UTD Ratio deco discussion

[The Chairman]

I don't let it run my ascent though, that's my responsibility.

I don't get it. Why the aversion to a PDC??? You're letting it do all the work, and then you rely on a system that's based on a guess, some arrogance and wishful thinking to ascend? What's the upside? I can see the downsides, but I really can't see the upside. Not trying to be combative either, but you're relying on electronics and actual research when you use your PDC for the entire dive.

 

[boulderjohn]

Interesting, so all of those things your friends miscalculated I use a computer for: average depth, ascent rate, and I run a stop watch on it when I start my ascent. In my opinion, computers/digital bottom timers are much better at this than the brain. .

For those who did not read the post I linked to in #161, I compared two situations. One was the one I mentioned a couple of posts ago (and mentioned in the part I quoted here), and the other was a situation in which two rebreather divers did not realize they were making an ascent that was far, far slower than their dive plan called for. In both cases, then, the two groups made mistakes that should have impacted their ascent profiles. Here is the difference.

1. The first group was using bottom timers and computers in gauge mode, as was required by their agency, so they had no idea they had made the mistakes. Not knowing they had made the mistakes, they followed their plan without adjustment and got bent.
2. The second group was using computers, but they did not notice their mistake either. In fact, with the dive was over, they still did not realize it was a mistake! Their computers knew it, though, and they made the adjustments for them. Fortunately, they did not assume the computer was wrong because it did not match what they thought they had done and ignore it.

 

[mikeny9]

I don't get it. Why the aversion to a PDC??? You're letting it do all the work, and then you rely on a system that's based on a guess, some arrogance and wishful thinking to ascend? What's the upside? I can see the downsides, but I really can't see the upside. Not trying to be combative either, but you're relying on electronics and actual research when you use your PDC for the entire dive.

To be fair, that's strictly your opinion. I don't share the same belief as you. You think RD is based on a "guess, arrogance, and wishful thinking," which simply isn't true. No offense, but most of the posts I've seen on here about RD are pretty ignorant. I would respect opinion's about RD from people if they took a class and dove it for awhile than those who simply read a thing or two about it. Most UTD people I know avoid Scubaboard for that reason alone.

In several UTD classes I've taken, students are taught exactly how RD is derived, what science it's based off of and where it comes from, and how it becomes an "ascent strategy" rather than just another model to follow blindly. A model is just that, a model. As I mentioned before, the computer is computationally precise, and the numbers it produces are accurate to the numbers the model would produce, but the model itself may not be accurate. If a cooking recipe tells you to put chicken in the oven for 20 minutes on 400, but you notice it hasn't even turned colors yet after 20 minutes, do you take it out? A computer cannot make common-sense decisions, as I described in my Google Maps analogy.

I also rely upon electronics, research, my brain, and my buddy's brain for my ascent. I used to set my computer to Buhlmann GFs and compare my own ascent strategy against the computer. If anything was terribly different, either I was off or the computer was. In practice this hasn't happened after 150+ dives, and I just set it to gauge mode now because I like the extra information it shows when you remove the stop and gas info.

For those who did not read the post I linked to in #161, I compared two situations. One was the one I mentioned a couple of posts ago (and mentioned in the part I quoted here), and the other was a situation in which two rebreather divers did not realize they were making an ascent that was far, far slower than their dive plan called for. In both cases, then, the two groups made mistakes that should have impacted their ascent profiles. Here is the difference.

1. The first group was using bottom timers and computers in gauge mode, as was required by their agency, so they had no idea they had made the mistakes. Not knowing they had made the mistakes, they followed their plan without adjustment and got bent.
2. The second group was using computers, but they did not notice their mistake either. In fact, with the dive was over, they still did not realize it was a mistake! Their computers knew it, though, and they made the adjustments for them. Fortunately, they did not assume the computer was wrong because it did not match what they thought they had done and ignore it.

The first group you mention in "#1" miscalculated their average depth, ascent rate, and amount of time at their last stop. A bottom timer tells you all of these things. No one that I know of is against using a computer/bottom timer to calculate average depth, show you an ascent rate, and have a stop watch. Group #1 sounds pretty foolish to me if they didn't have these things.

