Partial Pressure. Does he/she breaths more

[quoggle]

A question has arisen. Bl**dy sport diver tainees always asking questions.

Trimix

A diver is diving on a

50% Helium
40% Nitrogen
10% Oxygen

Partial pressure of Helium 0.5
Lowering the partial pressure of Oxygen to 0.1
Lowering the partial pressure of Nitrogen to 0.4


Oxygen is Toxic at 1.4 bar

My understanding (limited at best) is that they mix helium to lower the partial pressure of Oxygen from 0.2 to 0.1.

Now the question
Does the diver breathe more? (breaths per minute)
Does the diver get more Tired? and why?

Thanks Dave

[Garf]

Originally Posted by quoggle

Now the question
Does the diver breathe more? (breaths per minute)
Does the diver get more Tired? and why?

no, becuase they black out on the surface.

[IanRMartin]

In my limited understanding the helium is to reduce the Nitrogen (Narcosis)

[Steve Jones]

Oxygen is Toxic at 1.4 bar

Not correct

My understanding (limited at best) is that they mix helium to lower the partial pressure of Oxygen from 0.2 to 0.1.

Now the question
Does the diver breathe more? (breaths per minute)
Does the diver get more Tired? and why?

The mix that you suggest is not breathable at the surface (oxygen PO2 of 0.1)

However as you descend then this will of course increase (PO2 of 0.6 at 50 metres) and then be breathable, and as such will not need to take extra breaths, nor will the person get more tired.

Steve

[Ivon]

"Bloody instructors"

[Steve Jones]

Originally Posted by IanRMartin

In my limited understanding the helium is to reduce the Nitrogen (Narcosis)

Correct

[nigelH]

Originally Posted by quoggle

Trimix
A diver is diving on a
50% Helium
40% Nitrogen
10% Oxygen

As has been pointed out this is a hypoxic mix so you can only use it at depth. It also looks like cheap-and-nasty heli-air which is always the wrong mix for the depth.

Now the question
Does the diver breathe more? (breaths per minute)

No. The diver, and more significantly the diver's body, cannot sense the difference.

Does the diver get more Tired? and why?

No. Actually less. A nitrox at depth becomes more viscous and more work to breathe. Helium is generally much better for that.

The problem is narcosis. I'm out of my skull over 40m deep on air because the dissolved nitrogen is making a mess of my brain. Helium isn't so bad so that 50% of Helium means that at 60m (7bar total) I am only breathing 7*0.4=2.8bar of Nitrogen. That's the same as diving air to 25m and even I'm still reasonably together there.

HTH

[nickb]

Helium is normally added to displace enough nitrogen so the resulting blend will have an acceptable equivalent narcotic depth.

As we normally dive with a maximum partial pressure of oxyegen of 1.4 during the 'working' phase of the dive, diving deeper than 57m requires that a proprtion on the oxygen also be displaced, again helium is the gas normally used.

Human beings who normally live at or around sea level require a minimum PPO2 of around 0.16 to sustain life, so a bottom gas containing down to 16% O2 could be breathed during the entire dive, although decompression wouldn't be very efficient. Below 16% (apart from for a few Himalayan or Andean dwellers) could result in hypoxia if breathed for too long before reaching a sufficient depth to bring the PPO2 back to 1.6 (for 10/50 that's 6m)

[Janos]

Two things:

1) Most divers will drop the ppO2 by 0.04 whatever mix they are breathing provided that
2) they are breathing a mix with a ppO2 of at least 0.16.

So, on the surface,
- a diver breathing air (0.21) will breathe out air with a reduced oxygen content of around 0.17
- a diver breathing nitrox 32 (0.32) will breathe out nitrox with a reduced oxygen content of around 0.28

At a depth of 20m (3 bar)
- a diver breathing air (0.63) will breathe out air with a reduced oxygen content of around 0.59
- a diver breathing nitrox 32 (0.96) will breathe out nitrox with a reduced oxygen content of around 0.92

As long as they have enough oxygen (ie ppO2>0.16) then I would expect rate of breathing and tiredness to be unaffected. You might have trouble if you are exercising hard though.

Janos

[milldog]

Originally Posted by Garf

no, because they black out on the surface.

and drowned