Trimix pros and cons...

[Solly]

Hi, I want to know more about trimix use... if one uses air, the delimiting factor is the NDL due to 79% nitrogen and if one uses nitrox, the delimiting factor is the PPO2 which sets the MOD that the notrox mix can be safely used within.... what about trimix, what I see is that by having 21% oxygen, very low % nitrogen and a lot of helium one should be safe for either NDL and PPO2 ... but if this was correct all would have been using trimix .. so where is the catch? what am I missing?
thanks

 

[Rick Inman]

A few off the cuff.

Negatives:

• Faster gas means less forgiving if deco stop error is made.
  
• Cost of HE.
  
• Investment in training.

Positives:

• Reduces narcosis.
  
• Less dense gas reduces WOB (work of breathing) at depth.
  
• Makes people laugh at funny squeaky voice.

 

[Cave Diver]

Trimix generally refers to a blend of helium, oxygen and nitrogen, combined either in a normoxic or hypoxic mix.

Normoxic means that it uses a "normal" amount of O2, usually in the 18-21% range

Hypoxic means an O2 amount that is lower than this.

Helium is an inert gas, just like nitrogen. However it does not have the same narcotic effect that nitrogen has. This is the primary reason for using helium in a breathing gas mixture. There are several other gasses that share this same property, but they tend to be a bit more exotic and expensive.

Trimix is typically used for deeper depths, below the recreational limits of 130'. There are common mixes that are used, depending on the target depth, just like nitrox. Two of the considerations when determining mix is how much O2 is needed to keep the PPO2 in a reasonable range and the maximum amount of nitrogen that can be used while keeping the narcotic effect in check. Helium then makes up the remaining balance of the breathing gas.

Generally understanding and diving trimix isnt much different than nitrox; most of the same basic principles apply. There are some specifics that need to be covered by a good instructor before using it. Cost of mix, expense of training and less forgiving deco are some of the drawbacks.

There are also blends called heliox (helium and oxygen) and triox, that are used by some people.

 

[Meng_Tze]

Hi, I want to know more about trimix use... if one uses air, the delimiting factor is the NDL due to 79% nitrogen and if one uses nitrox, the delimiting factor is the PPO2 which sets the MOD that the notrox mix can be safely used within.... what about trimix, what I see is that by having 21% oxygen, very low % nitrogen and a lot of helium one should be safe for either NDL and PPO2 ... but if this was correct all would have been using trimix .. so where is the catch? what am I missing?
thanks

With diving there are always several delimiting factors. With air they are just as much the same as with Nitrox. Both can be limited by NDL, PO2/FO2. The same goes with trimix.

If you go beyond NDL, you end up in mandatory decompression diving. If want to go beyond a certain depth, you need to start looking at your FO2 and MOD. If you want to avoid certain risks like CNS, OTU loading, etc, etc, you have to start looking at time, depth, MOD and such in combination.


Indeed the benefits for He are to replace N2, and thus N2 loading in the tissues. This can lead to longer bottom times, inclusive of decompression. Another use of Trimix, is the ability to lower O2 content. This allows for gas usage at far greater depths. But deeper dives rack up decompression times very fast.... this leads to longer exposure times, fatigue, dehydration etc, etc.

All these factors have to be looked at in combination when deciding a dive on trimix.

 

[Solly]

what does 'less forgiving deco' mean?

 

[Cave Diver]

what does 'less forgiving deco' mean?

Helium is a lighter gas. It saturates into the tissues faster, it comes out of saturation faster. Your deco stops on trimix need to be more precise, or the gas can come out of tissue solution too fast, causing DCS.

 

[Cave Diver]

As a side note, there are people who advocate and use helium mixtures for shallower dives as well.

 

[Solly]

I can understand this, if you are able to sustain your buoyancy well and control your ascent rate such that you precisely do your stop then why not use it at shallower depth.. but the question here would be, why add more constraints if you can do with another mix or plain air and take the precautions not to exceed the limiting factors...
I think all kind of gas mixtures will impose certain measures on the user to use them safely, wither time constraint, depth constraint or more precise stops/ascent rate...etc.

Many thanks for the info, it helps a lot to understand more

 

[Cave Diver]

I can understand this, if you are able to sustain your buoyancy well and control your ascent rate such that you precisely do your stop then why not use it at shallower depth.. but the question here would be, why add more constraints if you can do with another mix or plain air and take the precautions not to exceed the limiting factors...
I think all kind of gas mixtures will impose certain measures on the user to use them safely, wither time constraint, depth constraint or more precise stops/ascent rate...etc.

Many thanks for the info, it helps a lot to understand more

Cost is one reason. Helium is not cheap. It also typically requires a booster pump to mix the gas. Oxygen is added to the cylinders first, then an appropriate amount of helium is boosted on top of that, then finally the cylinders are topped with air. As a result, not every dive shop is equipped to mix it, or has anyone on staff qualified to do so.

Once you start moving into the tech arena, you find more divers with home fill stations, compressors, boosters, and cylinders of oxygen and helium and the knowledge to mix and use it themselves.

Those that use it at a shallower depth are typically advocating that even as shallow as 100' some people are showing substantial impairment from nitrogen narcosis.

 

[TSandM]

I think the important point is that ANY inert (non-metabolized) gas that you breathe at depth will have to be offgassed as you come up to the surface -- so there is no gas you can put in your mix that won't have a "no-decompression" limit. Helium does the same thing that nitrogen does -- dissolves into the tissues as you dive, and comes back out as the pressure is reduced on ascent -- although there is some controversy about the relative behaviors of the two gases in decompression. But the reasons for using helium are reduced narcosis, reduced work of breathing, and reduced weight of gas (affecting buoyancy characteristics of the tanks you are carrying), not any increase in no-deco time, or reduction in required decompression for staged decompression diving.

Helium's cost will always limit its use in diving. I'm one of those people who DOES do some deeper recreational diving on helium mixes, but at about $40 a fill for 25/25, I don't do it very often