UTD Ratio deco discussion

[boulderjohn]

I will try to explain the altitude issue a different way. The key is this: it is not the pressure at any given depth that matters; it is the change in pressure as the diver ascends. In basic deco theory, you understand that a diver has to keep tissue pressure at a safe gradient relative to ambient pressure. As the diver ascends and ambient pressure decreases, the diver's tissue pressure must decrease enough to stay within that safe gradient range. At altitude, the decrease in ambient pressure as the diver ascends occurs at a different rate from sea level, with the biggest differences occurring in the shallowest depths.

Water weighs the same at any altitude, so the deeper the diver is on a dive, the greater the percentage of that water weight in determining ambient pressure. As the diver ascends and encounters less water weight, the atmospheric weight becomes more and more a factor.

To illustrate, let's compare the change in ambient pressure at an altitude of 6,000 feet as the diver ascends from what would be 6 ATA at sea level. At that altitude, the atmospheric pressure is about 0.8 ATA, so at the same starting depth (170 FFW), the diver is at 5.8 ATA. That 0.2 difference will be maintained throughout the ascent, but it becomes a greater and greater factor as you ascend,

• At 170 FFW, the pressure is only 3% less than at sea level--barely worth considering.
• At 136 FFW, the diver is now at 4.8 ATA, still only 4% less than sea level
• At 102 FFW, the altitude diver is still only at 5% less pressure than at sea level.
• At 68 FFWW, the altitude diver is at 6.7% less pressure than at sea level.
• At 34 FFW, the diver is at 10% less pressure than at sea level.
• At 20 FFW, diver is at 12% less pressure than at sea level
• At 10 FFW, the diver is at nearly 16% less pressure than at sea level.
• At the surface, the diver is at 20% less pressure than at sea level.

In summary, as the diver ascends, there is very little difference in ambient pressure compared to sea level for to first 100 feet of ascent--less than 4% compared to sea level. Over the next 30-40 feet, the change is about the same as it was over the previous 100 feet. Over the last 20 feet, the difference is dramatic--the same as the previous 150 feet of ascent.

You can see that reflected in the summaries of the two Buhlmann profiles in my last post. There is no real difference between the Buhlmann sea level profile and the altitude profile until you get to the last two deco stops, at which point the difference becomes very significant as the profile strives to keep the diver's tissue pressure at a safe gradient compared to the ambient pressure.

 

[boulderjohn]

The same kind of change I mentioned above in regard to tissue pressure gradient occurs in the growth of bubbles due to Boyle's Law. I put in the numbers a few posts ago. If we were to chart it out again, you would see the same thing: a bubble grows slowly at the greater depths and then expands dramatically over the last 20-30 feet. As I illustrated above, that change is ever more pronounced at altitude.

A bubble moving from 170 feet to 34 feet at sea level would triple in size.
In the last 34 feet, it would increase to 6 times its size.

A bubble moving from 170 feet to 34 feet at 6,000 feet would increase in size 3.2 times--not much difference.
In the last 34 feet, it would increase to 7.25 times its size, a huge difference.

Again, this is reflected in the difference in the Buhlmann altitude profile.

 

[RayfromTX]

John, can you address my question about the benefit of starting the dive at altitude and if or why that isn't considered in the calculated deco obligation? Does it just get totally wiped out at depth or is there some reason that I'm not understanding for it's exclusion from the algorithm? I may sound dumb but I really want to understand it.

 

[RainPilot]

My best guess is that since it is a deco dive, the compartments are saturated, this may take longer but then the de-saturation part becomes an issue.

Also, once you have been a couple of feet under for a few minutes you would be back to a sea level sat state anyway.

 

[boulderjohn]

John, can you address my question about the benefit of starting the dive at altitude and if or why that isn't considered in the calculated deco obligation? Does it just get totally wiped out at depth or is there some reason that I'm not understanding for it's exclusion from the algorithm? I may sound dumb but I really want to understand it.

The desktop software I use (Multi-deco) asks you how long you were at the altitude prior to the dive, and that is calculated into the dive profile. I do not know any dive computers that do that.

If you have been at altitude long enough to achieve equilibrium, then you will indeed start at a lower level of nitrogen than would a person at sea level. During the dive, though, you will be adding nitrogen at the same rate you would at sea level. As Brendon just said while I was typing, when your tissues are saturated a specific depth, then they are saturated at that depth, and it won't make any difference. After that, those tissues will have to stay within a safe pressure gradient upon ascent, as described above.

 

[RainPilot]

The Perdix keeps checking its pressure gauge even when off, its part of the "auto-on" feature. I would be very surprised if it doesn't keep track of that somehow.

