Helium offgassing rate

[andibk]

Dr. Deco
In some of the technical manuals I have read it states that helium on gasses 2.78 times faster then nitrogen. What is the off gassing rate of helium? Would the off gassing rate be the same as the on gassing rate?
What is the exact efficient ratio of oxygen over air while decompressing? I have been told the efficientcy ratio is 1.41:1. I have heard of other figures bouncing around and figured you would give us the straight scoop.

Bruce

 

[Dr Deco]

Dear andibk:

Halftimes

The rate that you mentioned is that derived by Dr Buhlmann for his tables. He died before the test program was completed, although there are tables based on his method developed after his death. This method was given in Professor Buhlmann’s book “Tauchmedizin” (Diving Medicine) and is referred to as the ZH-L16 Algorithm.

From the ratio of the square root of the molecular weight of nitrogen to the square root of the atomic weight of helium, we obtain the value of 2.65. This is based on Graham’s Law of Diffusion, and it means that helium will on gas 2.65 times faster than nitrogen.

In addition, helium will off gas 2.65 times faster than nitrogen. The longest halftimes for nitrogen will be longer (by 2.65) than those for helium within any given algorithm.

Oxygen

The washout of inert gas when oxygen is substituted is dependent on the partial pressure of inert gas in the tissue to the partial pressure in the arterial blood. There are some circumstances when the exchange is not “perfect” and is the result of vasoconstriction from the oxygen. If there is some activity of the diver, the vasoconstriction will be offset by the generation of carbon dioxide.

The degree of activity of the diver (I believe) is a big factor in determining the exchange of inert gas with oxygen breathing. Naturally this activity cannot be too great or it could lead to oxygen convulsions.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[andibk]

Dr. Deco
If the helium is coming out just as fast as it goes in then why the longer decompression?

Bruce

 

[Dr Deco]

Dear Bruce:

Uptake and On Gassing

On gassing is a simple process involving only solubility of a gas in a fluid (lung gas to blood to tissue). It is essentially a completely reversible process. It is governed by solubilities and the flow of blood ( = perfusion).

Elimination and Off Gassing

Off gassing is a more complex process and involves solubility (tissue to blood to lung gas). Added to this is the irreversible process of dissolved tissue nitrogen to gaseous bubble nitrogen. Once the nitrogen enters the bubbles, the pressure drops to ambient (plus something for the Laplace pressure from surface tension).

Because decompression involves the prevention (to a large degree) of bubble growth, it is necessary that this process proceed slower than on gassing. It is controlled by more than simple solution processes and blood flows.

The Half times

The difference of 2.65 referrers to the spectrum of half times. These times are a mathematical construct and are used to provide gas loads for differing situations. They could also be based on solubility rather than diffusivity. They may not actually exist in reality at all times. The limits for the dive are governed by the m-values or “safe supersaturation limits” for that particular compartment.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[cnidae]

Dr Deco once bubbled...
Dear Bruce:

On gassing is a simple process involving only solubility of a gas in a fluid (lung gas to blood to tissue).

So what your saying is because the solubility of helium is far less than nitrogen your decompression time will be less when diving a helium based gas, is my interpretation correct?

 

[BRW]

Postees,

Helium and nitrogen trade off for NDLs depending
on depth and time. For most NDLs in the shallow
zone, nitrogen is better. For deeper exposures
helium is better. For deep and deco diving,
helium wins, especially when coupled to
strategic mix switching on the way up. Overall
nitrogen is not the divers friend, even though
it wins the recreational NDL contest.

It's a matter of mass transport coefficients,
that is, solubility multiplied by diffusivity.
And the amount of gas stored in tissue/blood
safely, per unit volume. Both can be estimated
from sat experiments and reported data.

For shallow and short exposures, slow diffusing
nitrogen allows longer NDLs (than helium).
While for longer and deeper exposures, rather
insoluble helium wins the NDL race (longer
NDLs than nitrogen)

To see this, you need a computer and mass
transport equation. While this is probably
Greek to most of you, it's well know and
understood by physical scientists. Take
my word, in addition to what others have posted.

BTW, there are Haldane tissue halftimes that
can be linked to mass transport coefficients
very nicely. It's easy -- start with any
arbitrary set for He and/or N2 and scale both
sets relative to each other by their respective
mass transport coefficients. Nothing gained
from this in Haldane deco frameworks, and NDLs
come out the same (as per the above discussion)

Cheers,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Leader

PS -- I will be gone shortly, and those of
you who are tired of this will get a
break. Thanks, though, for allowing me
some quality time on Mike's website
:wacko:

 

[DeepTechScuba]

I'm not tired of this. I hope you will post more often.

At various points in time, someone or other bursts forth like a supernova with new theories and new insights, and older ideas and older ways therefore must inevitably give way to the new. I have read a lot of new information in various scuba publications by your or about you, Bruce. Its good to hear from you first hand as well.

You sound excited about NAUI's new dive tables. I love NAUI as well. But like any old tech diver, I fear the new, especially the totally new. But it is good to hear you shed new light on things.

Don't stay away too long. Thanks again.

 

[Kendall Raine]

What is your take on different physiological effects of hyperbaric N2 v. He? That is, is there anything to the notion that hyperbaric N2 induces red blood cell rigidity whereas He does not? How about asymetric effects on blood filterability (Cross and Pimlott's 1987 and 1989 papers to EUBS) between hyperbaric N2 and He? Same question for complement system activation? While Graham establishes relative diffusivity relationships in vitro, does Graham, or Fick for that matter, work for both on and off gassing in vivo with different inert gasses?

 

[G~Diver]

Dear Bruce:

The Half times

The difference of 2.65 referrers to the spectrum of half times. These times are a mathematical construct and are used to provide gas loads for differing situations. They could also be based on solubility rather than diffusivity. They may not actually exist in reality at all times. The limits for the dive are governed by the m-values or “safe supersaturation limits” for that particular compartment.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
Advanced Decompression Physiology Seminar Information

If Helium is less soluble then N2 how come the half times of He are 2.65 faster then N2 - as I think of it, if it less soluble it will take Helium longer time to dissolved , longer time to get to saturation > > longer half times.
Where is my mistake?

 

[Kevrumbo]

G~Diver, the misunderstanding is don't interpret solubility versus diffusivity/on-gas/off-gass half times as diametrically opposed constructs in this instance; there is a dynamic of both going on with regards to the physical properties of Helium and Nitrogen in the Mass Transport/Biomass NDL's Paradigm. . .

BRW post#9
BRW post#4