M Values and Gradient Factors - Explained

[Garf]

Originally Posted by dave archer

Garf, interesting post.

A few questions spring to mind:

1) Why do all common GFs total 100, e.g. 10/90, 20/80... Is there a reason why people don't tend to choose pairs which don't total 100, such as a more conservative GF of 10/80?

Well, there are still shedloads of divers diving gradient factors of 100/100 as they are using models based on pure buhlmann. Standard decoplanner gradient factors are 30/85. I've dived the 10/80 you suggest but you are starting your deep stops so low you end up racking up more time in the shallows. I've known people run Factors like 30/130, so there really is no one catch-all that suits everyone. That's one of the things I love about Gradient Factors, it really pushes home the simple truth that all deco theory is just theory, and not a one stop shop for avoiding problems. the only real common one I am aware of that adds up to 100 is 20/80, and that just works becuase, well, it works!. you have a level of conservatism at the top end, and you are starting your stops fairlydeep, but not so deep you will be penalised for it.

2a) If you breathe 100% O2 to deco at 6m and upwards, does this mean you are at 100% of the M value? Or greater? As you have no N2 in your breathing gas.

2b) Is the bubble formation point purely driven by difference between ambient N2 pressure in breathing gas and tissue N2 pressure? I understood there was also some linkage with ambient total pressure, otherwise switching to a rich or 100% O2 mix would be like ascending upwards rapidly, and 100% O2 would never be safe... somewhat confused about this bit

Cheers,
Dave.

These two questions are both linked, and in all honesty I wasn't sure how best to answer it until I had phoned one of my team, listened to them dribble for a bit, then gave up and call Mark Powell.

There are two processes going on here and you need to be careful not to confuse them. the M value is a theoretical value, for a given tissue compartment, where the pressure differencial between inert gas tissue pressure and ambient pressure becomes so great you end up with a bubble. This means if you ascend too quick, you get bent becuase the pressure of inert Nitrogen in your tissues is so much lower than the ambient pressure of nitrogen around you.

The gas you are breathing provides a different purpose. The Nitrogen will come out of your tissues at a rate determined by the process described above. However, the gas you are breathing then controls how efficiently that gas is transported out of your body.

The difference in these two procecces is why we have two areas that interest us in deco. there is the part where we are "opening the Oxygen window" (I can see that being another thread in itself) and getting the benefit of the gas by maximising the PPo2. And there is the section of the decompression curve where we are "pushing the pressure gradient" to maximise the velocity at which the gas is coming out of our tissues.

So, to answer your questions, no your are not necessarily at 100% of the M value at 6M, regardless of what gas you are breathing. some of your tissue compartments might be, some might not. It all depends on the difference between the inert gas pressure in your tissues and the ambient pressure. Secondly, no, again bubble formation is primarily driven by the process described here rather than the gas you are breathing. Now, notice I said "primarily", and I didn't say "solely driven" becuase, well, there are other things at work. The gas you are breathing, depending on who you are listening too, may have a dramatic effect on bubble formation. For example, if you swap to a gas at depth containing a significantly higher proportion of Nitrogen than you bottom gas, then.....oh, I'm just not going there as it's something of interest to people who dive deeper than me and I don't pretend to understand it.

Hope that helped

G

[neilh]

Originally Posted by Garf

the M value is a theoretical value, for a given tissue compartment, where the pressure differencial between tissue pressure and inert gas pressure becomes so great you end up with a bubble. This means if you ascend too quick, you get bent becuase the Nitrogen pressure in your tissues is so much lower than the inert pressure around you.

Isn't the M-Value more to do controlling the relationship between tissue pressure and ambient pressure rather than the inert gas pressure? Changing ambient pressure is going to have a corresponding effect on the inert gas pressure as well, but given that you can also change the inert gas pressure by changing the amount of N2 in the mix, my understanding was it was mainly about the ambient...

[Garf]

Originally Posted by neilh

Isn't the M-Value more to do controlling the relationship between tissue pressure and ambient pressure rather than the inert gas pressure? Changing ambient pressure is going to have a corresponding effect on the inert gas pressure as well, but given that you can also change the inert gas pressure by changing the amount of N2 in the mix, my understanding was it was mainly about the ambient...

rewritten - hope that makes more sense

Check the following for further reading...

http://www.dive-tech.co.uk/resources/mvalues.pdf

http://www.dive-tech.co.uk/resources/deepstops.pdf

[Mark Powell]

Originally Posted by Garf

There are two processes going on here and you need to be careful not to confuse them. the M value is a theoretical value, for a given tissue compartment, where the pressure differencial between tissue pressure and inert gas pressure becomes so great you end up with a bubble. This means if you ascend too quick, you get bent becuase the Nitrogen pressure in your tissues is so much lower than the inert pressure around you.

The gas you are breathing provides a different purpose. The Nitrogen will come out of your tissues at a rate determined by the process described above. However, the gas you are breathing then controls how efficiently that gas is transported out of your body.

I've tweaked Garf's comments above (edit: which he has now edited).

