UTD Ratio deco discussion

[orange_diver]

I learned RD from AG about 10 years ago and although i no longer tech dive I've followed all the recent findings, bought a petrel 2 a couple of years ago and dive recreationally with their 45/85 profile. If i did return to tech it would be running 2 shearwaters at 50/70 and I would never look back.

In class I recall AG saying altitude had zero effect and did not need to be taken into account when crafting your RD profile. The again, we also learned deep stops and s-curves and the oxygen window so I would put all that into the same circular file.

Interestingly, I've attached a snapshot of a transcript from a talk George Irvine gave in 2002 when asked specifically about diving at Lake Tahoe at 6000'. Perhaps it was even boulderjohn that asked the question.

his reply sounded very similar to the line AG was towing in 2005... PM me if anyone wants the full paper.

 

[decompression]

Interesting quote......I guess the jury is still out if there is a requirement to adjust RD for altitude, there's certainly a lack of research. I find it interesting that the one RD profile I ran without accounting for altitude was more conservative than the Buhlmann profile with altitude.

 

[boulderjohn]

I find it interesting that the one RD profile I ran without accounting for altitude was more conservative than the Buhlmann profile with altitude.

Please explain with specifics.

 

[Kevrumbo]

Atmospheric pressure decreases with altitude. Therefore surfacing with the same supersaturation ratio at diminished atmospheric pressure would only be possible with lower inert 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.
http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/2750/969023.pdf?sequence=1

Because of the reduced atmospheric pressure, dives conducted at altitude require more decompression than identical dives conducted at sea level. The air decompression tables, therefore, cannot be used as written. Some organizations calculate specific decompression tables for use at each altitude. An alternative approach is to correct the altitude dive to obtain the equivalent sea level dive, then determine the decompression requirement using standard tables. This procedure is commonly known as the “Cross Correction” technique and always yields a sea level dive that is deeper than the actual dive at altitude. A deeper sea level equivalent dive provides the extra decompression needed to offset effects of diving at altitude.

So try the following "Cross Correction" formulas within the Ratio Deco 2.0 paradigm:

For a given altitude A, the atmospheric pressure Pa (in atm) at that altitude is

Pa = (1 atm) * exp(5.255876 * ln(1 – (C * A))).

where C = 0. 0000068756 / 1 foot = 0. 000022558 / 1 meter,
depending on whether the altitude is given in feet above sea level or in meters above sea level.

With a calculated Pa (Pressure at Altitude determined from the above equation) and given Da (actual depth at Altitude in ffw), we have the general equation below giving Theoretical Ocean Depth (TOD), and with it we can use the dive tables that are based upon diving in the ocean, such as the UTD Min Deco/NDL Table or Cascading Ratio Deco Schedules:

TOD = Da * (1 atm / Pa) * (33 fsw / 34 ffw);

or TOD = Da * (1 atm / Pa) * (10 msw/10.3 mfw).

Figure out your adjusted depth deco profile using RD methodology, compare it with other deco software algorithms at altitude for the given actual depth and bottom time, and add more conservatism as necessary especially if you plan to incorporate deepstops to control fast tissue supersaturation- i.e. extending out a shallow O2 deco stop to further reduce the resultant decompression stress on the slow tissues, especially when surfacing at a lower ambient pressure at altitude.

 

[RayfromTX]

Please help me to see where I am wrong in my logic.

At 6,000 feet asl my nitrogen saturation before ascent is lower than it would be at sea level. At some depth below the surface I am effectively at the equivalent pressure that I would be at sea level and if I stayed there long enough I would reach a saturation point equal to that at sea level.

If instead, I continue to descend I start to saturate my tissues further and eventually I build a deco obligation that must take into account the altitude at which I will surface.

Imagine for a moment I was able to begin my dive at 6,000 asl and surface at sea level. I start the dive with less than sea level saturation levels and the bottom time could be longer than normal because of this if I was able to magically surface in a sea level environmental pressure.

It seems that y'all have been talking about the penalty at the end without considering the benefit of the lower than normal tissue saturation at the beginning. I heard this alluded to by a couple of people including the snippet by George Irvine. Isn't this why some people have stated in the past that altitude can be ignored? Help me understand why that is wrong.

 

[Kevrumbo]

Please help me to see where I am wrong in my logic.

At 6,000 feet asl my nitrogen saturation before ascent is lower than it would be at sea level. At some depth below the surface I am effectively at the equivalent pressure that I would be at sea level and if I stayed there long enough I would reach a saturation point equal to that at sea level.

If instead, I continue to descend I start to saturate my tissues further and eventually I build a deco obligation that must take into account the altitude at which I will surface.

Imagine for a moment I was able to begin my dive at 6,000 asl and surface at sea level. I start the dive with less than sea level saturation levels and the bottom time could be longer than normal because of this if I was able to magically surface in a sea level environmental pressure.

It seems that y'all have been talking about the penalty at the end without considering the benefit of the lower than normal tissue saturation at the beginning. I heard this alluded to by a couple of people including the snippet by George Irvine. Isn't this why some people have stated in the past that altitude can be ignored? Help me understand why that is wrong.

I still don't see why.

Let's just compare bubble growth. If a diver ascends from 136 FFW to the surface at sea level, we could calculate bubble growth according to Boyle's Law (P1V1=P2V2):

5*1=1*V2
5/1=V2
5=V2​

The bubble would grow to 5 times its size.

Here is that same bubble growth calculation at 6,000 feet (0.8 Atmospheres)

4.8*1=0.8*V2
4.8/0.8=V2
6=V2​

The bubble would grow to 6 times its size.

