Helium and deco obligations

[Rhone Man]

There have been a few different threads recently canvassing whether helium can cut your deco obligations as well as help with narcosis, so I thought a thread on that topic might be a better place to bottom out the issues.

From the comfort of my armchair I ran three dives on my planning software, each to 180 feet for 30 minutes, and each using EANx40 as a deco gas.

On air: total runtime comes to 1 hour, 55 minutes.
On trimix 21/40: total runtime comes to 1 hour, 32 minutes.
On heliox (21% O2): total runtime comes to 1 hour, 41 minutes.

I then ran three more dives, each to 230 feet for 20 minutes, again using EANx40 as a deco gas.

On hypoxic (19% O2) air (I know, why would you?): total runtime comes to 2 hours, 02 minutes.
On trimix 19/55: total runtime comes to 1 hour, 36 minutes.
On heliox (19% O2): total runtime comes to 1 hour, 46 minutes.

And I didn't even have to rinse my gear down afterwards.

 

[Gilldiver]

Not all that surprising.

Here is another one:

1) Do a dive to say 160' for 30 minutes on air and any deco gas 32, 36, 50, or 100% Use any one of them but only one.

2) Now do the same dive with 21% O2 and any percentage of He up to 35% and the same deco gas as in #1

3) Compare run times. Both should be within 2 or 3 minutes of each other.

This works for any He% less then 35% as long as the total of the inert gasses equal 79% (He% + N2%=79%) and you keep the O2 at 21%. This is how we started diving He mixes in the early 90's on air tables. We dove it as air and used O2 for the 20 and 10 foot stops.

 

[ianr33]

What kind of Farm Animal Stroke are you only using one deco gas on a 230 foot dive ?

If you put some Helium in the deco mix the run time will be longer (With 40% nitrox as the deco gas you are not absorbing any He at all,so deco time is less)

About the only thing new I learnt from my Trimix class was how run times are shorter using nitrox for deco when the backgas is trimix

Where did you decide on doing your Advanced Nitrox?

 

[Rhone Man]

What kind of Farm Animal Stroke are you only using one deco gas on a 30 foot dive ?

If you put some Helium in the deco mix the run time will be longer (With 40% nitrox as the deco gas you are not absorbing any He at all,so deco time is less)

About the only thing new I learnt from my Trimix class was how run times are shorter using nitrox for deco when the backgas is trimix

Where did you decide on doing your Advanced Nitrox?

The joys of only taking e-risk by doing unsafe diving practices on the computer... ! Another thing I learned playing around that was weird to my mind, is that the leaner your deco gas (assuming you use only one) it seemed to lower your total runtime because you switched gases deeper. Old hat to veteran tec divers I'm sure, but counter intuitive to me.

Doing my Adv Nitrox in Feb in Dominican Republic with Pirates Cove. I was supposed to be doing it with John Chatteron, but sadly he just e-mailed to say he has to pass me on to somebody else because he has a conflicting commitment. Still looking forward to learning a lot. Maybe John will be around if I later progress to trimix.:depressed:

 

[texdiveguy]

The joys of only taking e-risk by doing unsafe diving practices on the computer... !

Doing my Adv Nitrox in Feb in Dominican Republic with Pirates Cove.

Amen to the e-diving!!

This He game is the old story of you can't have the whole pie.

Good luck on the class.

 

[Chernish]

The joys of only taking e-risk by doing unsafe diving practices on the computer... ! Another thing I learned playing around that was weird to my mind, is that the leaner your deco gas (assuming you use only one) it seemed to lower your total runtime because you switched gases deeper. Old hat to veteran tec divers I'm sure, but counter intuitive to me.

I am certainly not a veteran tec diver, but maybe I can make it more intuitive to you

Helium is the main responsible for the amount of deco.
By switching to a Helium-free mix, you maximize the He gradient and thus maximize the speed at which Helium is leaving your body.

It is the same reasoning than when you switch from air to O2 to do your deco, with Helium instead of Nitrogen: you want to take it as soon as possible during your deco. The limits are drawn by the immediate physiological effects (hyperoxia, narcosis).
Of course in the case of Nitrox you also have to take into account that you are loading your body with another inert gaz.

Cheers!