Getting off the Helium?

[Juz]

Ok, what are your views on getting off the Helium?


Example: 65m dive for 25 mins using 15/50

Would you complete the entire dive with the He in the loop or would you get to 25m and flush the loop with air (or similar) to remove the He.

My deco planner would take 10 mins off the deco if I did this.

My goal here is not to get out quick however, it's to get out clean, so if I left the VR3/tables set to 15/50 and flushed with air, I'd actually be doing extra time in the water to ensure a clean deco.

BUT, and there always is a BUT!

How concerned should I be about the PPN2 spike. The PPN2 of the loop would be:

65m using 15/50 = 2.55
30m using 15/50 = 1.11
25m using air = 2.2

This is clearly lower than the PPN2 at depth, but at 30m it is a spike.

Comments and thoughts please, including what you actually do.........................

Juz

[nigelH]

Originally Posted by Juz

Ok, what are your views on getting off the Helium?

I'll happily blow the loop to pure O2 at 6m but I wouldn't bother if it involves Nitrogen.

Anyway... Isn't there a problem that if something goes wrong and you have to switch back onto the helium mix you stay supersaturated on slow N2 and resat on fast He which predesposes you to a vestibular bend? I can just see that being a hoot with my tendency to motion sickness.

[Mark Chase]

General consensus in OC diving and in CCR diving is to keep an ever reducing PPNitrogen during deco. Spiking the Nitrogen is apparently restricting the smooth off gassing of the helium and can cause problems with counterdefusion in the inner ear on big dives.

Whilst I now tend to follow this mantra I haven’t managed to get a firm grasp on the science behind it. On one hand I have been spiking Nitrogen for ages on OC diving often swapping from stuff like 18/45 to 32% at 40m without incident (as far as I am aware). I was told (by Mark Ellyot) that this was because I wasn’t doing aggressive enough dives to make the problem apparent. On the other side if you look at the GUE standard gas profiles that they use for DOTF you will note the careful reduction in PPNitrogen that is perhaps a significant part of how they get out of the water so fast. Also think beck to OC days when there was a trend to multi switch gas at gas switch stops. The idea of swapping back and forth between back and deco gases on a switch to reduce the shock to the system of the sudden increase in PPN.

So regardless of the complex science behind the idea it would appear that the more advanced / aggressive divers have been looking at and dealing with this issue for some time and have been reaping the benefits of reducing the PPNitrogen in terms of cleaner deco and reduced stop time.

The trend now is to stay on the He throughout deco and to flush the He with pure 02 on the shallow stops. Again what information I am able to digest on this subject all seems positive. Even GUE are agreeing on trimix for deco and I believe the hyperbaric doctors were looking at the benefits of 50/50 Heleox for the treatment of bends rather than pure 02.

So whilst I openly admit to hanging on the shirt tails of the experts in this rather than actually being one It does seem to be the way to go.

As for doing a gas switch to air dill during deco reducing deco? Does it? Or is it just the case that we are forced to use outdated decompression software that doesn’t deal with He in a favourable way. I have a sneaking suspicion that if we didn’t live in such a litigations climate requiring millions to be spent on proving any idea safe, there would be a package on the market that had some radical reductions in stop time for keeping He in the mix. Arguably VPM is doing this but isn’t this is related to the bubble theory rather than anything else?

Whatever, it’s the path I am following at the moment. But as always I am open to ideas

ATB

Mark Chase

[DALESDIVER]

Basically what Mark said, is the correct procedure, STAY ON THE HELIUM!

Helium is a freindly gas, it reduces the amount of CO2 produced and off gases cleaner than N2, We have played about with different procedures and the best is Heliox all the way up, including bailout gases of 50/50 and 80/20.

99% of tables that we are using at current time run on the old idea that He is bad for us, so it gives you credit for jumping off it ASAP, the more modern approach, following research by Dr Bill Hamilton is it gets you out of the water quicker on a heliox mix than trimix, his tables for 10/90 have been dived and they get you on the boat while the computer wants to be at 15m...

Looking back to o/c, I used the GUE method and tables and gases, and was always out at least 20-25 minutes before the guys switching back to 32 @40 etc, I always switch to a trimix at deco, keep the N2 as low or lower than in the bottom mix and it works wonders.

As a comarison of gases, on the flying enterprise last year, two of the team dived 10/52 with linked 4th cell, I dived 10/90 linked in, and for the same bottom time I had two minutes different in my deco, but twice as clear a head, and less fatigue after the dive due to a cleaner off gassing due to no N2 in the mix, I dont flush the loop to pure, as its already at 87% on its own at1.4 at 6m, only time I run 100% is if driving manual, or had a very hard dive.

[Juz]

Cheers guys, it confirms what I already thought.

Juz

[vid]

Originally Posted by DALESDIVER

Looking back to o/c, I used the GUE method and tables and gases, and was always out at least 20-25 minutes before the guys switching back to 32 @40 etc, I always switch to a trimix at deco, keep the N2 as low or lower than in the bottom mix and it works wonders.

hi, this might not be the right forum (seeing as my question is specifically o/c), but i've never heard the GUE methods described in this way - what's the best source to get more info

vid

[Mark Chase]

Originally Posted by vid

hi, this might not be the right forum (seeing as my question is specifically o/c), but i've never heard the GUE methods described in this way - what's the best source to get more info

vid

I believe I am correct in saying:

This refers to the use of standard gas ranges for back gas and deco and then using the principles of ratio deco for your ascent profiles.



If you look at the PPNitrogen at each gas switch and compare with back gas PPN you will see the picture.

Ratio deco is a little more complicated but as I personally consider it dammed aggressive for general diving except for the unbendable few or young fit divers I wont post the details of how that works. If you use Decoplanner on 30/90GF it will give you the basic shallow profile but you must also consider the GUE deep stops and 5min (or is it 6? I forget) ascent from 6m to the surface.

Personally I used Decoplanner OC with 20/80GFs as I couldn’t give a toss about the extra 10-20mins in the water.

ATB

Mark Chase

[vid]

thanks mark, the maths makes sense...

having searched the web, i still haven't found much more on the ratio deco - presume it's covered in some of the later technical moducles of some training organisations?

vid

[And]

Originally Posted by vid

thanks mark, the maths makes sense...

having searched the web, i still haven't found much more on the ratio deco - presume it's covered in some of the later technical moducles of some training organisations?

vid

Hi

Its just a rule of thumb for working out a rough deco plan.

The idea is that, as you get used to doing similar dives over and over, that you get an idea of the deco you need to do. Added to that some small adjustments to take advantage of higher po2's in certain parts of the deco, you end up with a deco schedule.

It only works in a relatively small range of dives, and really only useful, in the form it is taught, to use with standard deco gasses, so you can't use it in the popular nonDIR '32%/80%' deco gas configuration.

You learn it in Tech1, which is only one step away from DIRF, so its at the beginner level really.

Andy

[aquaholic]

Vid,

Ratio deco is part of the GUE tech courses.
If you have a dig through the DIR forum it might be covered on their somewhere..

Anke

Originally Posted by vid

thanks mark, the maths makes sense...

having searched the web, i still haven't found much more on the ratio deco - presume it's covered in some of the later technical moducles of some training organisations?

vid