High oxygen content question

[SillyQuestions]

Hi all,

Currently working on a project in a UK cave.

8 sumps, deepest depth being 9m (27ft) , average depth being about 4m (12ft)

Sump lengths are between 10m (30ft) and 190m (620ft). Averaging about 60m (200ft).

Now my problem arises that I'm currently diving on air. However, effectively doing 8 short dives in a row to shallow depths, surfacing after each one, then doing the same again on the way out actually racks up a fair amount of deco, as well as being outside what many tables/computers are set out to understand properly.

I don't have much experience with high O2 contents, I definitely have no qualification. But I could get high content fills if I asked. So, say I got an 80% fill, MOD of 30ft working on a ppO2 of 1.6 (which would only be for a short time anyway), planned the dives on vPlanner with 80% and then set my computer to 80%, what dangers am I clearly overlooking?

I've heard serious deco divers mention oxygen breaks during deco, would I be at risk of needing one of these? Why would I need one of these?

If nobody is willing to answer these questions, any chance anybody could direct me to some online/easily accessible book reference that may allow me to research myself?

Thanks

 

[The Chairman]

What PDC are you using that racks up a lot of deco?

 

[Akimbo]

Just to provide perspective, the only decompression tables I am aware of that deal with shallow long duration dives is the US Navy’s Table 9‑9 Air Decompression Table, Page 9-65. This table indicates a 380 minute/6.3 hour dive at 30' requires a 1 minute stop at 20'. Anything shallower is no-decompression. Divers have made saturation dives shallower than 30' for weeks and surfaced without stops or O2.

A 480 minute/8 hour dive to 30' has a 42 minute stop at 20'. See page 9-64, of the US Navy Diving Manual, 15 April 2008, Revision 6. That drops to 9 minutes of decompression on O2. Older Navy Manuals called the table Extreme Exposure and was primarily intended for emergency entrapment situations.

Here is a link to the Manual of about 28 MB in Acrobat/PDF format:

US Navy Diving Manual

 

[SillyQuestions]

It's the constant ascent and descent I think the computer doesn't like. It records them as 16 different dives with very little surface interval.


It's an apeks quantuum, old model.


Sent from my iPhone using Tapatalk

 

[The Chairman]

Maybe you should try an Oceanic?

 

[Akimbo]

Maybe you should try an Oceanic?

Or just a depth gauge and watch. My question is what is your objective for using Nitrox when you are in a no-decompression stop depth range anyway? This may be a case where a computer is more trouble than it is worth.

 

[EFX]

My Mares Puck DC adds a penalty for repetive dives and for dives done on multiple consecutive days. Your DC may be doing this but with such shallow depths I don't understand why it's asking for deco stops. I don't have the experience with this dive profile to tell you for sure. There was a DM who posted she got bent soon after a repetive dive doing rapid ascents and descents chasing runaway divers. In her case she overwhelmed the fast tissues by her rapid ascents. In your case I would think if you keep your ascents very slow (< 30 ft/min) your fast tissues would offgas OK. Going on nitrox will reduce ongassing but the high dive time might put you over on max O2. In that case when you get to 80 or 90% go back to air.

 

[rjack321]

You could spend all day bouncing between 27ft and the surface and not incur any deco. Seriously. There is no deco on this dive no matter how long it is. Check in something like vplanner, even on conservatism level4 an 800 minute dive at 27ft remains a no stop dive.

A bottom timer and tables will "solve" this deco problem.

 

[Mr Carcharodon]

Is this a problem because your ascents are too rapid for your computer's taste? The total dissolved gas for a single shallow dive is not enough to worry about but you can create bubbles with a rapid ascent. Additional rapid ascents make new bubbles and grow the ones that are already there. It is possible to eventually have a problem. RGBM has a penalty for just this sort of thing. I have no idea if that is the algorithm used by APEX. And for that matter it is not clear if to me if this was validated in RGBM so they may not have it quite right. Subjectively I know doing lots of rapid ascents from the shallows is one of the surest ways to feel really bad after a day of diving. So there does seem to be something to the idea. Maybe try cutting your ascent rate to 3 meters/minute and see if your computer is more happy.