50% for decomrpession?

I'm hoping to go tech soon and I've been doing some research on technical diving. One thing that I see UTD does is use 50% and 100% Oxygene for technical dives. According to you're standard gasses, you use 50% oxygen for decompression on dives to 160'. My question is why in the world would you decrease the amount of oxygen in your decompression gas as you go deeper?

It seems to me that the deeper you go, the higher amount of oxygen you want in your decompression mix, with 100% be ideal. One instructor I'm talking to about maybe taking technical training with says he uses 100% and 80%. He usually uses 80% in case surface conditions are bad, and its a little more forgiving than oxygen. I get that, and that makes sense to me, allowing a little bit of safety cushion.

But it really seems like you're going to still be on gassing with only 50% oxygen in your deco mix. Is the other 50% nitrogen or helium?

Is there something I'm missing?

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Permalink Reply by John Walker on January 25, 2009 at 10:34am

Guy, can you share that video ?

Guy Shockey said:

Heh Doug, I have actual video of the 02 window at work. It looks like tiny little people carrying tiny little buckets of 02 in the door and carrying tiny buckets of inert gas out the door. Of course, you can only see the tiny little people after about 15 cervezas....... otherwise it looks a lot like video of a microscope slide..........:-)

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Permalink Reply by Martin Chaffey on June 13, 2009 at 3:36am

Well said...
Guy Shockey said:

Josef, I am going to assume you are not a troll and your questions are legitimate.

1) For starters, run, don't walk to get a new instructor. If someone is telling you they use 80% and 100% for deco gases on the same dive, the only tech dive they should do is to the bottom of the pond on a golf course to collect golf balls.

2) If someone is unable to manage buoyancy at 20' with 02, they shouldn't be doing any technical diving at all, let alone teaching it. The argument about 80% 02 versus 100% 02 has been beaten to death. The concept of a "buffer" has been perpetuated by divers who can not maintain buoyancy and are attempting to compensate for lack of skill. If you can't do the skills, don't do the dive. This isn't PADI open water anymore. Get the training, develop the skills, practise, and use the right tools for the job. There are no short cuts in Technical diving or some widow somewhere is going to get a FTD florag bouquet in the mail one day. 02 comes in large bottles with about 2200 psi. Tank an Al 80 and fill it to 2200 psi with that same 02 (partial pressure fill) Now fill that same bottle to 3000 psi by topping it with air. What do you have? Do you think that this was coincidence?

3) Using lower 02 percentages at deeper depths effectively raises the pp02's of that gas. (google "Dalton's law") Using these PP02's at deeper depths also helps with removal of bubbles from your body. (google "Weinke") These two principles apply to all standard deco mixes used at their appropriate working pp02's. Doing a dive to 160" and waiting until 20' for any meaningful accelerated deco is an approach referred to as "bend em and mend em" In this case, you would have a lot of "mending" to do and I think the results would not be pleasant. (google "Buhlmann")

Josef, outside of a formal class or presentation, the internet is not the place to learn decompression theory and practise. Sign up for the RD class via this website then arrange to take a class from an instructor who knows what they are doing (see instructor list on this website). If you value your life as much as I value mine and your wife hates flowers as much as mine does, get the proper education and training and enjoy the very cool things you can do as a technical diver.

Guy

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Permalink Reply by Toine Peeters on December 8, 2009 at 9:04am

Decompression Gas Choice
Why we do not use 80/20 - Adopted from The Bakers Dozen
http://www.gasdiving.co.uk/pages/misc/kit/gaschoice.htm

1. [PDF]
British Sub-Aqua Club
File Format: PDF/Adobe Acrobat - Quick View
using BSAC 88 tables. The recommendation for using Nitrox 80 as a decompression gas was not in opposition to using 100% oxygen but more to take advantage of ...
www.bsac.com/core/core.../download.asp?id...Use+of+100%25+Oxygen

http://ftp.rta.nato.int/public//PubFullText/RTO/MP/RTO-MP-062///MP-...

Teaching Oxygen Tracking Article 95-2
http://www.iantd.com/articles/95-2gurr.html

Why IANTD Uses EANx80 article
http://www.subaqua.co.uk/Why_IANTD_Uses_EANx80_article

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Permalink Reply by Sergi Perez on December 10, 2009 at 1:57pm

I would like to add another conception about deco with 50%, and is the minimum gas you need to share gas with other diver ascending from the bottom, until you arrive to next available gas source, is lees if you plan to start deco with 50% than 100% oxigen (21m vs. 6m deep).

Be carefull with the technical instructors that can not give you a consistent explanation.

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Permalink Reply by Arno Pijnappels on June 16, 2010 at 4:22am

Of course this was mentioned above earlier. This was refferred to by the term rock-bottom.

Sergi Perez said:

I would like to add another conception about deco with 50%, and is the minimum gas you need to share gas with other diver ascending from the bottom, until you arrive to next available gas source, is lees if you plan to start deco with 50% than 100% oxigen (21m vs. 6m deep).

Be carefull with the technical instructors that can not give you a consistent explanation.