does it make sense to dive trimix in single tank

[TSandM]

I can absolutely guarantee that my headaches in Ginnie had nothing whatsoever to do with ascent rate; nor did my ascent rate differ on Monday's dive. Further, I know of no physiologic mechanism by which one would get headaches from ascents rates that are either too fast OR too slow. You DO get headaches from CO2 retention. I have had them from underbreathing, and I have had them from overworking on scuba. Helium clearly helps, and there is good theory to support why - download the DAN Technical Diving Symposium proceedings. One of the presentations was on flow characteristics in the bronchial tree. I thought it was fascinating.

 

[Kevrumbo]

It can be a dump question, but let me give it a shot anyway. I saw someone filling a single tank with trimix at LDS today. Don't know what the mix is, but my question is: does it make sense to dive trimixes for recreation with single tank? Say 30/30 or 21/35. Single tank will mean NDL/MDL dive. 30/30 is a interested one, MOD of 100ft, same as 32%, why use it at all, double or single? does it give you longer NDL/MDL time? 21/25 has MOD of 150ft, obviously limiting factor is single tank capacity. so people use it to do a bounce kind of dive to 150ft? What is the deal?

Helium has shorter NDL's than Air or Eanx 32 . . .so unless you're diving doubles with Trimix and Oxygen deco, it doesn't make practical sense to use helium mixes in a single tank by the above parameters.

Per the NAUI RGBM Deco Tables out since 2003:

Below are the comparative NDL's for Ean 32 and Helitrox; and example deco profiles for bottom times at 30m and 33m, for Ean 32 and Helitrox w/ & w/o O2:

Attached Files

• 
  RGBM_Eanx32_Helitrox_NDL.pdf (108.6 KB, 32 views)
• 
  RGBM_Eanx32_Helitrox_deco_30m.pdf (168.2 KB, 53 views)

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

 

[tflaris]

It can be a dump question, but let me give it a shot anyway. I saw someone filling a single tank with trimix at LDS today. Don't know what the mix is, but my question is: does it make sense to dive trimixes for recreation with single tank? Say 30/30 or 21/35. Single tank will mean NDL/MDL dive. 30/30 is a interested one, MOD of 100ft, same as 32%, why use it at all, double or single? does it give you longer NDL/MDL time? 21/25 has MOD of 150ft, obviously limiting factor is single tank capacity. so people use it to do a bounce kind of dive to 150ft? What is the deal?

Sure. Offshore Jax spearfishing. Mix to keep your head clear and 32 percent as a travel gas and deco gas if needed.

 

[rongoodman]

UTD treats 32% and 25/25 the same in the recreational NDL range, with Minimum Deco only required.

 

[PfcAJ]

I treat 3030 and 32 the same, personally.

 

[Rainer]

Ditto. It's what GUE teaches. I've had no issues with such profiles (with or without O2 acceleration).

I treat 3030 and 32 the same, personally.

 

[Kevrumbo]

UTD treats 32% and 25/25 the same in the recreational NDL range, with Minimum Deco only required.

I treat 3030 and 32 the same, personally.

Ditto. It's what GUE teaches. I've had no issues with such profiles (with or without O2 acceleration).

Sometimes even within a prescribed general deco profile, you have to tailor it to your own individual physiology. The GUE rule-of-thumb of diving 30/30 triox like Eanx32 utilizing MinDeco/ascent rules resulted in the "chokes" for me (I still had too much residual Helium in numerous microbubbles that overwhelmed my lungs' alveolar bed), after two repetitive dives to ave depth 24m with a 90min SIT a few years ago. I now use Oxygen for deco on the last repetitive dive when using recreational triox...


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.

 

[PfcAJ]

You get bent a lot, bro. What's up with that?

 

[Kevrumbo]

You get bent a lot, bro. What's up with that?

No . . .just that everyone likes bringing up the same episodes from years ago.

Again, For the Record:

One episode (2008) with acute/worsening symptoms upon presentation, clinically diagnosed and treated with HBOT [Hyperbaric Oxygen Therapy]:
Type I Bends Hit in Chuuk

One time suffering from the chokes after 30/30 triox repetitive dives with resolution of symptoms (shortness-of-breath); did not seek or require medical treatment (2006).

So officially two medical HBOT table 6 treatments stemming from Chuuk incident above in 2008, together with six to eight "practice sessions" to 2 ATA pressure, simulating patient treatment on dive accident victims, as part of the Catalina Hyperbaric Chamber Volunteer Crew Training Regimen 2009-10.

I'm not an expert . . . just an educated former patient and victim of past adversity & hubris who knows better now because of the experience. . .

 

[PfcAJ]

So like 1.5 times being bent?

Deco is highly questionable, lets face it. Its important to figure out what works for you. Glad you've figured it out.