Speaking of Oxygen...

[DocRCH]

By using 100% O2 before a dive, you are potentially runnign up your CNS clock for no real benefit. Also, ther is a potential problem depending on how fast you descend that you could be setting yourself up for an O2 "hit". If you descend below 20 fsw, your pO2 would exceed 1.6. You will on-gas N2 as you descend and will not lessen the amount of N2 in your tissues before you ascend.

Robertut:

 

[Rick Murchison]

Originally posted by DocRCH
By using 100% O2 before a dive, you are potentially runnign up your CNS clock for no real benefit. Also, ther is a potential problem depending on how fast you descend that you could be setting yourself up for an O2 "hit". If you descend below 20 fsw, your pO2 would exceed 1.6. You will on-gas N2 as you descend and will not lessen the amount of N2 in your tissues before you ascend.

Robertut:

Whew!
1. Potentially running up your CO2 clock. Yep... 20% per hour. Is that significant? It is if you're going to make a dive that uses up the other 80%. If not, it isn't
2. No real benefit - in the example given in my post earlier, I used a 30 minute pre-ox routine (10% CNS clock) before a half-hour dive to 66 feet on air, and get a 33% decrease in required offgassing for the 30 minute tissue using a super conservative bubble threshold. If a 33% decrease in required offgassing is "no real benefit" then you're right.
3. Your contention that PO2 in the blood would increase with a descent due to what you breathe before the descent, while seeming intuitively true, is without merit. PO2 doesn't increase with rapid descents unless you keep breathing a gas that provides a higher PO2 at the gas/blood interface than the dissolved PO2. As your blood is incompressible, the dissolved PO2 can only be increased by *adding more molecules* - just increasing the surrounding pressure with no more gas available to go into solution doesn't increase the PO2 in a liquid. So your max PO2 after pre-ox would be 1.0 ATM, and that would not increase until the PO2 in the breathing gas rises above it - no matter how fast you descend (for air, that'd be about 130 feet).
4. You will ongas nitrogen. Correct - but by starting with less nitrogen, you end the dive with less nitrogen, and have less nitrogen to offgas. The question is whether it is worthwhile, and whether any useful research has been done on the subject. Rough math indicates it may be of substantial benefit for normal recreational profiles, but I don't know if any experiments have been done or not.
Rick

 

[DocRCH]

the "rule of thumb that" I use is that it takes about 2 min for the gas that you are breahting to begin to replace the previous gas, and really it takes about 4 min. to reach and end point. So, after about 4 to 5 min you are saturating your tissues with the mix you are breathing. If you are in the water for any length of time, you will be diving that profile. I don't believe that breating 100% O2 "buys" you any more time. If you descend to say 60 to 100 feet in 3 min you may be asking for trouble. IMHO.

Robert:doctor:

 

[Rick Murchison]

Originally posted by DocRCH
the "rule of thumb that" I use is that it takes about 2 min for the gas that you are breahting to begin to replace the previous gas, and really it takes about 4 min. to reach and end point. So, after about 4 to 5 min you are saturating your tissues with the mix you are breathing. If you are in the water for any length of time, you will be diving that profile. I don't believe that breating 100% O2 "buys" you any more time. If you descend to say 60 to 100 feet in 3 min you may be asking for trouble. IMHO.

Robert:doctor:

I understand your opinion, but the physics just doesn't support it. If you take a liquid with a gas load [tension] at a given pressure, you can then submit that liquid to any increase in pressure you want to and the dissolved gas tension will not change [without additional gas at a higher pressure available - and in the pre-ox proposal this is not the case, but rather just the opposite, going from an oxygen pressure of 1 ATM to something between .2 and .4 ATM in the breathing gas for the dive]. It just doesn't have any way to get above 1.0 until reaching a depth where the PPO2 is above 1.0 in the breathing gas.
Rick

 

[omar]

Rick,

You are right for any tissue group between 10 to about 90 minutes for this example. I should have used the good old pencil and calculator instead of trying to cheat one of the deco programs.

I still don't think that I would prebreath oxygen for extended periods prior to a dive because of the vasoconstriction and irritation to the lung tissues caused by PPO2's greater than 0.5 atm. However, for a short period it appears to have a benefit. Now I understand why the question about any studies that have been done.

omar

 

[Rick Murchison]

Originally posted by omar
Rick,

You are right for any tissue group between 10 to about 90 minutes for this example. I should have used the good old pencil and calculator instead of trying to cheat one of the deco programs.

