Mixed Gasses - Basic Question

[The Kraken]

I've a basic question for those of you who dive tri-mixed breathing gas.
When developing a mixture for a specific depth I understand that you will plan your PO2 not exceed, say, 1.4 for that particular depth in order to avoid oxygen toxicity.

Now, when some of the nitrogen is replaced by helium in order to avoid nitrogen narcosis, do the same factors apply to the determination of the percentage of the gas? IE, do you look for a fG of the nitrogen at a specific percentage and then add the amount of helium so that you'll get the PG of nitrogen based on the same depth?

This is probably a very convoluted way to ask the question, but it's be best I can do at the time.

 

[dagodiver]

That is a perfect way to ask.
yes i set my EAD at around 100' so no matter how deep i go it will feel like a 100' dive according to the Nitrogen.
this equates to a 3.18pp of Nitrogen.
Hope this helps.
MM

Edit to fix o2 thanks.....

 

[The Kraken]

Thanx, Dago,
I've not advanced beyond the standard nitrox course. I've just been reviewing the physis of the gasses and am now really starting to UNDERSTAND how everything works in relationship to everything else.

My thinking was, even if flawed from ignorance, that if one set the Oxygen pressure to replicate a shallower dive, then one would do the same with the Nitrogen and just replace the displaced Nitrogen with Helium.

Thanx much . . .

Don't worry. I'm not going to be doing any diving that would require such mixes until I am trained satisfactorily.

Dennis

 

[novadiver]

That is a perfect way to ask.
yes i set my EAD at around 100' so no matter how deep i go it will feel like a 100' dive according to the Nitrogen.
this equates to a 3.18ppo2 of Nitrogen.
Hope this helps.
MM

It's PPN2

 

[The Kraken]

"this equates to a 3.18ppo2 of Nitrogen"

"It's PPN2"

Yep, it would be, wouldn't it?

 

[grazie42]

Kracken said:

My thinking was, even if flawed from ignorance, that if one set the Oxygen pressure to replicate a shallower dive, then one would do the same with the Nitrogen and just replace the displaced Nitrogen with Helium.

Maybe I´m misunderstanding you and the answers to your question so I´ll just try to explain how I plan/mix my mix and leave it to you (and others) to determine if the way I put it makes sense...

When planning a dive I (for bottom mix):
-Determine "maximum" depth, which will determine O2% (as when diving nitrox with a PO2 of 1,4)
-The % 02 gives me an EAD/EAN. I like my EAN @ 90-120 ft depending on the dive, sometimes even lower.
-The difference between desired EAN (90-120 ft) and the actual EAN of the "nitrox" determines the %He of the final trimix as Nitrogen is replaced by He.

To my mind your thinking was not flawed but rather correct about He replacing Nitrogen...
Hope this doesent confuse more than it helps...

 

[msandler]

I've a basic question for those of you who dive tri-mixed breathing gas.
When developing a mixture for a specific depth I understand that you will plan your PO2 not exceed, say, 1.4 for that particular depth in order to avoid oxygen toxicity.

Now, when some of the nitrogen is replaced by helium in order to avoid nitrogen narcosis, do the same factors apply to the determination of the percentage of the gas? IE, do you look for a fG of the nitrogen at a specific percentage and then add the amount of helium so that you'll get the PG of nitrogen based on the same depth?

This is probably a very convoluted way to ask the question, but it's be best I can do at the time.

Yes, He displaces O2 and N. You mix for a target oxygen and nitrogen partial pressures (as you described). The reason you do not want to go overboard with He is, it will actually prolong Deco on shorter dives. Though it is not readily soluble in blood, it is about 3 times the rate of absorbtion in tissue compartments.

 

[FreeFloat]

Yes, He displaces O2 and N. You mix for a target oxygen and nitrogen partial pressures (as you described). The reason you do not want to go overboard with He is, it will actually prolong Deco on shorter dives. Though it is not readily soluble in blood, it is about 3 times the rate of absorbtion in tissue compartments.

So He is biologically inert (which is the whole point of course) therefore you ongas and offgas it in similar fashion to N, just quite a bit faster.

Would it be theoretically possible to mix a gas requiring less N deco time than He deco time??

Please forgive the newbie question............. I'm not deco trained either

 

[O-ring]

Yes, He displaces O2 and N. You mix for a target oxygen and nitrogen partial pressures (as you described). The reason you do not want to go overboard with He is, it will actually prolong Deco on shorter dives. Though it is not readily soluble in blood, it is about 3 times the rate of absorbtion in tissue compartments.

Where are you getting this information that He takes longer to deco from?

 

[Charlie99]

Where are you getting this information that He takes longer to deco from?

Please note that msandler said "The reason you do not want to go overboard with He is, it will actually prolong Deco on shorter dives" (emphasis added).

If you search on "mass transport limit" you might find further info and perhaps an explanation by BRW.

edit: added link--- mass transport limit