Differences in Ratio Deco

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

boulderjohn

Technical Instructor
Scuba Instructor
Divemaster
Messages
31,647
Reaction score
29,789
Location
Boulder, CO
# of dives
1000 - 2499
I struggled with how to present this topic, because it has enough facets that I considered making it a couple of different threads. I am just exploring here and hoping for thoughts. I do not have a clear position--I stand in the perplexed middle.

My training in this is strictly UTD. After initial training from my instructor, I completed the full class with Andrew Georgitsis. Before taking that class, for more than a year I asked a lot of challenging questions to my instructor, to Andrew, and to Jeff Seckendorf, the Director of Training. I was highly suspicious of it, and I wanted more information that would satisfy me before I entrusted myself to it on aggressive dives. When I finally took the class, some of the presentation was clearly aimed directly at me--Andrew addressed me directly on some of his points. Not wanting to disrupt the class, I pretty much just took notes.

As for my knowledge of GUE, I got this from personal correspondence with JJ. I asked him the same questions I was asking Andrew and Jeff, and he very graciously gave me extended and detailed responses.

It was clear to me that there is a significant and growing difference between the two versions of RD. I will start with one of those differences here--the fundamental purpose and nature of RD.

JJ emphasized that RD is not a deco algorithm. It was created to enable divers to create an approximation of an established decompression algorithm. Because it is a linear mathematical process, it cannot perfectly recreate a nonlinear deco algorithm. It does successfully provide a good approximation within established parameters, and it can be used for that purpose within those parameters. When used, a diver should be able to validate it in comparison to the established algorithm.

Although in my discussions with UTD it was agreed that RD is not an algorithm, in the same email it said, in almost these words, that it is an algorithm. It is the primary planning tool for all dives, not because it accurately approximates an established algorithm, but because it is superior to established algorithms. In my RD class, RD derived ascent profiles were sometimes compared with various software profiles, not to show the similarity, but rather (it seemed to me) to celebrate the difference.
 
The use of RD at altitude was the primary reason for my challenges. I do most of my deco diving at high altitudes, and I was concerned that there was no altitude adjustment incorporated into RD. I was also concerned that our dive group had suffered 4 fairly serious cases of DCS in about a year of diving at altitude.

When I asked JJ about this, he said that RD was developed at sea level using sea level-based algorithms. He said they had done no such development at altitudes, and had no idea how effective it would be. He concluded that he would not use it at altitude until such a process had been completed.

UTD's position is that altitude has little to no effect on decompression, and RD has enough of a safety factor built in to make any such differences inconsequential. Andrew specifically said that if people are getting bent diving with RD at altitude, then there is some other reason for it.
 
The use of RD at altitude was the primary reason for my challenges. I do most of my deco diving at high altitudes, and I was concerned that there was no altitude adjustment incorporated into RD. I was also concerned that our dive group had suffered 4 fairly serious cases of DCS in about a year of diving at altitude.

When I asked JJ about this, he said that RD was developed at sea level using sea level-based algorithms. He said they had done no such development at altitudes, and had no idea how effective it would be. He concluded that he would not use it at altitude until such a process had been completed.

UTD's position is that altitude has little to no effect on decompression, and RD has enough of a safety factor built in to make any such differences inconsequential. Andrew specifically said that if people are getting bent diving with RD at altitude, then there is some other reason for it.

Wat?!

Can you please show me where he said that? That's lulzy.
 
Wat?!

Can you please show me where he said that? That's lulzy.

It was within the RD class I took. He was responding to the question of using RD without adjustment for altitude. He said that RD has enough conservatism in it that it does not need to be adjusted for altitude. If someone gets DCS while using RD at altitude, then a factor other than altitude is the cause.
 
The use of RD at altitude was the primary reason for my challenges. I do most of my deco diving at high altitudes, and I was concerned that there was no altitude adjustment incorporated into RD. I was also concerned that our dive group had suffered 4 fairly serious cases of DCS in about a year of diving at altitude.

Well I remember one of those cases (which you described here) and your profile was AGGRESSIVE to say the least. IIRC one of these dives was to 150ish max on 25/25 where you averaged 130ffw for 25mins, and only did something like ~12mins of deco at altitude to boot.