I still don't think that I would prebreath oxygen for extended periods prior to a dive because of the vasoconstriction and irritation to the lung tissues caused by PPO2's greater than 0.5 atm. However, for a short period it appears to have a benefit. Now I understand why the question about any studies that have been done.

omar

Thanks Omar,
I thought you'd crunch the numbers too.
Interesting, isn't it?
Rick

 

[WYDT]

I do not recommend anyone try this!!

Any deco that would be worth shortening is going to approach the O2 CNS limits anyway.

With Decoplanner: For example I do a 80min bottom time @ 90ft. with EAN 34. I'll have a deco profile like 1@30 1@40 and 14@20 on O2 that puts my CNS at 80%

I don't think I'd want to mess around with saturating my tissues with O2 predive to save a couple minutes of deco, if it even would!!

O2 hit submerged in water = DEATH!

Why don't you email JJ & George and ask them if they have tried it. They are pushing the limits of deco theory on every dive.

 

[omar]

First off,

This is a poor choice of gases. I would use 30/30 for the bottom mix and 50% for deco for this exposure. The CNS used in the DecoPlanner clock would be considerably lower with these gases. OR even 34% and 50% for deco is a much safer choice. I can tell you from experience that I would much rather use 50% than 100% just from the simple fact that I don’t get the oxygen boogers from 50%.

Next instead of using O2 straight for your deco you can take a “back gas” break, if you are using the right gas, that will in effect reset the O2 clock. If you are familiar with DecoPlanner you know that the CNS clock algorithm that is used is extremely conservative and is cause for the most debate and questions on the Dplan list. I do not use Dplan for the CNS calculation.

Lastly, the question was not about reducing the deco obligation. It was about reducing the risk of DCS. From that aspect the prebreathing of oxygen on the face of it appears to have a positive benefit. This is what brings up the interest in this method. As Rick originally asked is any one aware of studies that have examined this.

omar

 

[WYDT]

Omar, I don't agree with your "Poor choice of Gases" statement!

I don't really think trimix is needed for a 90ft dive. Not even GUE recommends mix above 100ft. You're right about the 50/50 lowering the O2 CNS in decoplanner but I'd rather do the deco with O2 (much more efficient) and since I'm not on O2 for anywhere near 20min nor going near the CNS max a backgas break isn't necessary. In fact using O2 at 20ft and 34% backgas gives fully half the deco time (14min vs 28min) that using Mix and 50/50 does so I'm not sure where you're coming from.

The whole point I was trying to make was that if someone actually tried this little experiment of prebreathing O2 to try to reduce deco OR "the risk of DCS" it would not really help and could be VERY dangerous! "Risk of DCS"?? Don't you think Risk of O2 TOX is much more dangerous??? I do. Maybe that's why we haven't seen any studies on the subject??

DSAO!

 

[omar]

Try it. You will feel a lot better doing mix instead of 34EANx.

Also try setting the GF to 20/90 on DecoPlanner and run both of them. Check out the tissue pressures. With mix and 50% you will be cleaner than the EANx and 100%. Also DecoPlanner does not give you a reasonable set of stops for this dive. I would add a couple of stops from 60 to 30 as well. So I would do a little more deco time and come out even cleaner. I do not try to optimize the dive for short deco but for a clean deco. And, in some cases the deco is longer.

And If the CNS clock gets to such a high level with your gas choice ~80% versus ~48% with the gases I recommended why is it not a better choice? Just because the deco is a little longer? As I said above the tissue PP is lower when you get out with the deco schedule for the mix and 50% versus the 34 and 100%.

You need to select the gas for the dive plan. This is a classic case of not using the proper gas and blindly basing it on what a deco program spits out. (I know, I did it earlier in this thread). In addition this is a very theoretical model for O2 exposure. If it was valid than how would the WKPP be able to get to >1,000% (or more) on their dives?

How do you know that prebreathing oxygen will not reduce the risk of DCS? What study are you referring to? As it is you are just speculating. As Rick pointed out the tissue PP is lower for the dive example used. This is how it could reduce the risk of DCS. And I said that the prebreathing time would need to be balanced to offset the effects to the lungs. These are the interesting points about this.

FYI, The 30/30 is being recommended by GUE for dives up to 120'. Which is considered a moderate dive. And take a look at page 49 of the Fundamentals of Tech Diving by JJ as well for mix diving.

omar