In the second group, you mentioned in the other thread they had an additional 10-15 minutes of deco. What was the average depth and total time spent at the bottom? They would have to be ascending stupidly slow to incur another 10-15 minutes of deco. In any case, were they using a bottom timer / computer to show their ascent rate? Or were they as foolish as group #1? If an RD diver was ascending stupidly slow like group #2, they should treat some of that time as bottom time and readjust their average depth, which would add to their total deco obligation and change the ascent profile.

 

[Patoux01]

To be fair, that's strictly your opinion. I don't share the same belief as you. You think RD is based on a "guess, arrogance, and wishful thinking," which simply isn't true. No offense, but most of the posts I've seen on here about RD are pretty ignorant. I would respect opinion's about RD from people if they took a class and dove it for awhile than those who simply read a thing or two about it. Most UTD people I know avoid Scubaboard for that reason alone.

Many people that talk about RD here actually have more experience than you with it, I believe...

 

[mikeny9]

I wish that were true. Unfortunately most people haven't taken any of the classes, nor dove RD. Anyone who thinks it's just based off of a guess, arrogance, and wishful thinking clearly hasn't taken the classes. Have you? Also, how do you know how much experience I have with RD? How much experience do these people that you mention have exactly?

 

[decompression]

Have to agree with mikeny9, I think very few have taken a RD course and certainly less the 2.0 version. That said, many on SB do have considerable Deco experience/knowledge. The debate will continue and that's always informative, at the end of the day, it's a tool, your choice how you use it.

 

[boulderjohn]

I asked you what UTD teaches about planning a dive at altitude, and this is what you wrote:

t's not taught directly as a part of RD, but it is taught that you shouldn't assume RD works for all altitudes. I've never had the chance or the interest to do any altitude diving, but personally 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.

So help me to understand that process I should use in planning a decompression dive at altitude. Is the following accurate? If not, where is it incorrect?

1. Pick any decompression program, including DecoPlanner or any of the programs that are part of Multi-deco and select one at any of its conservatism settings or GFs, even though I have previously been taught that all of those programs are incorrect at sea level.
2. Look at what those programs suggest at the altitude at which I plan to do my dive.
3. Adjust it in some way according to my understanding of Ratio Deco.

I had a former UTD diver do something along those lines recently. Tell me if what he did was appropriate.

1. He agreed to dive with my buddy and me using his new Shearwater computer on Buhlmann with GFs of 50/80. We made sure all his settings were the same as ours.
2. Our dive was to about 200 feet, ub t it was not a super long dive. According to both our surface plan (discussed with him ahead of time) and what the computers showed, our first stop would be at 60 feet.
3. That is when his Ratio Deco training set it. He couldn't do it. So he did his first stop at 150 feet and then continued to stop every 10 feet after that.
4. This changed what his computer wanted him to do considerably, and it put him completely out of synch with us. We could see him throughout our deco, but he as well below us.

 

[boulderjohn]

The first group you mention in "#1" miscalculated their average depth, ascent rate, and amount of time at their last stop. A bottom timer tells you all of these things. No one that I know of is against using a computer/bottom timer to calculate average depth, show you an ascent rate, and have a stop watch. Group #1 sounds pretty foolish to me if they didn't have these things.

In the second group, you mentioned in the other thread they had an additional 10-15 minutes of deco. What was the average depth of the time spent at the bottom? They would have to be ascending stupidly slow to incur another 10-15 minutes of deco. In any case, were they using a bottom timer / computer to show their ascent rate? Or were they as foolish as group #1? If an RD diver was ascending stupidly slow like group #2, they should treat some of that time as bottom time and readjust their average depth, which would add to their total deco obligation and change the ascent profile.

