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 Marc Blackwood on January 21, 2009 at 3:02pm

You're missing that for the deeper dives, decompression gases aren't substituted. Rather, more are added.

The lower FO2 gases you are questioning with are breathed deeper during the decompression schedule, in addition to the 50% from 70-20 and Oxygen from 20 up.

edit: I think I may have missed something in your question. Are you asking why 50% for 150 rather than 100% for 150? Another concept with UTD is 'rock bottom,' which is a name given to the minimum amount of gas each diver in the team must reserve in order to complete dive (surface) while sharing with another team mate. It accounts for one complete loss of gas failure. If you lose your backgas, you and I can get to the first decompression bottle on my backgas. 50% as opposed to 100% allows for the PO2/MOD limited gas switch to occur deeper, and thus reduces the amount of backgas we must carry in reserve.

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Permalink Reply by Josef on January 21, 2009 at 3:12pm

So say on a dive to 130 feet (UTD Technical 1) I would use oxygen for a decompression gas. I understand that. On a dive to 160 feet (UTD Technical 2) I would use 50% for decompression from 70-20 feet (70 feet because that is 1.6 PPO2?) then oxygen from 20 feet to the surface (20 feet because that is 1.6 PPO2)? Is the oxygen the decompression bottle and the 50% the stage bottle?

Why wouldn't you just got to 20 feet and use oxygen (100% oxygen, 0% nitrogen)? No ongassing at all. Or at least 80% with very little nitrogen in it, and some safety buffer.

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Permalink Reply by Marc Blackwood on January 21, 2009 at 3:43pm

I may be overstepping my bounds as a student member, but:

So say on a dive to 130 feet (UTD Technical 1) I would use oxygen for a decompression gas. I understand that.

Yep.

On a dive to 160 feet (UTD Technical 2) I would use 50% for decompression from 70-20 feet (70 feet because that is 1.6 PPO2?) then oxygen from 20 feet to the surface (20 feet because that is 1.6 PPO2)?

Multiple decompression bottles aren't introduced until UTD Trimix 1 (the step after Tech 2). As taught, you'd be on 50% from 70' (which yes, is almost a 1.6 PO2) all the way to the surface.

If you were trained with two deco bottles, you could go to Oxygen at 20' (again yes, because of the 1.6 PO2) and get a cleaner decompression profile, but the Ratio Deco schedule will be the same because it has been figured for the single deco gas (50%).

Is the oxygen the decompression bottle and the 50% the stage bottle?

Those are both deco bottles. A stage bottle contains your bottom mix and allows you to extend your time at depth further than your backgas limitations.

Why wouldn't you just got to 20 feet and use oxygen (100% oxygen, 0% nitrogen)? No ongassing at all. Or at least 80% with very little nitrogen in it, and some safety buffer.

For Rock Bottom. You've probably heard the term DIR, and may have read that it's an holistic approach to diving, and that you thus can't pick and choose individual parts of it.

Looking ONLY at the decompression, 100% from 20' up is possibly better for a 150' dive than 50% from 70' up. But looking ONLY at the decompression is inadequate. There are other considerations (e.g. gas reserves).

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Permalink Reply by Guy Shockey on January 21, 2009 at 4:03pm

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 Douglas Steding on January 21, 2009 at 4:07pm

Josef said:

Why wouldn't you just got to 20 feet and use oxygen (100% oxygen, 0% nitrogen)? No ongassing at all. Or at least 80% with very little nitrogen in it, and some safety buffer.

Remember, that ongassing/offgassing is a function of the pressure gradient for an individual gas in your mix and in your body, and it can also be a function of the pressure differential between your body and ambient.

Breathing 50% not only accomplishes the reduction of rock bottom Marc Blackwood mentions, it also allows you to offgas helium deeper in your profile:

Switching to EAN50 at 70 feet would result in you maximizing the helium gradient (0% helium in your mix) and allow you to offgas helium at depth.

At the same time, you've opened the oxygen window and maximized your PPO2, which in theory allows a vacancy to be created in your blood as oxygen is metabolized, allowing more offgassing. But this seems voodoo to me and I'll really believe it when I see it . . .

