23 replies to this topic

[PerroneFord]

Note: Split off from this thread. - ww

Tech is a catchy phrase. I will use it when I am referring to anything that is outside of the standard limit to open water diving. IE overhead environment by limestone, Steel or deco obligation. Others might use it for gear configurations or mixes. I think of anything from 40% down as Recreational. Anything above this as a more technical mix, for whatever it's intended use. It requires better diver skills and extensive information to use correctly. It is still recreational diving.

I would submit that mixes with less than 21% 02 are technical as well, and also deserve more care.

[peterbj7]

Especially since they almost certainly contain 3 principal gases (ie. ignoring those occupying less than 1% of the whole), and the normal tables/computers don't accommodate this situation.

And of course because such a gas is likely to be unbreathable near the surface, necessitating another gas.

I would classify both of these as "technical", though intime they may come to be deemed "recreational". Nitrox followed that path.

[Capn Jack]

Well, I've read through some deco stuff, and some deep diving stuff, and I think I have a BASIC grasp of partial pressures etc., but would someone help explain in clear terms why less than 21% O2 is needed for deeper diving?

I think a lot of people would be interested, and it would be great for me to have it explained by one of our resident gurus - plus we can start throwing around terms like "hypoxic nitrox" at HH and impress the crowds listening in to our diving discussions!!

[Scubatooth]

mark

it comes down to that mixes less then 21% cause the tissues in the body to go hypoxic when will cause you to loose conciousness within a couple of minutes. Eventually if it goes long enough will cause your heart & lungs to fail because its 02 demand isnt being meant of the body.

for diving the deeper we go the partial pressure of o2 goes up which is also bad but at deeper depths less 02 content would be equal to a lower eqivilant 02 pressure at a shallower depth.

im just coming from what i know based of my medical training so far, as far as diving i havent gotten that far yet so im not sure

[PerroneFord]

OK, in basic terms...

At ocean level we breathe air. Which is comprised of about 21% oxygen. Expressed as .21/1.00. With each Atmosphere we descend, we consume a greater volume of oxygen. So at 2 ata it's .42, at 3 atmospheres its .63, etc. When the fraction of oxygen reaches a certain level, somewhere beyondd 1.6, our bodies tend to have a reaction to oxygen. It is toxic in concentrations much higher than this, depending on length of exposure to it.

Thus when you take Nitrox class, they tell you to limit your exposure to 1.4 ata during the working portion of the dive and 1.6 on decompression. Those numbers are common ones but not absolute. Some people are more sensitive to oxygen and some less so.

Well, what depth do we reach 1.4 ata with air? Answer 192ft
What depth do we reach 1.6 ata with air? Answer 224ft
What depth do we reach 1.4 ata with EAN32? Answer 114ft (this is why nitrox is unsuitable for deep diving)

Of course those who understand narcosis understand that you will probably be narced out of your MIND before you reach 192ft on air. So we have two problems. We will not be thinking clearly, and our oxygen content is becoming dangerous. So what we need is less oxygen so we don't convulse. But that would raise the nitrogen, and make us more narced. So we need magic gas to battle the narcosis. Enter helium, which has far less narcotic value than nitrogen or oxygen.

So, by reducing our oxygen content, we can dive deeper without the risk of oxygen content getting toxic. And by using helium we can reduce the narcotic effect.

So what's a hypoxic mix? Well, when we reduce the percentage of oxygen to 16% or less, we can use that mix quite deep. 16% oxygen doesn't reach 1.4 ata until 262ft and 1.6 at 305, however, at sea level its percentage is .16. Remember air at sea level is .21. Well, .16 is the threshold for when we might lose consciousness because our body is starved for oxygen. So we cannot breathe this mixture on the surface. Thus the mixture is called "hypoxic".

So in deep mixed gas diving, the mixture you are breathing may not be safe to breathe on the surface. This is why when you do buddy checks for divers carrying hypoxic mixes, it's done at depth. Generally 70-120ft. And you descend with a "travel mix" or a mix that is safe to get you from the surface to a depth where your main tanks are breathable.