Maybe @Shearwater or @lynnpartridge can shed some light?

 

[decompression]

You are counting the total minutes of deco and assuming that is the only thing that matters. The phrase "more conservative" implies "safer," but what makes it safer? Look a the comparisons I made in post #214. I will repeat the summaries here:

Comparison of RD to Buhlmann at sea level:

1. RD first stop at 120 feet; Buhlmann at 90.
2. RD time on ascent between 180-80 = 9 minutes; Bulmann = 5
3. RD 70-60 time = 10 minutes (oxygen window theory); Buhlmann = 4
4. RD total ascent time to 50 feet = 19 minutes; Buhlmann = 9
5. RD total time at last two stops = 20 minutes; Buhlmann = 25
6. RD total Run time = 83 minutes; Buhlmann = 78

Comparison of RD to Buhlmann at 6000 feet:

1. RD first stop at 120 feet; Buhlmann at 90.
2. RD time on ascent between 180-80 = 9 minutes; Bulmann = 5
3. RD 70-60 time = 10 minutes (oxygen window theory); Buhlmann = 3
4. RD total ascent time to 50 feet = 19 minutes; Buhlmann = 8
5. RD total time at last two stops = 20 minutes; Buhlmann = 36
6. RD total Run time = 83 minutes; Buhlmann = 87

The ratio deco schedule does the decompression MUCH deeper and for MUCH longer time at those deeper depths than the Buhlmann profiles, especially at altitude. Have you looked at the threads on the most recent research regarding deep stops? Those threads have pretty substantial evidence that spending 10-11 more minutes of that deco below 50 feet and 5-16 fewer minutes from 20 feet up is NOT the way to go if you are looking for safety. It is absolutely not what I would choose to do.

It's a good comparison between the two, it certainly shows the similarities, although, the points that are in bold are part of shaping the S curve and O2, you can redistribute the Deco where it benefits the diver the best. So, there's the ability to make the 2 profiles very similar.

I think people can now see RD and Buhlmann at sea level and altitude are not as "radically" different.
Again, there's more time at deep stops which is still evolving, again can you run a profile with RD as you learned it to see the difference?

Yes, I've read all the threads........let me remind you that I provided the profiles of RD as information, DO NOT make this about me, I could care less how you dive. I'm not a banner carrier for RD, please stop trying to convince me. People doing these types of dives have pretty good idea of what's involved and can make decisions for themselves.

I should of kept my monkey at home........

 

[boulderjohn]

The Perdix keeps checking its pressure gauge even when off, its part of the "auto-on" feature. I would be very surprised if it doesn't keep track of that somehow.

Maybe @Shearwater or @lynnpartridge can shed some light?

The Perdix knows it is at altitude when it is turned on--it has a sensor for that. If it turns on by itself at the beginning of a dive, it assumes that the diver is still at the same altitude as when it was last turned on. That is a relatively new feature for Shearwater. It used to be that when their computers turned on by themselves at the beginning of a dive, they assumed they were at sea level. This caused problems for people diving at altitude, for they would turn on the computer to make the settings adjustments, go about their business finishing their preparations, then have the computer turn itself off just before they got in the water. This would lead to incorrect depth readings.

I know this because it happened to me, and I discussed it with Shearwater prior to their making the change.

The Shearwater does not, however, have any way of knowing how long you were at that altitude before you turned on the computer. If you were diving in Santa Rosa, NM (4,600 feet), you might have driven down from Santa Fe (7,200 feet) or up from El Paso (3,700 feet) just prior to the dive.

 

[boulderjohn]

I think people can now see RD and Buhlmann at sea level and altitude are not as "radically" different.

We evidently have pretty different opinions on what constitutes a significant difference.

The main difference between what you showed and what I learned is that we did deep stops at 75% of maximum depth. RD is currently doing 66%. We did S-curves, too.

I am curious about the S-curves. Back when I took the official RD class from Andrew, I had previously been arguing about the S-curve with him. It was based on a study that had been discredited. If you read Mark Powell's Deco for Divers, you will see that he says the theory of faster off-gassing via the oxygen window has no validity (2008 edition, pages 58-59). When I talked with Jarrod Jablonski about it, he said GUE recognized that the study's conclusion on that point was wrong, and they have since discontinued the S-curve. When I took the class from AG, he said that it did indeed look like that study was wrong, but he came up with two new reasons to support the S-curve, neither of which I can remember.

So what is the current thinking on this? What is the justification for doing longer deco at the 70 and 60 foot stops than at the 50 and 40 foot stops?

 

[mikeny9]

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