Helium or Air for CCR deco

[johnmaneely]

Hi All,
Any of you got ideas about whether its best to stay on a trimix diluent for your deco, or switch over to air when the depth is suitable. The run times are generally simaliar(trimix a little longer)but what about actual physiological advantages? Anyone know of any research thats have been undertaken? doppler info etc. Speaking to people I know they reckon just stay on the trimix, just wondering if there's any evidence to back it up.
John

[bang-on]

<font color='#0000FF'>john,
i have a simplistic approach to this.
eg; 17/35 70mtrs 48% n2
i avoid allowing the fn2 to rise,only fall.if you changed dil to air @ 30mts the fn2 is going from 48% to 67%.
fhe decreases as depth becomes less and o2 increases but i would normally increase setpoint from 1.3 to 1.4 for last stops.
so my thinking is reduce fn2 =less narcs
use same dil for entire dive so no big shifts in fraction of gases
and just let the pp do the work,although deep stops are a must.run a few comparisons on your deco software and see what you think.if anyone has a reason for changing,besides saving on deco i would like to read it.
cheers
barrie

[Rob Evans]

I've always been inclined to stay on the gas - there have been arguments in the past re: counterdiffusion between PHe and PN2 when going from a gas dil to Nitrox. It's been going on for ages in the OC world without (AFAIK) any detrimental effects and I've not seen any scientific testing to prove one approach over the other, but I'm tending to side with the DIR crowd on this one. Ie stick with the Helium in progressively lower concentrations until you hit the magic 6m &amp; then give it the old O2.

The beauty IMHO of CCR is that you can do this in a veeerrry smooth curve, for the simple reason the box is maintaining 1.3 PO2 the whole time.  Therefore for a sample mix of 10/52 Heliair, on ascent from say 45m (max depth is irrelevant for the purposes of the illustration), the mix you're breathing will be:

Depth    O2 / He
45         26 / 43
40         29 / 41
35         32 / 39
30         37 / 36
25         43 / 33
20         52 / 28
15         65 / 20
10         81 / 11
Followed by the O2 flushes at 6m.
EDIT: Looks like nobody spotted my deliberate mistake then...Phew

A nice curve of steadily decreasing PHe &amp; PN2, no sudden jumps to high PN2/no He &amp; no unwanted narcs. There is a penalty for the strong mix when calculating using Buhlmann which I'm quite happy to put up with cos there's really only a few minutes in it if you plan the dive on 100% for the 6m &amp; above stops.

Cheers
Rob

[bang-on]

Originally Posted by [b

Quote[/b] (Rob Evans @ Jan. 28 2004,22:30)]but I'm tending to side with the DIR crowd on this one. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Ie stick with the Helium in progressively lower concentrations until you hit the magic 6m &amp; then give it the old O2.

<font color='#0000FF'>rob,
agree with the thinking &quot;helium is best&quot; but i think that is just a school of thought rather than anything gue teach,maybe it is?
i think ccr takes a key consideration for o/c divers out of the equation &quot;helium costs&quot;.

does anybody have any other thoughts?

cheers
barrie

[Rob Evans]

AFAIK there's been no empirical data published one way or the other. It seems that since Buhlmann there's been no heavy research carried out regarding recreational deco, especially as regards Helium - so for the moment we're stuck with best-guess to a large extent.

I'd be interested to see what the DIR crowd can add to the question - I agree with the premise, mostly because it makes sense to me - is there any data that has been published?

[DALESDIVER]

Rob,
&nbsp; &nbsp; &nbsp; DECAP (Bill Hamilton), have been utilising heliox only, for all the dives on ccr deeper dives, also using bailouts of heliox, aka 50/50 &amp; 80/20 offboards, nitrogen is a bad gas as it were, so by using a gradient profile for deco starting at 2/3rds dive depth with 10/90 heliox as onboard dill. you would be pulling 1 min every three till 1/3 dive depth then upping the stop times with a max of 16 @ 6 after 25 @ 80m...
It works , but the VR3 is still at 15m when you are back on the boat!

Also by eliminating nitrogen it is easier to offgas the helium with a steady but controlled ascent, as all the tables we are getting as rec. divers are still using o/c style stops, it is worth resaerching this a bit, if you put v-planner vpm-b on the standard level it is very near decap, but it still gives you very long stops at 6m...

People are currently trying to get Bill Hamilton to publish his work, so that we can all benefit from it.

[Dominic]

I thought some Nitrogen was a good thing on deep dives to cure HPNS..?

[Alan57]

I use a 10.5/50 heliair mix without changing or flushing on every dive I do.
I choose it because it gives a low narcotic depth, it is easy to garage mix (without worrying about O2 cleaning) and the run times with DDPlan are not excessive. The low cost of the helium used also helps.
It can be topped off with air several times and you still know what the percentage breakdown of the constituent gases are.
I do not worry about HPNS as there is still plenty of N2 in the mix and do not change diluent on the ascent as the CCR reduces the N2 and He parts by boosting the O2 with a set point of 1.3

[johnmaneely]

Thanks all. I think just sticking with the 10/50 all the way to 6m then running on 02 to make the best of the times. Seems little point changing dil when theres a lack of evidence for doing so. Also avoids big shifts in gas pressures, which seems sensible.
John