As a bit of curiosity, the WKPP divers in Wakulla Florida use a mixture of about 10% oxygen and 90% helium. They do buddy checks at 190ft or so. And because the mix has no nitrogen in it at all, the narcosis level is no different than breathing air at sea level.

The only drawback is that each set of tanks they fill is about $80!

Edited by PerroneFord, 10 August 2006 - 09:29 PM.

[ScubaDadMiami]

Well, I've read through some deco stuff, and some deep diving stuff, and I think I have a BASIC grasp of partial pressures etc., but would someone help explain in clear terms why less than 21% O2 is needed for deeper diving?

I think a lot of people would be interested, and it would be great for me to have it explained by one of our resident gurus - plus we can start throwing around terms like "hypoxic nitrox" at HH and impress the crowds listening in to our diving discussions!!

For deeper diving, the concern is not the percentage of oxygen in the mix as much as it is the exposure to the body of oxygen molecules. The deeper you go, the more packed in the molecules get when you breath from the regulator or CCR. This can become poisonous to the body if the concentration gets to be too high for too long. So, the deep diver observes the number of molecules, known as partial pressure, and the time of exposure, in planning dives.

Once you get deep enough, even regular air gets to be poisonous. So, the diver dilutes the amount of oxygen down by adding other gases, that are not poisonous, to the tank. Since nitrogen is a component of air, the diver could add nitrogen to the mix. However, this becomes narcotic with depth. So, nitrogen is not a good option. For this reason, helium is typically used since it is virtually non-narcotic at depths that most divers will dive in the deep range.

As you dilute the oxygen down, the percentage of oxygen drops. If you get it low enough on the surface (the basic accepted low limit is 16 percent), this is not enough for our metabolism. Even lower, and we die from the exposure.

The problem comes when the diver is going to do a very deep dive. In that case, the diver has to put in a very low mix of oxygen in the tank. At depth, with all of the molecules so close together, this will be just fine for breathing. However, at the surface, this will not be enough oxygen to sustain life. So, in these instances, the diver will have to carry more than one tank of gas, breathing one from the surface down to a given depth, and then the diver will switch to the lower mix from that point down. On the way up, the diver switches back to the travel tank from a given depth in order to keep the partial pressure of oxygen high enough to sustain life.

Some example of commonly used open circuit mixes include 18 percent oxygen with 45 percent helium and 37 percent nitrogen (18/45), 15/55, and for pretty darn deep dives, 10/70. There are lots of other commonly accepted offshoots from these mixes.

You learn about this in detail once you get into technical decompression and trimix diving. An interesting class.

[Scubatooth]

perrone good information, no thanks i will keep my $80

ok so that there is something clear hypoxia as defined medically is when a persons hemoglobin saturation rate goes below 95%. it is below this saturation point that the body starts to thave the problems i listed above


96-100% is Normal (hemoglobin only carries 98% of 02, plasma proteins carry the other 2%)
91-95 mild hypoxia (o2 percentages 19.11- 19.95%)
86-90 moderate hypoxia( 02 percentages 18.06 - 18.9%)
<85% severe hypoxia = (o2 percentages 17.85%)

the 02 percentages listed about would be the equvilant in a mix.

FWIW

Tooth

[ScubaDadMiami]

The only drawback is that each set of tanks they fill is about $80!

For a 10/70 mix of 240 cubic feet, it is about $170. You get a grand total of about 25 minutes of bottom time from a mix like this.

Of course, for a CCR tank, this same fill would be just over $20, and you could get about three good dives out of it. The only drawback there is the price of the CCR. But heck, enough deep dives, and you end out saving!

[Scubatooth]

Howard

quit tempting me the CC run and hide everytime you mention that TLA



Tooth

[PerroneFord]

The only drawback is that each set of tanks they fill is about $80!

For a 10/70 mix of 240 cubic feet, it is about $170. You get a grand total of about 25 minutes of bottom time from a mix like this.

