Trimix Versus HeliOx and Deco time

JS1scuba

Heliox vs Trimix on Constant Set Point CCR diving is much less of a problem than when you compare Heliox vs Trimix on OC. While I would not run a schedule like the one you describe here switching to a 1.6 atm PO2 at 6 meters it's a sure way of not getting bent. It's old school approach to decompression. (if using Buhlmann) Ascend to 45 msw (expand bubbles) do some decompression then at 6 msw treat with oxygen. Since we know this is agressive if they shaved it all down to 200 min total run time they were probably fine.

Here is a link to a PDF where I make the comparison of

10/90 heliox 1.3 set point Hamilton Kenyon Bubble Model HKBM
16/45 trimix 1.3 set point Hamilton Kenyon Bubble Model HKBM
10/90 heliox 1.3 set point / 100% oxygen Buhlmann (raw)

http://www.nitroxdiver.com/Training/...le-compare.pdf

It's pretty clear that using a bubble model for this helps keep the CNS down, and gives a nice clean decompression. The differences between the heliox and the trimix decompression on the bubble model are reasonable.

Profiles generated with NAUTILUS DIVE PLANNER

Cheers

__________________
Joel Silverstein, VP COO
Tech Diving Limited a Division of Scuba Training and Technology Inc.

http://www.nautilusdiveplanner.com
FREE NAUTILUS DIVE PLANNER DEMO - REGISTER HERE
KISS # 275

diveoceanos

Hi Lance,

It is not possible actually to plan a dive at 75m for a little more than 200 mins under any circumstances. (Given there is only helium as inert gas in your loop, i.e Heliox).

I played a little bit with a self-developed deco software and here is what I found:

It would be possible to make this dive if the following conditions are met. Unfortunatelly these are lying outsite the commonly acceptable level of risk for the prudent diver:

1. Plan the dive with a descent rate of 10 mpm (You are at 75m at minute 8).
2. Stay at the bottom until 60th minute run time (stay down for 52 minutes).
3. Ascent rate at 10 mpm also.
4. Maintain PPO2 at 1.8 and switch to 1.6 at 8 meters (Would you accept this as a calculated risk??).

You will end up with something like 205-210 minutes. And that is Modified Buhlmann (no GFs). For Helium I would like to add some GFs though. Therefore unless if you are a super trooper diver and ready to accept the risk of getting your brain cells oxidised and rusted you wouldn't be able to do this dive in 3,5 hours.

Cheers,
Sotos

solocavediver

Hi,

I stuck this thru a VPM program. It will do the dive, but only if you imitate a Trimix dive by switching fairly soon to air. Here's a profile I came up with (NB it ain't recommended, it's VPM with zero conservatism, plus as the program rightly shows you have to worry about ICD with the sudden switch to air)

Cheers,

Charles.



Conservatism 0
Leading compartment enters the decompression zone at 60.3m

Depth Run Time Stop Time Mix ppO2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~
75.0 m 4.2 min 0.0 min Tx18/82 0.18 - 1.50 - 1.50 Segment ppO2 high!
75.0 m 54.2 min 50.0 min Tx18/82 1.50 - 1.50 - 1.50 Segment ppO2 high!
51.0 m 57.0 min 1.0 min Air 1.50 - 1.08 - 1.26 ICD Warning!
48.0 m 58.0 min 1.0 min Air 1.26 - 1.20 - 1.20
45.0 m 59.0 min 1.0 min Air 1.20 - 1.14 - 1.14
42.0 m 61.0 min 2.0 min Air 1.14 - 1.07 - 1.07
39.0 m 62.0 min 1.0 min Nx32 1.07 - 1.01 - 1.54
36.0 m 64.0 min 2.0 min Nx32 1.54 - 1.45 - 1.45
33.0 m 67.0 min 3.0 min Nx32 1.45 - 1.36 - 1.36
30.0 m 70.0 min 3.0 min Nx32 1.36 - 1.26 - 1.26
27.0 m 73.0 min 3.0 min Nx32 1.26 - 1.17 - 1.17
24.0 m 78.0 min 5.0 min Nx32 1.17 - 1.07 - 1.07
21.0 m 83.0 min 5.0 min Nx50 1.07 - 0.98 - 1.53
18.0 m 89.0 min 6.0 min Nx50 1.53 - 1.38 - 1.38
15.0 m 99.0 min 10.0 min Nx50 1.38 - 1.24 - 1.24
12.0 m 111.0 min 12.0 min Nx50 1.24 - 1.09 - 1.09
9.0 m 128.0 min 17.0 min Nx50 1.09 - 0.94 - 0.94
6.0 m 150.0 min 22.0 min Oxygen 0.94 - 0.79 - 1.59
3.0 m 189.0 min 39.0 min Oxygen 1.59 - 1.29 - 1.29

Drmike

me

__________________
Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you

Drmike

My approach to deco planning is: in areas we know very little, its best to be conservative.

Ergo, deep and bounce dives, Heliox dives (or near as damn it) I do very conservative deco. Who cares if I have to stay in the water a few hours longer than may be necessary- when you look at the risk V reward of a bad bend a few hours is not at all significant in my opinion.

I suspect habitual chasing of shortest deco times will greatly increase risk of injury eventually where as carrying out conservative deco times just means some bastards more likely to have ate all your damn food! (be careful when using hobbits as support divers!]

__________________
Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you

fireman

Dr. Mike--no old bold divers, absolutely.

This subject is interesting, I wonder what the comercial and military does, I believe they have both been using heliox for a long time. I'm sure rubicon has some info on the subject. I also believe that the models that we use are not really "good" with heliox. I suspect that it may be possible to do a profile with essentially a long slow descent--lots of short deep stops, and do a safe profile with time similar or shorter than a trimix profile. But I not going to do the research on myself by trying it. I also "wonder" about agressive gas switching, if it is possible to toggle back and forth between N mixes and heliox and avoid IBCD. I believe there should be significant advantages to heliox on CCR's, while avoiding open circuit disadvantages such as cost and multiple gas mixes.

__________________
Heres to you Capt. Bill
Never Forget, and stay safe everyone.

Drmike

If diving ccr Heliox (and avoiding risk of IBCD) your bail gas may also have to be Heliox so you may need a good sized booster - and cash. On deep dives and rapid descents Heliox can lead to HPNS so adding some N2 may be beneficial. Descending at 10m/min on a deep dive - no thanks, Ill live with EAD of 30m ish

__________________
Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you

Drmike

see Page 17B-45 Onwards in attached usn MK16 MOD1 deco tables for ccr Heliox dives setpoint 1.3


and

Rubicon Research Repository: Item 123456789/3552


looking at over 300mins for this dive (compared to 200mins quoted)


One point to bare in mind, somepeople have surfaced missing hours of deco and survived...doesnt mean to say you can or that the majority can or that they can next time....

Attached Images

manual-USNMk16.pdf (713.9 KB, 21 views)

__________________
Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you

teomannaskali

they also eat your food.

diveoceanos

Some results from my personal software are:

Depth: 75m, Time to Leave Bottom: 60th minute.
Descent Rate: 10 mpm, Ascent Rate: 10 mpm, Constant PP02: 1.3 ATA, Switch to pure Oxygen at 6m.

Diluent: Tx 16/45

Total Run time ----> 230 minutes (no GF)
Total Run Time ----> 260 minutes (GFlow: 50%)

Diluent: Heliox 10/90

Total Run Time ----> 310 minutes (no GF)
Total Run Time ----> 370 minutes (GFlow: 50%)