DCS from Trimix or air

[vjongene]

So far so good then 4) Why two expirations, what does it matter so long as you are in control

These are just the standards. I guess they don't want you to have to futz with your mask.

What if you lose control and get a runaway ascent?

Shouldn't happen of course when you are training at this level, but Murphy can strike any time. This is not an easy exercise. That was the point of my post in the first place.

 

[Charlie99]

I have a problem with this curriculum if it includes making "practice" emergency ascents from 130 feet to 30 feet. The way to do this drill is not to actually make the ascent from significant depth but, in shallow water, to swim horizontally for a distance and then to ascend from a shallow depth. The only time to make an actual ascent from these deeper depths at an emergency ascent speed is during an actual emergency because the ascent itself presents risks (over expansion injuries, etc.).[emphasis added]

It sounds like you are describing an out-of-air swimming emergency ascent. The original poster described lifting an unconscious diver at 8-10m/min (30fpm), and was concerned about the possibility that this controlled lift would become uncontrolled. Different things.

One regret I have about my PADI Rescue course was that our drills on bringing an unconscious diver to the surface were all done from relatively shallow depths and I ended up doing more a a swim-up rather than a controlled buoyant lift. IMO, swimming horizontally isn't an appropriate substitute for this sort of drill.

 

[Doc Intrepid]

"...His answer was that while this would certainly reduce the risks associated with nitrogen narcosis, the greater solubility of He in human tissues (and hence faster on- and off-gassing rates) would increase the risk of DCS associated with uncontrolled ascents. Since a significant part of the curriculum consists of performing emergency ascents (unconscious diver, buddy breathing, etc), he argued that the risks may outweigh the benefits.

So the question is, is there really an increased risk of DCS after an uncontrolled ascent while diving Trimix? And is the onset of symptoms more rapid than after a similar profile dived on air?" (emphasis added)

Victor,

Based on what I have read and understand regarding the use of helium mixes, your instructor's response was true as far as it goes. One of the primary tenets of my trimix training was that helium is an unforgiving gas with respect to ascents. If you cannot control the rate of your ascent (for whatever reason) there is an increased risk of DCS due to the helium in the mix (for the reasons your instructor provided). (As an aside, if potential students failed to demonstrate an adequate ability to ascend slowly and under control, my instructor refused to accept them as students.)

Given that your curriculum appears to include several fast ascents (I am not sure how 'rates of ascent' translate to the various adjectives: "emergency", "rapid", etc.), it would seem that these ascents may indeed be safer using non-helium mixes.

With respect to your second question above, a "similar profile dived on air" would not have helium in the mix, hence, no helium could come out of solution due to a fast rate of ascent. Ergo, there would be no symptoms of DCS based on helium bubbles if you were diving the same profiles on air. (Perhaps I misunderstood your question.)

If you're simply referring to how rapidly you would perceive symptoms of DCS, caused by helium versus nitrogen, while I'm unaware of any empirical data to base an opinion on (and I may be reading it wrong), onset of DCS will be perceived when bubbles exist to a degree that they cause symptoms that you recognize. DCS is DCS - even though in one case the bubbles may exist due to helium rather than nitrogen, I doubt you would perceive the symptoms any sooner. But I'm by no means an expert - others on the board may be able to offer a medically-based response.

[One organization funding research in this area is DAN, and I advise you to call them with your concerns.]

Doc

 

[vjongene]

With respect to your second question above, a "similar profile dived on air" would not have helium in the mix, hence, no helium could come out of solution due to a fast rate of ascent. Ergo, there would be no symptoms of DCS based on helium bubbles if you were diving the same profiles on air. (Perhaps I misunderstood your question.)

If you're simply referring to how rapidly you would perceive symptoms of DCS, caused by helium versus nitrogen, while I'm unaware of any empirical data to base an opinion on (and I may be reading it wrong), onset of DCS will be perceived when bubbles exist to a degree that they cause symptoms that you recognize. DCS is DCS - even though in one case the bubbles may exist due to helium rather than nitrogen, I doubt you would perceive the symptoms any sooner. But I'm by no means an expert - others on the board may be able to offer a medically-based response.

