DCS from Trimix or air

[vjongene]

I am getting ready to start working on a CMAS 3-star certification. This involves performing sets of drills in open water at a depth of 40 m, i.e. under conditions of significant narcosis. 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. 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?

Thanks already for your feedback. I know that there are many arguments for diving Trimix at depths over 30 m, the question here is purely whether there is a significant increase in the risk of DCS.

Victor J.

 

[mania]

Here you can find some information
http://www.cisatlantic.com/trimix/diving.htm
Mania

 

[Albion]

In simple terms fast ascents on helium are not a good idea. Helium is a light gas with small molecules, it gets into the system quickly and will expand rapidly too. So in a fast ascent those helium molecules will expand fast and bend you.

(Steps back and lets someone explain it more technically)

 

[vjongene]

Here you can find some information
http://www.cisatlantic.com/trimix/diving.htm
Mania

Thanks for the link, Mania. The topics covered there address mostly the question of how to decompress from deep dives on (presumably hypoxic) trimix.

So maybe my question should be re-phrased: is diving on normoxic trimix, in the 40 to 60 m zone and without travel or deco gasses, preferable to diving on air? And, given that emergency ascents will be part of the training schedule, does the use of trimix present an additional risk?

Please note that I am not trimix qualified, and have no intention of using it without training. The question has to do with the CMAS curriculum, and whether there would be good reasons to integrate basic trimix training with their deco training (not the case right now).

Victor

 

[Mako Mark]

Victor,

the overall trend in decompression thory over the last ten years has bee a movement towards slower and slower ascents. The introduction of deep stops are and ascent curve manipulation mean that although you ascend slower, you exit the water faster.

Here is a great article that will explain what I am talking aboutPyle stops

Your concern with the helium in trimix is valid, you need to be even more careful when ascending with trimix than with air due to the different soluability of the Helium.

Make sure when you are practicing those "emergency ascents" that you dont mistakeemergency for rapid, they are totally different. Practicing rapid ascents is askinfg for trouble no matter what gas you are breathing.

good luck with your class.

 

[vjongene]

Make sure when you are practicing those "emergency ascents" that you dont mistake emergency for rapid, they are totally different. Practicing rapid ascents is askinfg for trouble no matter what gas you are breathing.

good luck with your class.

Thanks, Mark. I have no intention of doing rapid ascents. The standards say that to pass the test you need to make your emergency ascent (dragging along an unconscious diver) at 8-12 m/min to 10 m, slow down, and make a safety stop at 3 m, so if I make a rapid ascent I flunk (aside from potentially getting DCS). But you know how things can go pear-shaped, and the question was really whether using trimix would make the risk even worse. The consensus seems to be that it would.

 

[DepartureDiver]

I just have to diasagree with your instructor. I think helium as gotten a bad rap in the past from not being understood and many tails have developed from it. First I doubt anyone going to that depth is so unskilled that an uncontrolled ascent would occur. Second, the fact that narcosis will occur without it could in itself result in mistakes that could lead to a DCI issue. While helium may get into your system faster of course it also comes out faster. For any time spent at depth, gas will be taken into the tissues and any rapid ascent would be bad. If such a rapid ascent occured it could also be dealt with. To not use helium based on his rationale is not justified. When using it properly it is possibly a more friendly gas.

 

[ScubaDadMiami]

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.).

The time to actually perform this skill is when it is better to get bent and be alive while rescuing another than it is to die underwater. So, having to make such a decision in real life should be an uncommon one (surfacing an unconscious diver, etc.). There are too many risks involved to do this drill from such a depth. That is the real problem.

When diving beyond 100 feet, it is wise to use some kind of helium based mix, such as 30/30 to the depths you are referring to here, to avoid problems with narcosis. So, your course curriculum is putting you in a no win situation: performing rescues with the possible influence of narcosis (which still has serious risks from making emergency ascents at these depths) to avoid potential helium issues, or using helium based mixes to limit the narcosis issues while possibly increasing risks from the ascent.

The goal is to practice a skill in a controlled situation so that, in the time of need, one takes limited risks to save lives, not to purposely take risks just to see if one survives.

 

[vjongene]

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.).

The time to actually perform this skill is when it is better to get bent and be alive while rescuing another than it is to die underwater. So, having to make such a decision in real life should be an uncommon one (surfacing an unconscious diver, etc.). There are too many risks involved to do this drill from such a depth. That is the real problem.

When diving beyond 100 feet, it is wise to use some kind of helium based mix, such as 30/30 to the depths you are referring to here, to avoid problems with narcosis. So, your course curriculum is putting you in a no win situation: performing rescues with the possible influence of narcosis (which still has serious risks from making emergency ascents at these depths) to avoid potential helium issues, or using helium based mixes to limit the narcosis issues while possibly increasing risks from the ascent.

The goal is to practice a skill in a controlled situation so that, in the time of need, one takes limited risks to save lives, not to purposely take risks just to see if one survives.

I am not here to defend the CMAS curriculum, which has been in place for many years. This may be part of the problem, too - the way people dive, and their attitude towards safety, has evolved over the years.

For clarity, here is the list of skills to be demonstrated for the Deco Diving and the Rescue parts of the curriculum.

Deco diving skills:

1. Perform detailed buddy checks on land and at 3 m, demonstrate both head-first and feet-first immersion techniques.

2. Descend smoothly to 30 m, stop at 30 +/- 1m, stay fully equilibrated for a few seconds without finning.

3. Descend to 40 m and maintain depth.

4. Do a full mask clearing - remove mask, put back on, then clear in no more than two expirations.

5. Be able to perform and interpret the full range of standard hand signals at 40 m.

6. Perform a buddy breathing ascent to 15 m while maintaining an ascent speed of 8-12 m/min, stop at 15 m for at least one minute while buddy breathing.

7. Ascend to 6 m and return to the immersion point using a compass only while maintaining this depth.

8. Respect all deco obligations incurred during the dive.

Rescue skills:

1. Bring an unconscious buddy up from a depth of 40 m, controlling the ascent speed (8 to 12 m/min) using both BCDs. Ensure that the buddy is breathing throughout the ascent to avoid an AGE (maintain a reg in his mouth, assist his breathing if necessary). Make a safety stop at 3-5 m, and ensure that there are no surface hazards.

2. Call for help when reaching the surface, and tow the unconscious diver for at least two minutes.

3. Once on land or on the boat, take all necessary measures to ensure the diver's immediate survival: correct positioning, administration of oxygen, CPR if necessary, protection against hypothermia, etc

 

[Albion]

I am not here to defend the CMAS curriculum, which has been in place for many years. This may be part of the problem, too - the way people dive, and their attitude towards safety, has evolved over the years.

Provided you are doing it all correctly Trimix would be an advantage, however the point is that it is training and you are doing some testing skills. The point of the training is to push you and if you are not up to it then things can go awry.

Deco diving skills:
1. Perform detailed buddy checks on land and at 3 m, demonstrate both head-first and feet-first immersion techniques.
2. Descend smoothly to 30 m, stop at 30 +/- 1m, stay fully equilibrated for a few seconds without finning.
3. Descend to 40 m and maintain depth.
4. Do a full mask clearing - remove mask, put back on, then clear in no more than two expirations.

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

1. Bring an unconscious buddy up from a depth of 40 m, controlling the ascent speed (8 to 12 m/min) using both BCDs. Ensure that the buddy is breathing throughout the ascent to avoid an AGE (maintain a reg in his mouth, assist his breathing if necessary). Make a safety stop at 3-5 m, and ensure that there are no surface hazards.

What if you lose control and get a runaway ascent?