Theoretical question about diving with Helium

[SouthEastFloridaDiver]

I'm just starting to learn about diving this trimix, and I have a rather theoretical question about diving with helium:

At sea level, the body is saturated with nitrogen equivalent to a partial pressure of 0.79 ATA. During decompression, the nitrogen levels have to be brought back again to some value close to the 0.79 ATA saturation, so that no bubbles can form.

For helium, the story is different: For all practical purposes, the body is saturated with helium equivalent to a partial pressure of 0.0 ATA (because there is close to zero helium in air), meaning there is no helium at all in body tissue. This means that during decompression, the helium levels have to be brought close to zero, so that no bubbles can form.

Is this correct, and is this one of the main reasons why diving with helium in the breathing mix has longer deco times than with air? Also, if in theory I would breath a mix with high helium content at sea level for a few hours or so, and then would suddenly stop, could I get DCS? I know that this has no real practical relevance, but I just want to see if I understand the physics correctly.

 

[david.tolan]

First, I am not a decompression diver, so this is conjecture based on a vague knowledge of physics and maths, but ...

Bubbles form because your blood is super saturated. Pressure at depth allows this level of Nitrogen or Helium to dissolve in your blood. When you release the pressure, it comes out of solution, eg opening a bottle of coke. (PADI DIVING 101)

Logically, it must be irrelevent what gas maintains the pressure on you. Some divers use Argon to fill their drysuits, then Argon maintains the pressure on them. The partial pressure of Nitrogen in their drysuits would be 0, The partial pressure of Nitrogen is their blood is high. Yet no formation of bubbles.

Also, if it were partial pressure in breathing gas was important for bubble formation, you could not decompress with Pure O2. Nitrogen in your blood is high, Nitrogen in your breasing gass is 0, yet there is no sudden formation of bubbles.

So partial pressure is not responsible for bubble formation.

The properties of Helium may be very different from Nitrogen, in terms of how long it takes to come in and out of solution in your blood, but the fact that there is almost 0 helium in air should not effect bubble formation.

Partial pressure differences will effect how quickly anything in solution can cross a membrane, but that's a whole other story. That's the reason that decompression on pure O2 is faster than decompression on nitrox or air.

Specifically, according to Fick's law the amount of material passing through a membrane is proportional to the difference in concentration between the two sides of the membrane.

If you breath a helium mix at the surface for several hours, you will disolve nitrogen in your blood. When you stop breathing it, there is no sudden drop in pressure. So no sudden formation of bubbles. It will pass out of your system slowly because of the partial pressure gratient, but I can't see it bubbling.

What do you decompression divers thing ?

D

<SNIP>
Is this correct, and is this one of the main reasons why diving with helium in the breathing mix has longer deco times than with air? Also, if in theory I would breath a mix with high helium content at sea level for a few hours or so, and then would suddenly stop, could I get DCS? I know that this has no real practical relevance, but I just want to see if I understand the physics correctly.

 

[cdiver2]

I'm just starting to learn about diving this trimix, and I have a rather theoretical question about diving with helium:

At sea level, the body is saturated with nitrogen equivalent to a partial pressure of 0.79 ATA. During decompression, the nitrogen levels have to be brought back again to some value close to the 0.79 ATA saturation, so that no bubbles can form.

For helium, the story is different: For all practical purposes, the body is saturated with helium equivalent to a partial pressure of 0.0 ATA (because there is close to zero helium in air), meaning there is no helium at all in body tissue. This means that during decompression, the helium levels have to be brought close to zero, so that no bubbles can form.

Is this correct, and is this one of the main reasons why diving with helium in the breathing mix has longer deco times than with air? Also, if in theory I would breath a mix with high helium content at sea level for a few hours or so, and then would suddenly stop, could I get DCS? I know that this has no real practical relevance, but I just want to see if I understand the physics correctly.

There are a lot of mixed gas, ccr deco divers on it
http://www.yorkshire-divers.co.uk/forums/index.php?

 

[BigJetDriver]

Okay, folks, let me see if I can boil down some elements of this to simple terms, because, in the end, it is NOT simple.

