Recreational Trimix classes

[detroit diver]

narcT once bubbled...


Why can't you dive 27/17 at 150 ft? PPO2 is only 1.5

I'm not being a smart a*%, but what are the physiological advantages of using such a high level of helium rather than say 17?

Or should I just shutup and take a trimix class

Conventional thought is that ppo2 should be no higher than 1.4.

The advantages of higher helium is that you have less nitrogen to breathe.

 

[baenglish73]

How do these numbers work? :dunce:

What is a 27/27.

Then what is an 80/20 (Strokemix)? :reaper:
I may have interpreted why it is bad, but the lingo is beyond me.

 

[Charlie99]

detroit diver once bubbled...
The advantages of higher helium is that you have less nitrogen to breathe.

If the goal is to have less nitrogen, why not go all the way and have a 30/70 heliox mixture?

(I don't want to hear about costs, but rather the relative desirability of heliox vs trimix for 150' near NDL dives).

Charlie

 

[RichLockyer]

Charlie99 once bubbled...
If the goal is to have less nitrogen, why not go all the way and have a 30/70 heliox mixture?

Cost outweighs the benefit.

 

[detroit diver]

baenglish73 once bubbled...
How do these numbers work? :dunce:

What is a 27/27.

Then what is an 80/20 (Strokemix)? :reaper:
I may have interpreted why it is bad, but the lingo is beyond me.

The first number is your oxygen content, the second your helium content, in percentages.

Ie. 21/35 would be 21 percent O2 and 35 percent He. Nitrogen is not represented in this number.


80/20 would be 80 percent O2 and 20 percent He.

 

[detroit diver]

He didn't want to hear about costs, so that's why I didn't respond!!!

RichLockyer once bubbled...

Cost outweighs the benefit.

 

[RichLockyer]

narcT once bubbled...
Why can't you dive 27/17 at 150 ft? PPO2 is only 1.5

Take a trimix class and a current nitrox class.
Current limit for PPO2 is 1.4 for the working portion of the dive and 1.6 for decompression. It is wise to go even lower, especially for extended exposures or repetitive dives. 1.0-1.2 would be a good limit for a week long liveaboard.

Drop the abject fear of decompression. There isn't some magic bottom time that will bend you... below traditional NDL, your ascent rate is your decompression stop. Beyond NDL, your ascent rate is simply slowed further. If you were to go to 100ft for 10 minutes and immediately surface, you would probably get bent, though you are well within NDL.

Once we drop the fear of decompression, we realize that we no longer need to optimize our O2 level for the specific dive. We can now dive a standard mix based on planned depth RANGE... 0-100ft, EAN32. 70-120ft, 30/30. 70-150, 21/35. 150-200, 18/45. 200-250, 15/55. 250-400, 10/70.
This means that:
1 - We don't get stuck with a tank that's "too hot" for the dive if conditions required a change in destination/site.
2 - We reduce our exposure to high pressure O2
3 - We can bank EAN32 and fill our tanks (except for the 30/30) by simply priming with helium and topping with EAN32. There is never a need to blend pure O2 directly into our tanks, and the bank can be filled from a CF blender.

 

[RichLockyer]

baenglish73 once bubbled...
What is a 27/27.

Then what is an 80/20 (Strokemix)?

27% oxygen, 17% helium.
80/20 is a unique case, which refers to 80% nitrox.

 

[DeepScuba]

Rich's post brings me to a question I've had for a while now. Not so much a question, but more than I just want some input to here the pro's and con's.....don't mean to hijack the thread.....

I used to opt for one long(er) deco dive on a site, vs two short(er) deco dives.

I then came across George Irvine's rant about doing two "milder" deco dive lessens the chance of a DSC vs one longer one due to the greater increase of deco time on the single longer dive.

My thought was alwasys centered around doing one longer one, doing the required deco and skipping the compressing/decompressing follow by compressing/decompressing for the second dive. There again, my thinking was, only doing it once was "kinder" to my body.

I've since changed to his philosophy, also noting I get more actual (total) bottom time and spending less time (per dive) getting "deep" into the "Deco-Zone".

Thoughts?

 

[Charlie99]

detroit diver once bubbled...
He didn't want to hear about costs, so that's why I didn't respond!!!

What software do you use to plan your dives?

I notice that for example, on a 140' 15 minute dive, that V-Planner gradually increases the deco requirements as the He% increases, with heliox deco times being signficantly longer than 30/30 or even 30/50.

This implies to me that eliminating all N2 is not an ideal goal.

What does your software or tables say?