Trimix, o2 %

[dmdoss]

Im reading the book Deep Diving. Kinda lost on a issue. In a trimix blend do you drop the amount of o2 from 21% to lower for a deep dive? IF so this bring me two more question. Is this to stop o2 tox? and does this not create a problem before you get to depth? Im really interested in deep diving, wanting to start this summer. Just trying to read up and study some before i start my classes.

 

[nadwidny]

Im reading the book Deep Diving. Kinda lost on a issue. In a trimix blend do you drop the amount of o2 from 21% to lower for a deep dive? IF so this bring me two more question. Is this to stop o2 tox? and does this not create a problem before you get to depth? Im really interested in deep diving, wanting to start this summer. Just trying to read up and study some before i start my classes.

Yes. Yes. No.

 

[dmdoss]

How low does the o2% have to be before you run into a problem?

 

[karstdvr]

Yes. Yes. No.

Actually the last no can need further clarification. Typically if using a hypoxic trimix and diving a still body of water like a lake or a sink,then you will drop down quickly,and more likely not need a travel gas. In situations where there is a strong current and exertion is going to occur,using a hypoxic mix as your travel gas can be bad. To illustrate a few years ago a guy was diving 10/60,and was using his bottom gas as his travel gas. The current was so strong that he passed out from hypoxia,fortunately some alert people didn't allow him to drown.

 

[Diving Doc]

in theory, the human body can function on 16% oxygen (thats approx the amount of 02 we exhale when we breathe). therefore it is THEORETICALLY safe to dive with this sort of mix. However, i would not want to try this for an extended period myself.
I guess it all comes back to your dive plannin, how long would it take you to reach the max depth etc etc. Personally i'd be inclined to use a travel gas for descent and ascent, but thats me!

this is verry much a personal opinion and im sure theres others out there with a different one, so watch this space.

 

[karstdvr]

this is verry much a personal opinion and im sure theres others out there with a different one, so watch this space.

I agree the opinions are varied,and sometimes this argument gets emotional (imagine that another emotional argument in tech diving). Generally I put all the factors of the dive together and make the best decision. If I am diving deep sink,or a cave like Diepolder,I never use a travel gas because only seconds expire from the time I drop below the surface,drop O2,and head southward,so it really isn't an issue. If I am tagging along with a trimix class where the students will linger at each drop,or there is going to be a lot of exertion involved,then the travel gas is something I use if I have a hypoxic mix,especially below 16%.

 

[pescador775]

Doss, the fraction of oxygen is beside the point. The answer to what you are asking is related to partial pressure of that gas. Thus, a fraction of 16% or even 10% is more than adequate for metabolism (it's safe to breath) at a depth of, say, 2 ata and deeper. With 10% fraction the partial pressure at 35 feet is one atmosphere which is fine. Some of the more knowledgable posters are referring to "travel" gas, eg, a so called "normox" or nitrox used to bridge the gap to deeper depths where a blend with much lower fraction is then safe to breath. The switch is made and the dive continues to target depths. As you alluded, divers who are sensitive to oxygen should keep the maximum PP below 1.4 ata. That concern enters into the decision as to what mix is to be prepared for that dive.

 

[theskull]

The general consensus among my buddies, with which I agree and have tested through experience, is that if you are going to be doing any real swimming at or near the surface you want at least 18% O2 in the mix. So if backgas is 18/30 we don't use a travel gas. If it's 17/50 we'll start the dive breathing our deep deco mix (EAN 36, 40, or 50).

As has already been mentioned, if the dive is going to start with a simple direct descent a travel gas won't be needed even with a 10/50 mix, because by the time you finish exhaling and take your first breath you will be 25 ft. deep or more and the ppo2 will be acceptable.

theskull

 

[Doc Intrepid]

How low does the o2% have to be before you run into a problem?

There is at least one other consideration (other than breathing low O2 gas close to or on the surface).

Your gas plan seeks to take into consideration the potential things that could go wrong, so you plan to reserve an adequate supply of gas to get both yourself and a stricken team mate to the surface in the event of an emergency.

In an emergency respiratory rates can increase...

If I'm diving on a wreck deep enough that I require 10/60 then I'm going to sling an 80 of a deco gas that allows me to use it as a travel gas - aside from other considerations this allows me to conserve bottom mix/backgas so that my gas plan more closely resembles thirds. (Where I dive it is critical that the diver follow the anchor line up - thus you need to get back to the anchor line before surfacing...)

This is highly subjective and other divers will do things differently, but the point is that you do not make decisions only based on potential for issues at the surface on low PPO2 mixes. There are other things to consider as well.

 

[H2Andy]

In a trimix blend do you drop the amount of o2 from 21% to lower for a deep dive?

not necessarily. you can keep O2 at 21% (or higher), but the helium that replaces some of the nitrogen will reduce the narcotic effects of nitrogen, thus reducing nitrogen narcosis.

you can also drop the O2 lower than 21% to increase the depth to which you can dive without worrying about oxygen toxicity.

Is this to stop o2 tox?

yes

and does this not create a problem before you get to depth?

if your mix is too "lean" to breathe at shallower depths, you will take a "travel" mix to get you to the proper depth, then you switch to the hypoxic mix. however, even leaner mixes tend not to be a problem if you will be descending quickly, as you will reach a "safe" depth rather quickly. of course, that's not the case in truly lean mixes for extremely deep diving.

generally, mixes of more than 18% O2 do not require a travel gas

How low does the o2% have to be before you run into a problem?

as pescador775 says, the issue is partial pressure, not percentage of mix, so you have to take into account what a particular percentage of O2 at the surface will provide you, partial pressure-wise, at depth

generally speaking a .18 ATA is considered the lowest partial pressure needed to keep you conscious

1.8 ATA is considered the highest safe partial pressure (by the Navy). other agencies suggest a 1.6 ATA maximum during the working portion of the dive, 1.6 ATA during deco stops

so, you are talking about keeping your O2 mixes at a partial pressure of between .18 ATA and 1.8 ATA for safe diving (keep in mind, the deeper you go, the higher your partial pressure will get)