Recreational Trimix

[mumphrey]

I did a recreational double dip on the Spiegel Grove today. Decided to use my doubles which were already loaded up with 30/30. Figured displacing all that evil nitrogen with heavenly helium would give me nice, long no decompression limits. Did that ever turn out to be a stupid move!

As it turns out, 30% NITROX would have yielded a NDL of 26 minutes at 100' vs. 14 minutes on the relatively pricy trimix. My "stoke" buddies got a 28 minute NDL on their 32% Nitrox and I ended up doing a lot of deco time on my fancy back gas.

I'm fully aware that the primary benefit of increased helium percentages is reduced narcosis as opposed to increased NDL bottom time. However, I was shocked to discover that more helium actually dramatically reduced NDL limits all other things being held constant.

Can someone please explain this to me?

 

[rjack321]

pick a different model.

 

[deepstops]

I did a recreational double dip on the Spiegel Grove today. Decided to use my doubles which were already loaded up with 30/30. Figured displacing all that evil nitrogen with heavenly helium would give me nice, long no decompression limits. Did that ever turn out to be a stupid move!

As it turns out, 30% NITROX would have yielded a NDL of 26 minutes at 100' vs. 14 minutes on the relatively pricy trimix. My "stoke" buddies got a 28 minute NDL on their 32% Nitrox and I ended up doing a lot of deco time on my fancy back gas.

I'm fully aware that the primary benefit of increased helium percentages is reduced narcosis as opposed to increased NDL bottom time. However, I was shocked to discover that more helium actually dramatically reduced NDL limits all other things being held constant.

Can someone please explain this to me?

I used to use quite a bit of 30/30 but I've backed away from it because I couldn't justify the cost.

As far as why the deco is shorter, helium goes into your tissue faster but it also comes out faster which is why your ascent rate should be slower but you don't necessarily stay in the water longer.

Generating tables on Vplanner will almost always give you shorter deco times on helium based gases.

 

[TSandM]

As Richard says, different models treat helium differently. Some penalize you for it, some give you credit for it -- but all, I think, require a careful and slow ascent to control bubbling. It really depends on whether the model is worried about rapid diffusion or recognizing that helium's poor solubility means you end up with very little of it to offgas.

 

[lamont]

Just dive 30/30 like its 32% -- although safety stops or minimum deco become mandatory (3-5 minutes from 50 feet at least -- and then whatever the 32% tables call for on top of that).

 

[Kevrumbo]

I did a recreational double dip on the Spiegel Grove today. Decided to use my doubles which were already loaded up with 30/30. Figured displacing all that evil nitrogen with heavenly helium would give me nice, long no decompression limits. Did that ever turn out to be a stupid move!

As it turns out, 30% NITROX would have yielded a NDL of 26 minutes at 100' vs. 14 minutes on the relatively pricy trimix. My "stoke" buddies got a 28 minute NDL on their 32% Nitrox and I ended up doing a lot of deco time on my fancy back gas.

I'm fully aware that the primary benefit of increased helium percentages is reduced narcosis as opposed to increased NDL bottom time. However, I was shocked to discover that more helium actually dramatically reduced NDL limits all other things being held constant.

Can someone please explain this to me?

From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth:

Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

http://www.scubaboard.com/forums/ask-dr-decompression/32090-helium-offgassing-rate-2.html

 

[bletso]

I use TX30/30 for most of my dives, but then with a rebreather it's very cost effective. As pointed out, different planers will give different profiles. Keep in mind though that slow ascents are mandated with He mixes.

Dale

 

[waterpirate]

This is where the disscussion of standard gasses vs. ideal gasses really gets interesting. If you had doubles loaded with 30/30 you could have planed your bottom time and done the deco. Or you could have bled some gass off and remixed to 32/20 or 32/25 which prolly would have been the "ideal mix" planned your bottom time and moved on.
I prefer the "ideal gas" route but that is just me. I also see the advantage of standard mixes for simplicity. I tell people all the time when talking about annylisers that you have to pick the annyliser you are going to trust. I trust my personal over the lds. The same is true of deco models and computers. You have to pick the one you choose to trust, regardless of what other peoples models yeild. From a team approach it should have been disscussed pre-dive and decided.
Not trying to be offensive but if you are diving 30/30 you should have asked and answered your own querry in class work for HE 101.
Eric

 

[Wookie]

Just dive 30/30 like its 32% -- although safety stops or minimum deco become mandatory (3-5 minutes from 50 feet at least -- and then whatever the 32% tables call for on top of that).

I think that is very dangerous advice. 30% He is too much to be ignored, or treated like nitrogen. Ask anyone who has suffered from a He bend if you can just blow off 30% He in a mixture. YMMV.

For the OP, I learned the same lesson on the Vandenberg last year. I had a bunch of 18/45 in my doubles I wanted to get rid of. I'd have spent a lot less time on deco diving air...

 

[GShockey]

I think that is very dangerous advice. 30% He is too much to be ignored, or treated like nitrogen. Ask anyone who has suffered from a He bend if you can just blow off 30% He in a mixture. YMMV.
.

That is not what Lamont said. Read his post again. He was very specific about ascent rates and deco.