Help understanding M values

[Blackwood]

In my book (and algorithm) M values have a dimension (pressure).

I like this simple definition: "The M-Value is the MAXIMUM TISSUE PRESSURE (nitrogen level) that is allowed to be left in the body after a dive." that I have found here

Alberto (aka eDiver)

That's the maximum surfacing m-value. If all you want is "NDL" that's fine, but is inadequate for stage decompression.

 

[DiveNav]

That's the maximum surfacing m-value. If all you want is "NDL" that's fine, but is inadequate for stage decompression.

I agree .... that is why I said "a simple definition"
For deco we play with the lines and GF values

 

[Blackwood]

I agree .... that is why I said "a simple definition"
For deco we play with the lines and GF values

Personally, I find the ratio to be more straightforward/simpler, especially when considering gradient factors, multi-level diving, etc..

 

[Mr Carcharodon]

M values are given in units of pressure. So for example the M value for the 4 minute compartment of nitrogen is 106.4 fsw in Buhlmann's ZHL16(1990) table. In this case fsw (feet of sea water) is used as a pressure unit. Pressure gradients are also pressure because they are just the difference between compartment pressure and ambient. Surfacing M values (M-naughts) of course are also in units of pressure. If you wanted to you could work in atmospheres, bar pascals, fsw, msw or whatever as long as you stay consistent. Standard practice has been to use fsw or msw which makes relating pressures during the dive easier.

So for example: M (fsw)(the maximum allowed pressure gradient)=dM(unitless)*depth(fsw)+M-naught(fsw) -- everything has units of pressure, except for deltaM which is a slope and is unitless.

The confusion may be that Haldane used pressure ratios in his work around 1908. Workman who originated M values, around 1965, used differences.

 

[gcbryan]

delete...slow typer.

 

[DC53]

In reality, pressure can be expressed in "FSW", psi, atm, bar and more. However, an "M" value should be expressed as a ratio.

That's exactly what I was thinking. M value is a ratio without units.

The divemaster teaching seems to want us to think in terms of FSW.... I get what they mean in terms of a gas pressure dissolved and presumably overstaturated to a degree felt to be acceptable for each compartment.....However they don't seem to say whether that is FSW value for a given compartment is oversaturated (ie fully saturated at a given depth, times the M value), or simply when a compartment reaches saturation at that depth.

Unless I were to be using a computer to plan a the dive profile, looking at each of the compartments, AND then also not using a computer to crunch the numbers for me while underwater, I have to say I don't see the practicality.... Maybe down the road (done with DM and onto Tech) I will see the value of it in dive planning.

 

[DC53]

M values are given in units of pressure. So for example the M value for the 4 minute compartment of nitrogen is 106.4 fsw in Buhlmann's ZHL16(1990) table. In this case fsw (feet of sea water) is used as a pressure unit. Pressure gradients are also pressure because they are just the difference between compartment pressure and ambient. Surfacing M values (M-naughts) of course are also in units of pressure. If you wanted to you could work in atmospheres, bar pascals, fsw, msw or whatever as long as you stay consistent. Standard practice has been to use fsw or msw which makes relating pressures during the dive easier.

So for example: M (fsw)(the maximum allowed pressure gradient)=dM(unitless)*depth(fsw)+M-naught(fsw) -- everything has units of pressure, except for deltaM which is a slope and is unitless.

The confusion may be that Haldane used pressure ratios in his work around 1908. Workman who originated M values, around 1965, used differences.

That seems consistent with my studying.

 

[Thalassamania]

Always go to a primary source, here's Ed Thalman's 1980 paper.

 

[Mr Carcharodon]

Your getting sidetracked with supersaturation. The whole idea is that you want to keep M<G. Where M is your M value, and G is pressure gradient. So the M is just a limit. If G is less than the limit you are OK. Think of it a lengths. If the length of your tissue pressure gradient G is less than the length of your M value you are OK.

Also G is the bridge between the dissolved gas models and the bubble models. It is the physical anchor to which all the senseless bubble model squiggles are tied.

What's the practical value? Well it does teach some deco theory rather than expecting you to take tables on faith. Which pretty much was the approach in open water. If you want something you can use in the water look into ratio deco, but get your Dive Master done first.

 

[The Chairman]

Your getting sidetracked with supersaturation. The whole idea is that you want to keep M<G. Where M is your M value, and G is pressure gradient. So the M is just a limit. If G is less than the limit you are OK.

I hate to quibble, but shouldn't your ideal be M>G??? If M<G, then you would be undergoing DCS of some kind.