Help understanding M values

[DC53]

I have a question about M values.

I have read a lot on the issue including reading Deco for Divers once and then a second time really closely, including other books and materials. Anyway I thought I had a good grasp on it.

However, now I am doing divemaster training and PADI literature is expessing M values in what seems an entirely different way and it does not seem to make sense to me.

My understanding of M values is, on a mathematical level....

The degree of supersaturation of a compartment at any given depth, divided by what is 100% saturation at that given depth. This gives you a measure of the rapidity in which the dissolved gas in the compartment will offgas. You need a number greater than 1.0 in order to off gas but on a practical level as the number increases to say 1.5 or greater the likelihood that there will be symptomatic bubble formation increases steadily due to increasingly rapid offgassing. Mathematically if this equation is correct then the feet in depth will drop out leaving you an absolute number rather than a number of feet or some other units. You are simply left with a ratio that gives you an (estimated) measure of allowable supersaturation.

However, PADI stuff wants to express it in feet salt water which makes no sense to me at all.

Anyone care to clarify this for me?

 

[Mike Boswell]

http://www.ddplan.com/reference/mvalues.pdf

 

[The Chairman]

Good stuff Mike. Thanks.

 

[Diver0001]

I have a question about M values.

I have read a lot on the issue including reading Deco for Divers once and then a second time really closely, including other books and materials. Anyway I thought I had a good grasp on it.

However, now I am doing divemaster training and PADI literature is expessing M values in what seems an entirely different way and it does not seem to make sense to me.

My understanding of M values is, on a mathematical level....

The degree of supersaturation of a compartment at any given depth, divided by what is 100% saturation at that given depth. This gives you a measure of the rapidity in which the dissolved gas in the compartment will offgas. You need a number greater than 1.0 in order to off gas but on a practical level as the number increases to say 1.5 or greater the likelihood that there will be symptomatic bubble formation increases steadily due to increasingly rapid offgassing. Mathematically if this equation is correct then the feet in depth will drop out leaving you an absolute number rather than a number of feet or some other units. You are simply left with a ratio that gives you an (estimated) measure of allowable supersaturation.

However, PADI stuff wants to express it in feet salt water which makes no sense to me at all.

Anyone care to clarify this for me?

pfffffff.....ok..... well let me try to give a rough idea of the mechanics.

Basically there are three "dials" in this machine....

one of them determines how much over-pressure a compartment can tolerate before bubbles start to form (the M-value)... which is what decompression algorithms use to determine the "ceiling".

one of them determines if a compartment is going to be on-gassing or off-gassing at a given depth.... this is a function of the "tissue loading" vs "ambient pressure" and can be seen as a switch that's turned either to "on" or "off" at any given depth at any given time. It's possible that some compartments will be on-gassing as others are off-gassing at a give point during a dive. The NDL displayed on your computer shows you the data for the "controlling" compartment (one of the 12 or 16 normally). Compartments that are not "controlling" (ie closest to their NDL) are calculated but not displayed.

and one of the dials determines the speed at which ongassing and offgassing occurs (the 1/2 time).

I hope I didn't lose you so far....

So because the M-value can be expressed as a "ceiling" I suppose you can express it in terms of a depth and ergo as fsw..... which is what I suppose you've read and didn't understand.

still with me?

R..

 

[Crowley]

to simplify: Great .pdf to read through but the introduction should clear up the OPs question:

For a given ambient pressure, an M-value is defined as the maximum value of inert gas pressure (absolute) that a hypothetical "tissue" compartment can "tolerate" without presenting overt symptoms of decompression sickness (DCS).

in other words, the M value can indeed be expressed as a measure of pressure, and msw/fsw is simply another way of writing the pressure "value" as opposed to Bar or ATA. Feet and metres of sea water don't have all those fiddly decimal points.

Cheers

C.

 

[The Chairman]

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

 

[Blackwood]

an "M" value should be expressed as a ratio.

Maybe PADI wants to know the surfacing m-value (i.e. relative to 1ATM edit or as Crowley notes, relative to any given depth/ambient pressure), so they may be asking for how deep you can go (i.e. the maximum dissolved gas pressure) without exceeding that ratio?


Hard to know exactly what they're looking for without seeing the confusing question/statement.

 

[Mike Boswell]

I think Dr. Deco (aka Michael Powell) could best be able to address the OP's question, since he helped develop and test the M-values for the DSAT RDP used by PADI.

 

[DC53]

pfffffff.....ok..... well let me try to give a rough idea of the mechanics.

Basically there are three "dials" in this machine....

one of them determines how much over-pressure a compartment can tolerate before bubbles start to form (the M-value)... which is what decompression algorithms use to determine the "ceiling".

one of them determines if a compartment is going to be on-gassing or off-gassing at a given depth.... this is a function of the "tissue loading" vs "ambient pressure" and can be seen as a switch that's turned either to "on" or "off" at any given depth at any given time. It's possible that some compartments will be on-gassing as others are off-gassing at a give point during a dive. The NDL displayed on your computer shows you the data for the "controlling" compartment (one of the 12 or 16 normally). Compartments that are not "controlling" (ie closest to their NDL) are calculated but not displayed.

and one of the dials determines the speed at which ongassing and offgassing occurs (the 1/2 time).

I hope I didn't lose you so far....

So because the M-value can be expressed as a "ceiling" I suppose you can express it in terms of a depth and ergo as fsw..... which is what I suppose you've read and didn't understand.

still with me?

R..

Yes still with you.

 

[DiveNav]

.... However, an "M" value should be expressed as a ratio.

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)