Getting of 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

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decoweenie

This reminds me of someone said something about making sure the person who gives you advice on deco will be there to push your wheel chair for the rest of your life.

So this is only sharing information, not deco advice...

1. I stay on the same trimix diluent all the way to surface
2. but I compute with an air diluent switch at 30m
3. and I use 10/100 GF factors

These steps give me relatively shortest shallow deco stops except for going with bubble-model or increase the GF-high above 100.

The reason is a long time ago I too was believing in all of the theory "benefits" about getting off Helium as soon as possible, and did the diluent switch on ascent, etc. One day, I just felt lazy and didn't do the switch, but followed the same table. Felt same or better afterward so just did that ever since.

Remember that deco profile is just a solid line drawn thru a gray zone to follow. You could be anywhere within said gray zone...

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aquaxel

i do similar.

use bottom mix all the way up. then goto high o2.

if necessary use o2 breaks on long o2 deco (12min. on / 6min. off ...) for the 6min. i use deepest non hypoxic mix. - lowest po2

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Mark Chase

I already put this on YD but it might get more discussion here:

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

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Scuba_Vixen

Excellent Post! Very well expressed.

If you look at Bruce Weinke's NAUITEC tables, they ride back gas up to the O2 switch. You get better contingency planning with 50/50 or 50/25 as a 70' bottle but the idea is still the same, better to have helium in the mix than nitrogen.

In Rebreather speak, we just have to raise the PO2 during deco and flush to O2 @ 6m and we've done the same thing.

A lot of OC divers have switched from Ean50 for the 70' bottle to 50/50 heloix or 50/25 Tx and report that they seem to feel much better post dive.


Stay on that path,


Darlene



Nitrogen .... The *Real* devil's gas

wrecked

Yep it does cause it, I was on a 70mtr dive last year on OC and switched over too 32% at 40mtrs from a 17/45 mix and I was violently ill about 2-3mins after. I felt better though I had some vertigo for the rest of the deco.

wizbang

But Wrecked, you do have a special set of circumstances. Your experience may be confined to a very small percentage of the population.

I find the use of air for flushing is ruled out by what I take for bailout. I plan on either a standard set of bailouts or a "best gas choice" and either way air dosn't make the list (very rarely) (for the depth or deeper and duration you specified).

That means I am running He all the way until O2 flushing.

Scuba_Vixen

I ran this profile in V-Planner to see what the PN2 (ead) looked like ....... No wonder you felt like shit! Look at the nitrogen slam when you went on 32%. Ead goes from 15m to 32m, and doesn't drop below the 15m level untill 8 more stops.


Dec to 70m (3) Trimix 17/45 18m/min descent.
Level 70m 16:07 (20) Trimix 17/45 1.35 ppO2, 28m ead, 34m end
Asc to 45m (22) Trimix 17/45 -9m/min ascent.
Stop at 45m 0:13 (23) Trimix 17/45 0.93 ppO2, 16m ead, 20m end
Stop at 42m 1:00 (24) Trimix 17/45 0.88 ppO2, 15m ead, 19m end
Stop at 39m 1:00 (25) Nitrox 32 1.56 ppO2, 32m ead
Stop at 36m 1:00 (26) Nitrox 32 1.46 ppO2, 30m ead
Stop at 33m 1:00 (27) Nitrox 32 1.37 ppO2, 27m ead
Stop at 30m 1:00 (28) Nitrox 32 1.27 ppO2, 24m ead
Stop at 27m 1:00 (29) Nitrox 32 1.18 ppO2, 22m ead
Stop at 24m 1:00 (30) Nitrox 32 1.08 ppO2, 19m ead
Stop at 21m 1:00 (31) Nitrox 32 0.99 ppO2, 17m ead
Stop at 18m 3:00 (34) Nitrox 32 0.89 ppO2, 14m ead
Stop at 15m 3:00 (37) Nitrox 32 0.80 ppO2, 12m ead
Stop at 12m 4:00 (41) Nitrox 32 0.70 ppO2, 9m ead
Stop at 9m 7:00 (48) Nitrox 32 0.61 ppO2, 6m ead
Stop at 6m 7:00 (55) Oxygen 1.60 ppO2, 0m ead
Stop at 3m 12:00 (67) Oxygen 1.30 ppO2, 0m ead
Surface (67) Oxygen -9m/min ascent.


Here's the same dive using 30/30 Tmx instead of Ean32. About 8 mins longer runtime, but look at the smooth decline to the PN2 (ead)

You'd get back on the boat after this dive feeling ready for the next one.

Dec to 70m (3) Trimix 17/45 18m/min descent.
Level 70m 16:07 (20) Trimix 17/45 1.35 ppO2, 28m ead, 34m end
Asc to 45m (22) Trimix 17/45 -9m/min ascent.
Stop at 45m 0:13 (23) Trimix 17/45 0.93 ppO2, 16m ead, 20m end
Stop at 42m 1:00 (24) Triox 30/30 1.55 ppO2, 16m ead, 26m end
Stop at 39m 1:00 (25) Triox 30/30 1.46 ppO2, 15m ead, 24m end
Stop at 36m 1:00 (26) Triox 30/30 1.37 ppO2, 13m ead, 22m end
Stop at 33m 1:00 (27) Triox 30/30 1.28 ppO2, 12m ead, 20m end
Stop at 30m 1:00 (28) Triox 30/30 1.19 ppO2, 10m ead, 18m end
Stop at 27m 1:00 (29) Triox 30/30 1.10 ppO2, 9m ead, 16m end
Stop at 24m 2:00 (31) Triox 30/30 1.01 ppO2, 7m ead, 14m end
Stop at 21m 2:00 (33) Triox 30/30 0.93 ppO2, 6m ead, 12m end
Stop at 18m 3:00 (36) Triox 30/30 0.84 ppO2, 4m ead, 10m end
Stop at 15m 5:00 (41) Triox 30/30 0.75 ppO2, 3m ead, 7m end
Stop at 12m 6:00 (47) Triox 30/30 0.66 ppO2, 1m ead, 5m end
Stop at 9m 9:00 (56) Triox 30/30 0.57 ppO2, 0m ead, 3m end
Stop at 6m 7:00 (63) Oxygen 1.60 ppO2, 0m ead
Stop at 3m 13:00 (76) Oxygen 1.30 ppO2, 0m ead
Surface (76) Oxygen -9m/min ascent.


If you're using Tmx dil, I can't imagine not staying on it untill the 6m O2 flush.



Darlene

Deepwreck

I won't say anything about OC so please apply this to CCR only.

I found that the software is penalizing for the use of HE. I also know that all the data is coming out in favor of HE, and what the computers are doing is this. They are giving you credit for the ongassing of HE and then when the time comes to offgas they are much slower that they shoud be.

I dive this way, and I would also say that you shouldn't dive anything until you are ready and well qualified.

For a dive to 250' I would use a lot of HE and would stay on the mix throughout the dive. I also know that if my Computer thinks that I'm still on HE then the offgassing is much slower that air would be. (go figure air offgassing faster that HE) So at around 100' I would tell my computer that I made a switch to Air, and stay on the same mix. (no flushing) The computer or Tables will reduce the time and get you out of the water faster and cleaner. In this plan I have felt better and I dive cold, dark water, for the wrecks.

To much N is a bad thing, and switching to N would help for deco, but also put you at rick for other bad things. Inerear.

Decodiver

Some very good comments above, especially Darlene's demonstrated Nitrogen slam.

I stay with He surface to surface, sub 100m dives I will change from say a 6/72 dil to a 20/30 at 45 metres but would never flush with air.

Just my Euro two cents worth.

Cheers,

Dave Cooper.

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