(1) It does not matter what inert gas you use. It will go into solution in your body under pressure, and it will come out of solution when you release that pressure. (The rates will differ, but this basic remains the same.)

(2) Partial Pressure of any gas DOES matter. Remember Henry? The amount of any gas IN a solution varies DIRECTLY with the partial pressure of that gas ON that solution. (Here YOU are the solution, and the gas is what you are breathing.)

(3) If the partial pressure of any gas in a solution (you) exceeds a certain critical ratio or over-pressure as compared to the ambient (or existing pressure) around you, that gas (no matter what it is) will come out of solution rapidly. It will form bubbles. Those bubbles will be in you. You will, as the famed Dick Rutkowski puts it, be in "Bubble Trouble"!

(4) The speeds at which various gases go into solution, and come out of solution vary. Also, total on-gassing and off-gassing occurs at different rates.

(5) Modern decompression models are now dealing with gas in the solution phase AND gas in the bubble phase, hence they are called "Bubble Models". (RGBM would be an example.)

(6) Finally, gradient matters. If you have NO molecules of a particular inert gas in the breathing gas you are inspiring, and you have a LOT of molecules of that gas in YOU (i.e. in solution), you can be said to have a steep gradient (comparison between the two), which will aid the off-gassing of that inert gas from your body. (Note: We are not breaking the critical over-pressure ratio here between you and ambient pressure.)

(7) Very little argon, for instance, is acquired through the skin (although there is some transfer) when you use it as a suit insulator. The critical exchange is between what you are breathing, and YOU.

Admittedly, this is an over-simplification, but these are the basics of the problem.

Hope that helps!

Cheers, and happy diving!

 

[BigJetDriver]

So partial pressure is not responsible for bubble formation.

David,

I must correct this one statement.

Partial Pressure is DIRECTLY responsible for bubble formation.

See my post above, but in short, the partial pressure of any inert gas on your system will determine how much of it you on-gas. Then, as ambient pressure decreases, the amount of it that you have in solution will directly affect the amount of bubble formation.

The critical over-pressure ration CAUSES bubble formation when exceeded, so that is "trigger" mechanism, if you will.

 

[Charlie99]

David,
Partial Pressure is DIRECTLY responsible for bubble formation.
.

Isn't bubble formation more directly related to the SUM of partial pressures of all inert gases?

 

[MookieMoose]

David,

I must correct this one statement.

Partial Pressure is DIRECTLY responsible for bubble formation.

See my post above, but in short, the partial pressure of any inert gas on your system will determine how much of it you on-gas. Then, as ambient pressure decreases, the amount of it that you have in solution will directly affect the amount of bubble formation.

The critical over-pressure ration CAUSES bubble formation when exceeded, so that is "trigger" mechanism, if you will.

I think there's some confusion here. There are two seperate and distinct pressures being refered to: 1) the pressure (and partial pressures) of the breathing mix, and 2) the ambient pressure imposed as a consequence of depth.

The partial pressures of the breathing gas, more specifically the difference between the partial pressures in the breathing mix and pratial pressures of the same gasses in you body, will determine if a gas will be absorbed into the body or adsorbed out into exhaled gas

The ambient pressure imposed by your current depth will determine the degree of saturation, or supersaturation, based on the amount of inert gas absorbed as mentioed above.

Both these pressures are critical when it comes to bubble formation

 

[Dr Deco]

Hello S E F Diver:

When all is said and done, one must only consider the relationship between the dissolved gases in the tissues and the external pressure.

The external [hydrostatic] pressure is the sum of the atmospheric pressure and the pressure of the water column [when diving is concerned]. When one is submerged and breathing, that nitrogen will dissolve in the body. When one ascends, the external pressure will be reduced. Dissolved gasses can then diffuse into the microbubbles [apparently] always present in the body [and every other fluid for that matter].

When the pressure of the dissolved gases exceeds the hydrostatic pressure and the Laplace pressure [from surface tension] of the microbubble, it will grow.

That’s it. It has nothing to do with what gases are already in the atmosphere.

Dr Deco :doctor: