at a safe & slow 1m/min rate. . .

 

[waterpirate]

It is my understanding that once you break the 150fsw mark on trimix, the easieast way to add safety is to include helium in your deco gas for the reason stated above, and keep a handle on how you feel.
Eric