There are two processes going on here and you need to be careful not to confuse them, the M value is a theoretical value, for a given tissue compartment, where the pressure differencial between inert gas tissue tension and ambient pressure becomes so great you end up with a bubble. This means if you ascend too quick, you get bent becuase the Nitrogen pressure in your tissues is so much higher than the ambient pressure around you.

The gas you are breathing provides a different purpose. The Nitrogen will come out of your tissues at a rate determined by the .... the difference in partial pressure between the nitrogen in your tissues and the pertial pressure of the nitrogen in the gas you are breathing. This difference is known as the inert gas gradient. By reducing the depth we reduce the partial pressure of the gas we are breathing and so increase the inert gas gradient. But if we reduce the depth too much we create to much of a difference between tissue tension and ambient pressure and can exceed the M-value.

By switching to a different gas we can reduce the partial pressure of the gas we are breathing and so increase the inert gas gradient but without reducing ambient pressure and risking bubble formation.

So switching to 100% at 6m increases the offgassing by increasing the inert gas gradient but doesn't push us closer to our GF or our M-Value.

[dave archer]

Garf, Neil, Mark thanks for clarifications, makes much more sense now.

So in short, at any given moment..

- altering depth alters likelihood of bubbles/bends, as it alters alters the ambient pressure, and so brings you closer to or further away from M value.

- altering gas has no impact on likelihood of bubbles/bends, but alters the rate of offgassing.



I knew that once upon a time, but it's a distinction that can become confused, in my mind at least.

Cheers,

Dave.

[Mark Powell]

Originally Posted by neilh

Can anyone help me get my head around M-Values for a couple of models?

My understanding of Haldane's 2:1 ratio is that a given compartment can tolerate a tissue tension of up to twice the ambient pressure. I.e. at 20M the M-Value is 6 bar (3 ATA x 2) - so you could get from 50M (6 ATA) with saturated tissues up to 20M. Is that right?

Haldane never used the term M-value. Haldane's 2:1 ratio mean that the tissues can tollerate twice the ambient pressure. So yes if you are saturated at 50m (6 ATA), then you can ascend directly to 20m (3 ATA) without exceeding the 2:1 ratio. If you use the M-Value terminology then a 'Haldane M-Value' would be twice the ambient pressure.

Originally Posted by neilh

For Workman's model I'm not entirely clear about how whether the result of M0 + (dM x depth) is the absolute tissue tension or not. From some of the bits I've read it seems that relative pressures might come into play with this model rather than absolute (i.e. 10M is 1 bar relative to surface pressure rather than 2 bar asbsolute) - but I'm not sure!

Can anyone shed any light?

Workman (and Buhlmann) took a different approach. The M-Value is defined by an equation that gives an M-Value that varies between compartments and with depth. Simplifying it the M0 term gives a baseline M-Value which is varied by the dM x depth term. This second term has the effect of increasing the M-value at greater depth which allows greater overpressuriation at depth. In addition faster tissues have a higher M0 and so can tollerate a higher overpressurisation than slower tissues.

Buhlmann use the same conceptual idea but used the variables a and b to produce the same effect as Workman.

[Mark Powell]

Originally Posted by dave archer

So in short, at any given moment..

- altering depth alters likelihood of bubbles/bends, as it alters alters the ambient pressure, and so brings you closer to or further away from M value.

- altering gas has no impact on likelihood of bubbles/bends, but alters the rate of offgassing.



I knew that once upon a time, but it's a distinction that can become confused, in my mind at least.

Cheers,

Dave.

That's a pretty good summary!

[neilh]

Originally Posted by Mark Powell

Haldane never used the term M-value. Haldane's 2:1 ratio mean that the tissues can tollerate twice the ambient pressure.

Fair point

Originally Posted by Mark Powell

Workman (and Buhlmann) took a different approach. The M-Value is defined by an equation that gives an M-Value that varies between compartments and with depth.

Ok - but how does the M-Value result relate to the tissue tension? If you calculate the ppN2 for a saturated tissue based on the FN2 in the gas being breathed and the ambient pressure what do you need to do to the M-Value you get out of the equation in order to compare the two?

[Mark Powell]

Originally Posted by neilh

Ok - but how does the M-Value result relate to the tissue tension? If you calculate the ppN2 for a saturated tissue based on the FN2 in the gas being breathed and the ambient pressure what do you need to do to the M-Value you get out of the equation in order to compare the two?

They should be in the same units so its a direct comparison. I don't have the paper in front of me to check but will confirm later.

[neilh]

Originally Posted by Mark Powell

They should be in the same units so its a direct comparison. I don't have the paper in front of me to check but will confirm later.

Ah. Perhaps that's where my problem lies

I'm calculating the tissue tension using FN2 x ATA which is therefore in bar.
The Workman M-Value data I have is in msw (or fsw).

So I guess what I'm looking for is how to get to values that are directly comparable

EDIT:
Am I completely off the mark or can I simply convert from msw to absolute pressure using msw/10 + 1 and absolute pressure to msw using pressure-1 x 10?