Multiple mixes

[rjack321]

I remember a recent article from Mitchel/Doolette recommending to do sudden changes of He/N2 only near the bottom or the surface to avoid isobaric counter diffusion which makes me imply that it applies to bounce diving as well. Did I get it wrong?

Link?

People have been doing 15/55 to 50% switches for decades ¯\_(ツ)_/¯

 

[lermontov]

I remember a recent article from Mitchel/Doolette recommending to do sudden changes of He/N2 only near the bottom or the surface to avoid isobaric counter diffusion which makes me imply that it applies to bounce diving as well. Did I get it wrong?

There was a thread on here a while back where simon called ICBD "bro science"

Help me understand the rule of fifths for IBCD

This great article gives you a quick and dirty understanding of the rule of fifths as a way to avoid Isobaric Counterdiffusion when diving Helium. Hello, The rule of fifths is a classic example of 'bro-science' that somehow became accepted as fact and continues to be perpetrated to this day...

scubaboard.com

 

[LFMarm]

Here is what I was referring to:

Doolette and Mitchell propose a more practical approach: minimizing the switch from trimix to nitrox on ascent or planning to perform these switches at depth or in shallow water to minimize supersaturation.

From Isobaric Counterdiffusion in the Real World and

1. Doolette, David J; Mitchell, Simon J (June 2003). “Biophysical basis for inner ear decompression sickness”. Journal of Applied Physiology. 94 (6): 2145–50. doi:10.1152/japplphysiol.01090.2002. PMID 12562679.

My takeaway is that the rule of fifth is bro-science, but isobairc counterdiffusion is real and because of that we should avoid trimix to nitrox switches that are neither deep nor shallow. Hence my question on doing for example a 21/35 flush on a 100 m dive.

 

[tbone1004]

Isn’t option 1 better to avoid isobaric counter diffusion?

if that is your primary concern then yes, but it comes at a severe penalty in total decompression time once you get up to the nitrox depths. If it makes you feel better you don't have to do a ful dil flush once you hit nitrox depths, you can do it in partial flushes, but by the time you hit 70ft the deco penalty of not flushing with no helium is pretty severe.

 

[LFMarm]

if that is your primary concern then yes, but it comes at a severe penalty in total decompression time once you get up to the nitrox depths. If it makes you feel better you don't have to do a ful dil flush once you hit nitrox depths, you can do it in partial flushes, but by the time you hit 70ft the deco penalty of not flushing with no helium is pretty severe.

Great point. One challenge of partial flush is that it’s hard to guess what mix you are actually breathing hence you would keep your bottom mix on your computer and still pay the big deco penalty.

 

[tbone1004]

Great point. One challenge of partial flush is that it’s hard to guess what mix you are actually breathing hence you would keep your bottom mix on your computer and still pay the big deco penalty.

sure, but at some point you are supposed to start doing "air breaks" so by that point you will have connected the new gas without helium and switched the bottom mix. Personally I have no issue getting off helium at 70ft, but certainly by the time you hit 40-50ft you should be well off of it and that's where most of the time is anyway. The other reality is there are very few of us doing dives with run times of 4+ hours at those depths where it matters anyway so for most people it's making a mountain out of a mole hill. If you aren't going deeper than 200ft it certainly doesn't matter so get off of the helium asap and switch it over on the computer