I flat out tell you (again) that was a rediculously aggressive dive. It was too deep on the wrong gases with inadequate deco compounded by not having enough experience to know that it would basically work for you. I'm not sure what George taught you for RD in UTD Tech1, but it was wrong, you misunderstood, or you chose to ignore it.
 
I've thought over the same things. If you look at pressure variations with altitude, it certainly seems as though it could be reasonable to make minimal to no adjustments to decompression, given a) the amount of deco we do, and b) the standard deviation in both total decompression and its distribution among decompression models.

But there ARE things about RD as taught by UTD that are not fitting well with what little science there is -- UTD/RD has a tremendous emphasis on very deep stops, and from reading things like the proceedings of the DAN conference on technical diving, it appears that not only is there little evidence to support doing that, what little there is says we ought to be moving upwards FASTER in those depth ranges.

In my GUE Rec Triox class (which covered all the classroom work for T1) we were taught that there are a number of methods of determining total decompression and the distribution of needed stops. We were also taught how to look at the profile generated by the software we were using, and make a rational analysis of it based on some underlying principles. Although I found it odd that GUE sells software that generates profiles they won't dive, I appreciated the non-denominational instruction on how to manage one's deco.

I personally use RD for the staged decompression dives I do -- which are not many, and not aggressive. But I run VPM on my X1 as a sanity check, and where VPM becomes vehemently opposed to my plan (which has been quite rare) I go ahead and do the deco it calls for. So far, so good.
 
UTD/RD has a tremendous emphasis on very deep stops

I don't do those "75%" stops anymore. I haven't done them for years since most of my buddies agree. My personal rule of thumb is come up 50ft from the bottom at 30ft/min. Then do a slow slide on the first deep stop. 1st "1min" deep stop is 60ft (2ata) off the bottom. Assuming a square profile - once you start massively multileveling (which I occasionally do up a wall) its more fluid but the principle remains - "75%" is just too damn deep unless you're doing 250ft+ dives.
 
In my GUE Rec Triox class (which covered all the classroom work for T1) we were taught that there are a number of methods of determining total decompression and the distribution of needed stops. We were also taught how to look at the profile generated by the software we were using, and make a rational analysis of it based on some underlying principles. Although I found it odd that GUE sells software that generates profiles they won't dive, I appreciated the non-denominational instruction on how to manage one's deco.

This brings me to another variation in the two programs, more of a slightly different evolutionary path, perhaps.

I have long been suspicious of the theory of the oxygen window and the resulting S-curve configuration of that section of the ascent profile. I asked a number of challenging questions about that privately within the UTD program, and I later asked JJ about it.

In the RD class I took (which, BTW, was only a few months ago), I was surprised by that part of the presentation. Andrew gave three reasons for doing an S-curve, one of which was the O2 window effect. He said that the O2 window theory was the least valuable of the three reasons, and there was some question about its validity. (Off the top of my head I don't remember the other two reasons--I had never heard them before.)

JJ also said there was doubt about the benefit of the O2 window effect. He said they are still teaching the S-curve because it has worked in the past and they are reluctant to change something that seems to be working. (In Deco for Divers, Mark Powell says that some DCS models are also based on that--"we aren't sure why, but it seems to be working.")
 
I don't do those "75%" stops anymore. I haven't done them for years since most of my buddies agree. My personal rule of thumb is come up 50ft from the bottom at 30ft/min. Then do a slow slide on the first deep stop. 1st "1min" deep stop is 60ft (2ata) off the bottom. Assuming a square profile - once you start massively multileveling (which I occasionally do up a wall) its more fluid but the principle remains - "75%" is just too damn deep unless you're doing 250ft+ dives.


I think the "first stop at 75%" thing is somewhat dependent on the bottom time. For longish bottom times (60-70mins at 150-160), I've been stopping at 75%, which is maybe 10ft deeper than the first stop prescribed by VPM (decoplanner).

Stopping at 75% after a shorter time (maybe 20mins?) might not really have a benefit.
 
https://www.shearwater.com/products/teric/
http://cavediveflorida.com/Rum_House.htm

Back
Top Bottom