Both groups made mistakes. When I was with UTD, we were told not to use the average depth feature of the bottom timer, because it included the ascent time. We were taught to check our bottom timers ever so many minutes and keep a running average in our heads. There is no question that they did realize that they had screwed up the ascent rate to the first stop, just as the second group did. The second group is also at fault because they were of the "the slower the ascent the better" school of ascending, which many people have said is the most common error in technical diving--they don't get off the bottom with the urgency they should.

But you missed the point. Both groups made mistakes. Both groups were unaware of the fact that they made the mistakes. The difference is that one group acted according to those mistaken beliefs, and the other group acted according to their computer's correction of their mistaken beliefs.

 

[boulderjohn]

To be fair, that's strictly your opinion. I don't share the same belief as you. You think RD is based on a "guess, arrogance, and wishful thinking," which simply isn't true. No offense, but most of the posts I've seen on here about RD are pretty ignorant. I would respect opinion's about RD from people if they took a class and dove it for awhile than those who simply read a thing or two about it.

I completed several hundred dives, including shallow training dives, in my UTD years. I was certified at Tech II. I took my Ratio Deco class from Andrew Georgitsis. That was a long time ago. I am honestly trying to understand what the current thinking is here, and this is your chance to show everyone what that is.

Below you will find two dive profiles from Multi-deco ZHL-16 C with GFs of 50/80, which is what I am currently using. The first is for a simple freshwater dive using 21/35 at sea level, and the second is the exact same dive at 6,000 feet. (I did it with 2 deco gases; I can change it to 1 deco gas if you prefer.) When I was with UTD, we would have done them both the same, but you say things are different now, and UTD will change practices at altitude.

Could you please post what those two dives would look like in current UTD practice? It doesn't have to be you who answer--any of the other UTD instructors reading this thread can fill in the information.

Elevation = 0ft
Conservatism = GF 50/80

Dec to 180ft (3) Triox 21/35 60ft/min descent.
Level 180ft 27:00 (30) Triox 21/35 1.32 ppO2, 85ft ead, 105ft end
Asc to 80ft (33) Triox 21/35 -30ft/min ascent.
Stop at 80ft 0:40 (34) Triox 21/35 0.71 ppO2, 30ft ead, 40ft end
Stop at 70ft 1:00 (35) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 2:00 (37) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 3:00 (40) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 4:00 (44) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 6:00 (50) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 8:00 (58) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 14:00 (72) Oxygen 1.29 ppO2, 0ft ead
Surface (72) Oxygen -20ft/min ascent.


Elevation = 6,000ft (s)
Conservatism = GF 50/80

Dec to 180ft (3) Triox 21/35 60ft/min descent.
Level 180ft 26:53 (30) Triox 21/35 1.32 ppO2, 85ft ead, 105ft end
Asc to 80ft (33) Triox 21/35 -30ft/min ascent.
Stop at 80ft 0:40 (34) Triox 21/35 0.71 ppO2, 30ft ead, 40ft end
Stop at 70ft 1:00 (35) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 2:00 (37) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 2:00 (39) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 3:00 (42) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 6:00 (48) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 7:00 (55) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 25:00 (80) Oxygen 1.29 ppO2, 0ft ead
Surface (80) Oxygen -20ft/min ascent.

 

[Kevrumbo]

As for diving at altitude, it makes sense that attempting to surface with the same sea level saturation value at a diminished atmospheric pressure can only be possible with a much lower inert gas load for a particular leading compartment; in-other-words decompression profiles have to be compensated with either longer shallow stop times and/or more conservative stop depths than the allowable Buhlmann M-values.

However IIRC, the original premise of not having to compensate for "Lake Tahoe" type dives was that altitude was conserved in the RD method calculation; IOW, the diminished atmospheric pressure at altitude was automatically translated to at depth in absolute atmospheres, so the tissue ongas/offgas rates would be the same with either the same or slightly less inert loading.

The trouble with the above assumption -that classic RD with deepstops needed no altitude compensation- IMHO, comes again from the results & implications of the NEDU Deepstops Study: you have a surfacing Slow Tissue Critical Supersaturation, compounded by reduced atmospheric pressure upon exiting the water. In this instance the risk for DCI would be much greater than the identical profile performed at sea level.