What is the Best Mix Ever?

[Soggy]

well when you weigh out the cost of the gas (He) and its decompression properties along with the cooling effect breathing He caused the body to go through..

My experience and understanding of helium is that it does not cause you to get cooled by the act of breathing it. Inflating a drysuit with helium does chill you (and potentially cause other problems like ICD and potentially skin bends).

Helium is a poor insulator because it is an efficient *conductor* of heat. Because helium does not hold heat well, it doesn't remove much heat from your body when you breath it. I believe that breathing an effective insulator, like argon, would actually result in more heat being removed from your system.

I am not a chemist, but this is my rough understanding of the process. I may be wrong.

 

[amascuba]

I don't feel narcosis effects at 120' (maybe I'm not that susceptible or just unaware). Which gas would put me at less risk of DS (assuming the same dive profile)?

A. 21/35
B. EAN 28
C. It's a tie

21/35 will give you roughly the same bottom time at 120fsw as air within NDL. You will have the advantage of having a clearer head. The disadvantage to helium is that it on gases AND off gases quicker, so you will have to have your buoyancy in check, particularly for the ascent portion of the dive.

I'm not where I can look at my tables, but I would guesstimate that EAN28 gives you roughly the same bottom time at 120fsw within NDL as air at 100fsw within NDL. You would be more susceptible to narcosis, but you would have a longer NDL at 120. Based on your question I would say that EAN28 would give you less risk of DCS within recreational limits.

I do agree with omar though, I would prefer the helium for dives over 100fsw just because of the narcosis issue. I would say at those depths that you do experience narcosis, but you have not been put into a situation where you notice it. It's easier not to notice it on warm, clear water dives that are relatively uneventful.

 

[Doc Intrepid]

Most divers would say they are either "not narked" or "felt no effects of narcosis" even when videotaped evidence illustrates they are significantly impacted by narcosis.

It isn't something you're always aware of - unless you have a problem, and even then you may not feel dizzy or vertigo or out of control - but you may exhibit behavior ranging from task fixation to bizarre decision-making.

JJ relates a story where even divers in the WKPP (who swore they were not impacted by narcosis) were found to be narked when pre-positioning stage bottles. JJ and George would be heading out and find stage bottles laying all over the place instead of neatly positioned as briefed. They'd look at each other and ask "who the hell swam nearly a mile into the cave to mess with our stages?" Ultimately it became clear that the divers placing the stages, who swore they felt no narcosis, were narked as hell under those circumstances. Thats when the WKPP went to using helium mixes exclusively, per the citation.

Assuming cost is no issue, go with choice A. 21/35. You have nothing to lose, and in the event of a catastrophic situation you will most likely respond with a clearer head...

 

[H2Andy]

I am not a chemist, but this is my rough understanding of the process. I may be wrong.

i believe you are right:

Dr. Jolie Bookspan:

"Helium has greater thermal conductivity than air. Undeniably, you lose more heat when surrounded by helium than by air, because heat conductance is the major factor in skin heat loss. Therefore helium is not used in dry suits. However, respiratory heat loss depends on heat capacity, and not at all on conductance. The thermal capacity of helium per gram is higher than that of air. However, there are fewer grams of helium for the same volume breathed because it is far less dense, making thermal capacity less compared to the same volume of air. Less heat would be lost breathing helium, so it should not chill you to breathe, as commonly thought. In a helmet or full face mask, your face may feel cool, making it hard to separate out the lesser loss through breathing.

Depth affects gas density, and so, heat loss through the breathing medium, and to be more confusing, you also need to account for interactions of respiratory heat loss through convection and evaporation. With helium you may also be more aware of the cold that is so common in diving, than when dulled by narcosis while breathing non-helium mixes. Remember too, it is generally not feasible to breathe air at depths where helium is used, so hard to compare in actual use.

The short answer seems to be that breathing mixtures of helium at depths encountered by technical divers does not seem to result in greater cooling than breathing air. Helium feels colder to your skin than air, but it carries away less heat when you breathe it."

http://www.uhms.org/Publications/publications list.asp

 

[Soggy]

i believe you are right:

Whew...At least I got one right today!

 

[omar]

I don't feel narcosis effects at 120' (maybe I'm not that susceptible or just unaware). Which gas would put me at less risk of DS (assuming the same dive profile)?

A. 21/35
B. EAN 28
C. It's a tie

You are narced at 120. There is a significant body of literature that documents this effect.

You will have a greater inert gas tissue loading with the 21/35. It does not translate to a greater risk of DS. Your dive profile will have greater bearing on that and you are more likely to screw up when you are narced.

 

[omar]

I did a quick estimate of the heat loss of 21/35 vs air at 120 based upon a 0.52 SAC (15L SAC). You will lose 11 calories LESS breathing 21/35 than air. In other words this is negligable as pointed out above.

 

[grazie42]

Originally Posted by pensacola
I don't feel narcosis effects at 120' (maybe I'm not that susceptible or just unaware). Which gas would put me at less risk of DS (assuming the same dive profile)?

A. 21/35
B. EAN 28
C. It's a tie

If DS is short for decompression sickness, then I would go with "A" as He is less tolerant of bad bouyancy control. Most would argue that you have no buisiness diving unless you can control your bouyancy (I agree) but the risk for DS would still be greater with "A", IMO.

I´d still use it though, or perhaps 30/30 and stay a few feet above 120...
The real benefit of tmx in this scenario is that you could do one "serious" dive (instead of 2 short) with deco etc and be able to go furter, into places you haven´t seen before...

 

[pensacola]

You are narced at 120. There is a significant body of literature that documents this effect.

So I guess I'm like the people who say "I can drive fine after four beers." I just don't know that I'm narked. Thanks for all the info.

 

[limeyx]

Most divers would say they are either "not narked" or "felt no effects of narcosis" even when videotaped evidence illustrates they are significantly impacted by narcosis.

It isn't something you're always aware of - unless you have a problem, and even then you may not feel dizzy or vertigo or out of control - but you may exhibit behavior ranging from task fixation to bizarre decision-making.
...

Assuming cost is no issue, go with choice A. 21/35. You have nothing to lose, and in the event of a catastrophic situation you will most likely respond with a clearer head...

What you have to "lose" with 21/35 is that (depending on what/who you believe), you need to really have your buoyancy down, and do a "proper"/slow ascent on the 21/35 compared to air.

For this dive, max 120 feet, average around 100-110 I would choose 30/30 and get the benefit of the He (narcosis) and the O2 (more MDL dive time)

for the shallower dive, either use up the rest of the 30/30, or top the tanks with 32%