Thanks for the detailed response, Doc. The question here was, during a rapid ascent, are you more likely to develop DCS symptoms if you are breathing trimix than if you were diving the same profile on air (the composition of the bubbles would of course be different). One of the things that the instructor was concerned about was that the greater off-gassing rate would result in a faster onset of DCS symptoms, and hence a smaller time window in which to prevent them (e.g. by administering oxygen).

 

[Charlie99]

I asked the instructor whether there were any plans in the CMAS curriculum to introduce the use of normoxic Trimix for diving in the 40-60 m range. His answer was that while this would certainly reduce the risks associated with nitrogen narcosis, the greater solubility of He in human tissues (and hence faster on- and off-gassing rates) would increase the risk of DCS associated with uncontrolled ascents.

Solubility and rate of diffusion aren't the same thing. Helium actually has LOWER solubility in human tissue than does nitrogen, by a factor of 3 to 5. OTOH, helium has a faster rate of diffusion, by a factor of about 2.7. Two different properties. The result of these two different properties is that trimx has shorter decompression from long, deep dives; but at the same time, trimix has shorter NDLs (or has more required decompression) than air or nitrox on shorter and shallower dives. I interpret this to mean that trimix has a higher risk of DCS from uncontrolled ascents from short dives.

One has to balance that increased risk of DCS were a fast ascent to occur, with the fact that narcosis would make an uncontrolled ascent more likely to occur.

Here's an excerpt of an article that touches on this issue:

In bounce diving, selection of helium or nitrogen as the inert gas is a trade between the lower solubility of helium, less than nitrogen by a factor of 3-5, and the greater diffusivity, about 2.7 times greater than nitrogen by Graham's law. For long exposure times, solubility factors dominate and helium is a better breathing gas than nitrogen. The solubility advantage should also hold up for short exposures if gas exchange is perfusion limited, while if diffusion is rate limiting, the 2.7 diffusion advantage of helium might be expected to outdistance its 3-5 solubility advantage over nitrogen, and so air (nitrogen) would be better for bounce diving. Goat experiments clearly show that nitrogen is better for bounce exposures of less than 20 minutes duration, that is, longer nonstop time limits compared to helium.

It sounds like they are still bending goats.

Decompression Dilemmas

 

[vjongene]

Solubility and rate of diffusion aren't the same thing. Helium actually has LOWER solubility in human tissue than does nitrogen, by a factor of 3 to 5. OTOH, helium has a faster rate of diffusion, by a factor of about 2.7. Two different properties. The result of these two different properties is that trimx has shorter decompression from long, deep dives; but at the same time, trimix has shorter NDLs (or has more required decompression) than air or nitrox on shorter and shallower dives. I interpret this to mean that trimix has a higher risk of DCS from uncontrolled ascents from short dives.

Yes, you are right. I should go back to my high school physics textbooks (or read my son's)... Thanks much for the explanations and links. In the specific case I was referring to, i.e. a short relatively deep dive followed by an ascent (bounce dive) the diffusion rate would be the determining factor, not the solubility. So my instructor was right, if not entirely for the right reasons - ths risk of DCS would be higher for helium than for air under these conditions.

 

[Tom Yerian]

All the answers these divers gave you are in the correct zone. You are basically dealing with two inert gasses, each having different offgas ratios,HE deco deep,and N2 deco on the shallower end. Helium in any content in a diving gas can present problems in proportion to..percent in mix, depth used,DIVER EXPERIENCE,possible emerg. situations,diver health and fitness, and other variables. Helium is not as forgiving as Nitrogen on a bad ascent. Due to Boyles law, if you take a hit on he at depth , you will be feeling it even befor you surface!Once you supersaturate He, being a lighter gas, bubble expansion increases quicker, hence feeling the hit befor you surface. Now your treatment depth ( wet or dry ) is deeper.Etc, Etc, Etc....So, even REC. mix can cause you trouble! When I use Any He, I always carry, 100% beside my usual 50%. .......yid.....Capt. Tom