UTD Ratio deco discussion

[CAPTAIN SINBAD]

I am curious to know why 66% of depths was chosen as the first-stop depth? It is still more logical than 75% stop depth but is there a particular gf lo that this stop is trying to replicate?

 

[boulderjohn]

One of the other changes in RD 2.0 is not only the 66% deep stop from 75%, but the way the S-curve is shaped.

Example with old RD using a 50% bottle and 30 min of total deco:
70': 5min
60': 5min
50': 2min
40': 2min
30': 3 min
20': 12 min
10': 3 min

Example with RD 2.0 using a 50% bottle and 30 min of total deco:
70': 3 min
60': 3 min
50': 2 min
40': 2 min
30': 5 min
20': 12 min
10': 3 min

In the profile he provided, decompression said it was RD 2.0. Here is the middle portion he created.

70-5(44), 50%
60-5(49), 50%
50-2(51), 50%
40-2(53), 50%
30-10(63), 50%

He indicated that the individual has some discretion with this. What are the guidelines?

 

[mikeny9]

Those numbers may very well be correct for those depths, I mentioned a Tech-1 profile only. Those stop times are beyond my limits. In general there is discretion in how you shape your deco and how much total deco you do, it really depends on the situation.

 

[mikeny9]

I am curious to know why 66% of depths was chosen as the first-stop depth?

...the result of the Italian study did have some minor changes in the UTD ratio deco approach (hence "ratio deco 2.0") and generally has the diver stopping a bit shallower on the deep stop portions of the ascent (66% vs 75%).

The actual reason has something to do with an increase in carotenoids (sp?) in the divers following profiles with the old RD. I believe the results of the Italian study are still not published due to an embargo as Dr. Simon Mitchell mentioned in another thread.

 

[boulderjohn]

Those numbers may very well be correct for those depths, I mentioned a Tech-1 profile only. Those stop times are beyond my limits. In general there is discretion in how you shape your deco and how much total deco you do, it really depends on the situation.

So...why? What is the reason to do the stops that way?

 

[Rainer]

I believe the results of the Italian study are still not published due to an embargo as Dr. Simon Mitchell mentioned in another thread.

The results are most certainly published. I've read it. The authors are just not free to disseminate the paper outside of the journal for a year. You can always subscribe if you want to read it.

 

[mikeny9]

So...why? What is the reason to do the stops that way

Which way? Generally RD tries to spend as much time at 1.6 PPO2 for the whole "oxygen window" thing while limiting on-gassing of the fast tissues, that's the goal anyway as I understand it.

The results are most certainly published. I've read it. The authors are just not free to disseminate the paper outside of the journal for a year. You can always subscribe if you want to read it.

Can you provide a link? If there is none, what journal is it published in?

 

[Rainer]

Can you provide a link? If there is none, what journal is it published in?

Print only:

Spisni E et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. DHM 2017;47(1):9-16.

 

[mikeny9]

Cool thanks, I'm going to try and get my hands on it.

 

[Kevrumbo]

Decompression models are based on pressure ratios, and not on absolute pressures. In determining how your body rids itself of excess nitrogen gas, decompression models rely upon the ratios of the pressures you experience at depth to the atmospheric pressure you experience after the dive. The key to not forming nitrogen bubbles in your body (and thereby avoiding DCI) is to keep those pressure ratios within tolerable limits.

For example:

If the 80min half-life tissue were leading for decompression, it's total inert nitrogen loading pressure that will allow a direct ascent to the surface is 1.6 ata from 20fsw. The supersaturation ratio relative to sea level is then: 1.6 ata/1.0 ata = 1.6

However at 9000 ft, the supersaturation ratio relative to the surface at altitude is:
1.6 ata/ 0.7 ata = 2.28

Therefore surfacing with the same "safe" supersaturation ratio allowable at sea level (1.6) but now at diminished atmospheric pressure would only be possible with lower inert nitrogen gas pressure in the same tissue. Hence the decompression times for dives at diminished atmospheric pressure need to be lengthened in comparison with those at sea level.

Another good example, here's a can of soda analogy showing the importance of pressure ratios in preventing bubble formation: take the can of soda into a recompression chamber and increase the pressure to the point where if you opened the can, the soda would be flat because the ambient air pressure would be great enough to keep the bubbles in solution. Likewise, if you open a can of soda during an airplane flight, it's likely to fizz more than if you had opened it on the ground because the airplane cabin is at a reduced pressure. How much the soda fizzes does not depend on just the internal pressure of the unopened can, nor on the external air pressure, but it instead depends on the pressure ratio between the pressure in the unopened can and the pressure of the surrounding air when you open it.

Thank you very much for your time. If I can indulge you a bit more, in your example 20 fsw at 9,000 feet is 1.6 ATA. Is that true or is it really 1.32 ATA in which case 1.32 ata/ 0.72 ata = 1.83
Why would we ignore the reduced atmospheric pressure when calculating absolute pressure?

A Nitrogen pressure of 1.6 ata at sea level 1.0 ata yields an allowable surfacing supersaturation ratio of 1.6 (1.6 ata divided-by 1.0 ata equals 1.6)

For illustrative purposes -A hypothetical Nitrogen pressure of 1.6 ata at 9000 ft altitude for a reduced atmospheric ambient pressure of 0.7 ata grossly does not equal an allowable surfacing ss ratio of 1.6 (1.6 divided-by 0.7 equals 2.28).

An actual Nitrogen pressure of 1.32 ata at 9000 ft altitude for a reduced atmospheric pressure of 0.7 ata does not equal an allowable surfacing ss ratio of 1.6 (1.32 ata divided-by 0.7 ata equals 1.83). You must ascend to an intermediate depth to decompress further in order to attain an allowable surfacing ss ratio of 1.6, or less for better margin.

At 13 fsw your calculation would go like this if I understand correctly. 1.12 ata/ .72 ata = 1.55.
Does that mean that the delta in the depth that your tissue can tolerate in your example is 7 feet?

For the Buhlmann Altitude Air Decompression Tables above 700m (2310ft), the last two deco stops before surfacing based on the 80min Tissue half-life leading the decompression is at 4m (13ft) and 2m (6.6ft). So yes, the delta depth is roughly 7 feet for an allowable staged M-value ascent to the last deco stop, and roughly 7 feet from the last deco stop to the surface.

OK, that's that part of the equation. What about the reduced nitrogen levels in your tissues at the beginning of the dive due to being acclimated to 9000 feet. Does that affect the algorithm for tissue loading. Why would we ignore that? When I input my altitude into my Perdix computer does it use that in it's calculations or does it only use it for surfacing pressure for the end of the dive?

For conservatism, the Buhlmann Altitude Air Deco Tables initially assumes and already accounts for both fast & slow tissue representative compartments, a sea level Nitrogen Pressure saturation of 0.8 ata without any acclimatization time (i.e. a scenario of an Emergency Rescue Dive Team going straight from sea level to altitude by helicopter, and ready to splash in). So when your Perdix ZHL-16 automatically adjusts for altitude, it doesn't prompt for or need to know your acclimatization time at that altitude.

(Reference, see also: http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/2750/969023.pdf?sequence=1)