Bühlmann ZH-L16 and M-values

[David Novo]

I am reading Deco for Divers and at some point it states that in in Bühlmann ZH-L16 "M-Values were mathematically derived from the half-times based on the tolerated surplus volumes and solubilities of the inert gases".

Can anyone please explain me the concept of tolerated surplus volumes?

Thanks.

 

[tursiops]

See the second paragraph of this fundamental reference for the subject: https://www.frogkick.nl/files/understanding_m-values.pdf

 

[David Novo]

See the second paragraph of this fundamental reference for the subject: https://www.frogkick.nl/files/understanding_m-values.pdf

So in that model, M-values were derived from half-times based on M-values? This sounds like an egg or chicken paradox...

 

[tursiops]

So in that model, M-values were derived from half-times based on M-values? This sounds like an egg or chicken paradox...

LOL. You need to read a bit deeper....and remember that a "tissue" can only hold so much gas (over ambient pressure) before it comes out...which is the problem that the limits are trying to prevent. Haldane's original work thought it was a factor of 2 over ambient, but he was looking at total gas pressures, not PPN2, which si what is relevant. Since N2 is 79% of air, that become an overpressere of 1.58. Keep reading!

 

[David Novo]

LOL. You need to read a bit deeper....

I do not doubt!

and remember that a "tissue" can only hold so much gas (over ambient pressure) before it comes out...which is the problem that the limits are trying to prevent. Haldane's original work thought it was a factor of 2 over ambient, but he was looking at total gas pressures, not PPN2, which si what is relevant. Since N2 is 79% of air, that become an overpressere of 1.58.

This is clear from what I read so far. I am just trying to understand the difference between M-values and the "tolerated surplus volumes" used to derive them.

 

[tursiops]

I do not doubt!



This is clear from what I read so far. I am just trying to understand the difference between M-values and the "tolerated surplus volumes" used to derive them.

The amount of gas I can shove into a tissue is the tolerated surplus volume (of gas). It is experimental. Any additional gas in not tolerated.

 

[dmaziuk]

This is clear from what I read so far. I am just trying to understand the difference between M-values and the "tolerated surplus volumes" used to derive them.

If you can read Deutsch and have an extra hundred $$s to drop on Tauchmedezin, please find out where Herr Dr Buhlmann pulled that stuff out from and report back. I too would like to know how he came up with magic fudge constant that, multiplied by tissue compartment's half-time, gives the almost-right M-values for both helium and nitrogen.

 

[emoreira]

In order to on-gas a compartment, there should be a differential pressure between the outside pressure and the pressure in the compartment. The same to off-gas. So, to off-gas a compartment the outside should have a lower pressure than the compartment. If the outside pressure is too low the gas in the compartment can start to form bubbles. The M-value is the maximum differential pressure that a compartment can withstand without forming bubbles during the off-gassing process.
This M-value depends on the compartment and the pressure. Workman modelized this as a linear function for each compartment.

 

[tursiops]

Workman modelized this as a linear function for each compartment.

As a linear function of what?

 

[emoreira]

As a linear function of what?

For each compartment the M-value is a linear function depending of the ambient pressure.