We would see the same kind of math in relation to the gradient between gas dissolved in the tissues and ambient pressure, meaning that it would take longer for dissolved gas to reach a safe pressure prior to final ascent.

​

I understand everything you said about the NEDU study, and I agree with it. I don't understand how it has any impact on what I wrote about the difference between ascending at sea level and ascending at altitude.

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.

 

[decompression]

Please explain with specifics.

My post #179

Using your profile, 180', 27 BT at sea level and 6000' altitude.

Your sea level dive (Buhlmann) was 38 mins of Deco, your 6000' dive was 46 mins of deco.
Ratio Deco was 44 min of Deco, with no adjustments for altitude. Sorry, 2 minutes of Deco "less" not more conservative, now that I had a closer look. Pretty close though as the Deco time wasn't added to for conservative factors (cold water, age, fitness etc).

But does that give an example of a change in RD since you were taught it? What would your profile look like using what you learned as RD?

 

[RayfromTX]

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?

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?

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?

 

[mikeny9]

I asked what sea level depth equivalent would you use in order to plan a 160' dive at 6000' elevation. You never answered

Either you're not reading my responses or not remembering them:

Because 160' at 6000 is definitely not 160' at sea level, the dive is outside the limits of my training regardless of bottom time.

You see I did answer, it's just not the answer you're looking for.

Since you never answered the question about the sea level equivalent depth, I had to come up with a number on my own in order to run a plan...

I did answer your question, you either didn't read it or didn't like it. So you're putting words in my mouth, cool.

You said you could not plan ANY dive to 160' at 6000' elevation (using RD or Buhlmann) because it would be more deco time that you are certified for.

You're not reading my response or not remembering again:

...the limits are no more than 30 min of deco, no more than 15 min on 100% O2, and no average depth greater than 160' (at sea level).

The limits are not just deco time, but depth as well.

The result was the conclusion that you could EASILY do a 200' dive at sea level and have less than 30 minutes of deco.

Maybe YOU could do a 200' dive at sea level, but if you actually read the limits I've my training, you would know that I wouldn't do a dive to 200' at sea level for any amount of time.

So, your statement that cannot do a dive to 160' at 6000' of elevation with less than 30 minutes of deco, using Buhlmann, is just wrong.

I never said you couldn't do it using Buhlmann, quite the opposite actually, again you're not reading my responses:

Perhaps it's possible for you to get 30 min or less of deco time and do a dive to 160' with VPM-B or Buhlmann GFs...

The original profile boulderjohn posted was a dive to 160', for 25 minutes, at 6000 ft of elevation. Knowing what I know about ratio deco, I can quickly determine that it'd be more than 30 minutes of deco (e.g. outside the limits of my training). Boulderjohn's profile he posted confirms this as well.

Well, given that I chose 200' as the sea level equivalent because you won't answer the question of what sea level depth is equivalent to 160' at 6000'.

You can conveniently put words in my mouth; I did answer the question, but you're the one who started talking about 200'. This part was confusing:

I just planned for 15 minutes at 200' (elevation: sea level)

The dive I planned and then talked about does not ever exceed 160'...

Anyway, there's no point in depth adjusting 160' for me because again it's beyond the limits of my training.

The dive I planned, with 25 minutes of deco, is counting the deco time starting from as soon as the diver leaves the bottom. I planned 15 minutes of BT and got a total runtime of 40 minutes. So, 25 minutes of deco (i.e. total ascent time).

Cool, good for you. I wouldn't do a dive to 200' at sea level.

You said that you can't do the dive but it's not because of Ratio Deco and that is clearly just nonsense.

If you would read what I posted, I can't do a 200' dive not due to ratio deco, but due to training limits.

I also wouldn't do a 160' dive at altitude for any amount of time, and that's because I do not understand ratio deco beyond 160'/170' at sea level. It's the same reason I wouldn't do a dive beyond 160' at sea level. This is is a limitation of my training, not a limitation of ratio deco itself. Like I said, if/when I take Tech II, I'll come back here and drop a profile.

And you still haven't answered boulderjohn's question: How did you determine that 130' at 6000' of elevation is the same as diving to 160' at sea level? What process did UTD teach you that you used in order to come up that number of 130'?

Hmm, for the 8th time now, UTD didn't teach me anything about depth adjusting for altitude. I did answer boulderjohn's question, but you're just not reading it:

I would look at some depth adjustment table or a computer like deco planner or multi deco to figure out what the depth adjustment should be, then just use RD according to the new depth.

Speaking of not answering questions:

Are these acceptable?

John, are the profiles I posted for 6000 ft elevation acceptable?

And my earlier question that you still haven't answered: What depth at sea level is equivalent to diving to 160' at 6000' of elevation? What number and how do you calculate it?

Again:

Because 160' at 6000 is definitely not 160' at sea level, the dive is outside the limits of my training regardless of bottom time.

Not the answer you're looking for sadly.

...when one would expect to learn about adjusting for altitude in the UTD RD curriculum.

Wow...

For the 8th/9th time, UTD doesn't teach anything about altitude diving in RD. I'll keep going I guess, how many can we get to, 20 times?

 

[boulderjohn]

My post #179

Using your profile, 180', 27 BT at sea level and 6000' altitude.

Your sea level dive (Buhlmann) was 38 mins of Deco, your 6000' dive was 46 mins of deco.
Ratio Deco was 44 min of Deco, with no adjustments for altitude. Sorry, 2 minutes of Deco "less" not more conservative, now that I had a closer look. Pretty close though as the Deco time wasn't added to for conservative factors (cold water, age, fitness etc).

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.