Then, as you come up, still breathing that EAN50, you've gotten rid of a fair amount of helium and are using pressure gradient to offgas nitrogen, i.e., you continue to depressurize yourself at a controlled rate and allow that depressurization to occur in a manner that the nitrogen bubbles stay small . . .

Make sense?

So, why not switch to O2 at 20 feet? The answer is that even at the Tech 2 level you are a baby tech diver. Handling 2 deco bottles introduces risk, and for those dives (20 minutes, 150 feet) the risk of switching to O2 at 70 feet (amplified by the fact you are a newbie tech diver) is not worth the additional advantage from breathing O2 at 20 feet (i.e., you are dealing with ~20 minutes of total deco, and switching to O2 may shave perhaps 5 minutes off that time).

Since the risk outweighs the reward (O2 tox at 70 because you are a new diver and f-d up the gas switch) O2 as a second deco gas isn't introduced until Trimix 1 (where there is significant benefit in using O2 in terms of reducing total deco time).

This clarify things?

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Permalink Reply by Guy Shockey on January 21, 2009 at 4:30pm

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 Josef on January 21, 2009 at 5:28pm

Thank you for the replies.

I am not a troll, just looking for answers. I am not trying to internet tech dive or learn. I am trying to gather information on the different methods of technical diving and wasn't sure why UTD was doing what they do so I thought I would ask to help me understand my choices.

Marc Blackwood. Your first response you said on an 150' dive you use 50% nitrox decompression from 70-20 feet then you use oxygen decompression for 20 feet and shallower. Then Douglas said you only use one decompression gas until trimix. Is a 160 foot dive a technical 2 dive or a trimix dive?

Guy Shockey. The instructor I spoke to does not use 80% and oxygen for decompression gasses on the same dive. He said he usually uses one or the other but mostly 80%. I'm sorry I wasn't clearer.

Douglas. Thank you for your response. Your explanation about getting rid of the helium makes sense to me.

Thank you all for your responses. They are helpful.

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Permalink Reply by Brian Wiederspan on January 21, 2009 at 6:14pm

Not to nitpick, but just to make sure we're comparing apples to apples, UTD Tech 1 is 130' using 25/25 and O2 for deco. UTD Tech 2 is 160' using 21/35 and 50% for deco.

Josef, you've been given some great answers so far. Make sure you continue your research and ask as many questions as you can.

As Marc mentioned in his reply to you, what UTD teaches is a holistic system and rarely does one component serve a single purpose. In the case of using 50% for deco gas, not only does it address the faster tissues deeper, it also helps your Rock Bottom, as you now only have to travel to 70', instead of 20'.

Please feel free to ask any other questions you have. There's a lot of misinformation out there, and rarely is something really what it appears to be at the surface.

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Permalink Reply by Marc Blackwood on January 21, 2009 at 7:11pm

Josef said:

Thank you for the replies.
Marc Blackwood. Your first response you said on an 150' dive you use 50% nitrox decompression from 70-20 feet then you use oxygen decompression for 20 feet and shallower. Then Douglas said you only use one decompression gas until trimix. Is a 160 foot dive a technical 2 dive or a trimix dive?
.

I brought up 50% + O2 when I mistakenly thought you were referring to dives deeper than the 150 foot range. I thought you were asking why UTD standard deco gases include those with less than 50% Oxygen. Ignore everything in my first reply before the word edit, as it doesn't pertain to your question.

There is no difference in deco methodology between 150 and 160. Same gases, same shaping, etc.. Only change is decompression time.

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Permalink Reply by Douglas Steding on January 21, 2009 at 8:41pm

Dude, I've been looking all over for 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 Lynne Flaherty on January 23, 2009 at 6:16pm

Hey, Doug, have you got a copy of Deco for Divers yet? It's got the absolutely clearest explanation of oxygen window in it that I have ever read. I used to think it sounded like nonsense, but in fact, it doesn't, in any of the three ways in which the term is used, and which Mark Powell discusses.