How do M-values change with respect to altitude?

[Hungry Lobster]

I'm learning about Buhlmann and other Haldane-like decompression algorithms. There's a point that I cannot understand, and that is not mentioned in the many documents I've found on the web: do M-values change with altitude and how?

The problem may not exist with A-series Buhlmann compartments, where M-values can be calculated using half times and the "zero-depth" pressure. However, what about B- and C-series compartments whose M-values are empirical? And how about other algorithms, where M-values cannot be calculated at all?

I'm starting to think that altitude does not affect decompression algorithms (except for the depth-to-pressure step).

 

[fdog]

My understanding is that an M-value expresses the:

• maximum supersaturation pressure
• a particular compartment
• at that ambient pressure

can experience.

When depth is ignored, and the obligation is calculated from a purely pressure perspective, I was taught that altitude will not have an effect on the instantaneous M value. This is to say, the M value at 3 ATA (6,000' altitude) is the same for the M value at 3 ATA (sea level).

Since altitude has an influence on the ambient pressure at a particular depth - for example, 60 FFW at 6,000' altitude will have a lower ambient pressure than 60 FFW at sea level - altitude will have an effect when regarded from a depth perspective.

So it all depends on how you look at it suppose, depth or ATA.

The practical application: Since identical depth profile dives at altitude vs. sea level will have different ambient pressures throughout the dive, they will have differing M values at each point in time, and thus differing schedules.


All the best, James

 

[Red Sea Shadow]

Buhlmann's M-values are based on absolute pressures, so can be used at altitude. His M-values were developed and tested for a broad range of
ambient pressure exposures, from high altitude diving to deep sea diving.

Excerpted from Erik Baker's "Understanding M-values".

 

[Hungry Lobster]

Thanks for your answers.

My understanding is that an M-value expresses the:

• maximum supersaturation pressure
• a particular compartment
• at that ambient pressure

can experience.

When depth is ignored, and the obligation is calculated from a purely pressure perspective, I was taught that altitude will not have an effect on the instantaneous M value. This is to say, the M value at 3 ATA (6,000' altitude) is the same for the M value at 3 ATA (sea level).

Since altitude has an influence on the ambient pressure at a particular depth - for example, 60 FFW at 6,000' altitude will have a lower ambient pressure than 60 FFW at sea level - altitude will have an effect when regarded from a depth perspective.

So it all depends on how you look at it suppose, depth or ATA.

The practical application: Since identical depth profile dives at altitude vs. sea level will have different ambient pressures throughout the dive, they will have differing M values at each point in time, and thus differing schedules.


All the best, James

Clear. However now I have a second question. In all the texts I've read, I've found the following formula:

M = M0 + ΔM d

Where d depth and ΔM is bar / meter (or bar / foot). Now, at the sea level, transforming this formula to use pressure instead of depth is pretty simple:

ΔM' = ΔM × 10 meters ÷ 1 bar
M = M0 + ΔM' (p - 1 bar)

Where p is the pressure and "- 1 bar" means: remove the sea level pressure.

But what about altitude diving? Can I use my formula with "p" unchanged (so that "(p - 1 bar)" may be negative)? Or do I really have to rely on depth?