Eliminating the Helium Penalty

[Tim1]

Up front, I practise diluent switching on deep dives where I would be using 10/70 or leaner. Consequently my run times are truncated.

Initially I hypothesised that the switch(es) were forcing the algorithm to recalculate however I proved to myself that this is not the case. I ran a spread sheet which calculates dynamic loop composition at 10 metre increments throughout the dive and entered this data as hypothetical diluent switches and compared the consequent run time with that produced by remaining on the same diluent thoughout. There was not an appreciable difference.

So, why the shorter run times created by a more 'aggressive' diluent switches? The change in the proportions of He and N2 in the mix and a consequent reduction in the helium penalty imposed by the algorithm would appear to be a reasonable conclusion.

I have been looking at the scant data around the use of heliox in bounce diving; some researched and some anecdotal. It is clear from Royal Navy data (where 20/80 is a standard gas) that there is an increased risk of a type 2 bend from heliox. Anecdotal evidence from practising heliox users suggests that slow(er) ascents are required, a low(er) low GF (ie. 15) is sensible and entering a low or lower He content reduces or removes the helium penalty. With this in mind I can understand the rationale for a switch from heliox to a helium free diluent however the probable IBCD hit may not be desirable...

For me, using a heliox diluent is in the context of a dual rebreather. This configuration is only necessary where it becomes impractical to either carry or have access to sufficient OC bailout. It would therefore be contra-productive have to carry additional gas for the purpose of diluent switching. So, heliox it is, top to bottom and back up again. But then, there is the issue of HPNS...

 

[Jneves]

First of all, I must apologise for my previous sarcastic words, I am sorry.

Rjack,
I should’t and I don’t give a rat’s ass who you voted for. This was out of line from my part.

I do get a bit concerned to see fellow divers jumping so easily to embrace any new study conclusion, no matter how limited scope research may be or how not directed at the type of diving we do. Please be careful. Take your time.
I am not at all against change. But it must be sustained and verified change.

There is, no doubt, a lot of merit to these studies but being partial, individually they are worth what they are worth. We need a lot more of them and we need the results to be validated by a lot of actual dives over a wide range of profiles before we can actually see them as a safe standard.
Rubicon’s paper is just one study on rabbits, and maybe we don’t even know what the profile was and exactly in what kind of tissues the bubbles were collected and the composition of the mix (was it very high He content or very high N2 content?), was it a dry dive or were the rabbits immersed in water (this makes a huge difference), the temperature, and so on.
Helium intake may be minimum in therms of volume of gas (as is N2) during a dive, and therefore in an explosive decompression may show very little in blood samples (if it didn’t have time to diffuse back into blood). The critical thing here is where the bubbles formed and are retained in what tissue.
I am not a physiologist, so I my wording will not be very technical, but as fare as I know the laws of physics have not changed nor will they in the foreseeable future. So He will keep diffuse into fast tissues fare faster than N2. It will also off-gas faster from fast tissues than N2 if perfusion remains the same. The issue here is that if you keep even a small fraction of He in the inspired mix AND you have, for any reason, to blow to the surface with still a small load of He in the nervous tissues, chances are you will get a neurological hit. The spinal cord in particular is very vulnerable to these pressure gradients. The uptake of He here is made primarily by diffusion. The spinal cord is not as irrigated as the brain is, the off-gassing has to be by diffusion too, hence the risk of bubble forming and growing in and also around the cord, where they will compress the cord itself, and/or the nerves irradiating from it, maybe to the point of rupture. If the neuronal connections are destroyed the damage will be permanent. N2 may do the same thing, but as it diffuses slower into nervous tissue the amount of gas available to form bubbles at the time of a sudden big pressure gradient is likely to be small.
I definitely have not seen any studies that have changed our understanding of the above. But if someone has please point them out to me.

In my post, I didn’t defend any particular procedure. I raised questions – as I said, food for thought.
I did not suggest that gas switching should be used just to reduce deco time. What I said is that in case of partially omitted deco the chances of a neurological hit are bigger than if no He was present in the breathing mix (and also off-gassed from nervous tissues). Hence the diluent switch to a non He based mix for the final stages of deco.

Hbm

Yes, I DO KNOW people that are paraplegic from He accidents. And I know a lot of people that had accidents on air, and none is paraplegic. Normally if one follows conservatively most algorithm generated schedules, and most important are able to complete them successfully, accidents are rare. However if you blow your deco, then the consequences differ widely from mix to mix (or stablished deco protocol with different mixes).

Tim1

Yes, I guess there will always be advantages and disadvantages in any mix (or combination of) ones choses to use. This is why one should evaluate and weight the risks – all of them, no matter of what kind! - in order to take a safe decision.

Now that I have stirred the calm waters, I will let the diving doctors and physiologists step in if they wish to do so.

Dive safe

JNeves

 

[bazSXD]

Dil switches used to be fairly common with some of the deep RB divers here in Oz, it fell out of favour due to the added risks of mixing up gas swaps. People were even using switching blocks for their dil.

I use a switching block and the majority of my customers also do - nothing wrong with a block. I also regularly switch to a non He gas on deco, normal air at about 35m. It all depends on the depth of the dive and how much faith you put in Isobaric Counter Diffusion, rich He mixes call for shallower non He gas switches.