Of course, for a CCR tank, this same fill would be just over $20, and you could get about three good dives out of it. The only drawback there is the price of the CCR. But heck, enough deep dives, and you end out saving!

** Note to Howard **

1. Those boyz aren't paying retail!

2. They are filling LP121s for their SCR rebreathers.

3. Their bailouts are AL80s with bottom mix, and they are dropping tanks (or rather their support divers are) with mixes around 1.0 ata for contingencies. Pretty neat system actually. Of course, the idea of carrying in a rebreather and 3 stages with 3 scooters just to be sure you can get BACK to your contingency gear is amazing...

[Walter]

OK, in basic terms...

At ocean level we breathe air. Which is comprised of about 21% oxygen. Expressed as .21/1.00. With each Atmosphere we descend, we consume a greater volume of oxygen. So at 2 ata it's .42, at 3 atmospheres its .63, etc. When the fraction of oxygen reaches a certain level, somewhere beyondd 1.6, our bodies tend to have a reaction to oxygen. It is toxic in concentrations much higher than this, depending on length of exposure to it.

Good info, but somewhat unclear. The .42 and .63 to which you refer are partial pressure of oxygen measured in atmospheres.

Thus when you take Nitrox class, they tell you to limit your exposure to 1.4 ata during the working portion of the dive and 1.6 on decompression. Those numbers are common ones but not absolute. Some people are more sensitive to oxygen and some less so.

More good information. I also teach a max 1.6 atm of O2 for emergencies. BTW, since we are referring to partial pressures, the measurements are not in ata or atmospheres absolute.

Well, what depth do we reach 1.4 ata with air? Answer 192ft

It's 187 ft

What depth do we reach 1.6 ata with air? Answer 224ft

It's 218 ft

What depth do we reach 1.4 ata with EAN32? Answer 114ft

It's 111 ft

Of course those who understand narcosis understand that you will probably be narced out of your MIND before you reach 192ft on air. So we have two problems. We will not be thinking clearly, and our oxygen content is becoming dangerous. So what we need is less oxygen so we don't convulse. But that would raise the nitrogen, and make us more narced. So we need magic gas to battle the narcosis. Enter helium, which has far less narcotic value than nitrogen or oxygen.

So, by reducing our oxygen content, we can dive deeper without the risk of oxygen content getting toxic. And by using helium we can reduce the narcotic effect.

Good info.

[PerroneFord]

Walter, I ran the MODs in V-Planner, not off the top of my head. Discrepancy between them is probably salt/fresh

[Walter]

That's the difference, but unless your computer/depth guage is calibrated in fresh water (rare) it doesn't matter.

[PerroneFord]

Mine is.. and since I dive freshwater 99% of the time, it's how I have v-planner set.

[Diverbrian]

Real life example:

I just dove the Daniel J. Morrel last weekend. The listed depth is at 200 ft. to the mud. I got down to 192 ft. (damn water levels are low in the Lakes again ).

I had 17/40 (my shop prebanks EAN36 and that made this a little easier mix to make) in my tanks on my back. One deco bottle was EAN36 and the other was good ol' fashioned oxygen.

I was using 17/40 (I was off by a percent due to gauge error on my oxygen percentage) to negate the possibily of a Central Nervous System toxicity hit at 200 ft. The other mixes were to accelerate decompression and out of this topic. A partial pressure of oxygen equal to 1.2 is little more favorable towards this effort than a PO2 of 1.48.

The disadvantage would be this. When I switched to backgas to climb the ladder, I suddenly felt like I climbed Mt. Everest in the water. I had left O2 and gone to a mix that wasn't as nice to my body. (Purpose was to hand up my deco bottles. I can't hand them up if I am breathing off of them.) If I had been forced to stay on that mix, I may have had some issues. For the most part, I don't worry about that until 16 percent O2, but still....

On a hypoxic mix, many times one is forced to use a travel gas on the surface as the lower oxygen contents don't support bodily functions quite as well. Translation: You don't want to become hypoxic and drown.

The advantage... you are reducing the risk of CNS toxicity below 200 ft.