Differences in Ratio Deco

[rjack321]

As he should if that's what he's teaching.

I still question it, though. If you know what you're doing, you know how long it will take you to reach your first stop. Ending BT at the stop level implies to me that the initial ascent time is an unknown quantity until after the fact, which is a symptom of either a) something unexpected happening - would be a red flag WRT deco, or b) inexperience.

Or it takes longer to get up to deep stop #1 due to the profile of the cave

 

[amascuba]

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.

We are almost certain that one of the divers (has been bent twice - as far as I'm aware it's been after doing his second T2 dive of the day - John can acknowledge or discredit this though) that you mention has a PFO or some other physiological defect that prevents him from completing a successful deco within same ascent profile as yourself. For our other two team mates (both on the same dive - there was a third team mate, but he had 50% and o2, instead of just 50%), it was almost certainly a user error as they were a little deeper than what they averaged and were slow getting off the bottom to their first stops.

I personally haven't believed the reasoning for adjusting for altitude. I haven't gotten my head around the logic since open water class, actually. I had even asked Rob during my fundies class about altitude and his answer was: "What's altitude?" Where we dive at in New Mexico, you will reach 1ATA at 6ffw. At the surface we are at .8ATA. On top of that, we begin the dive at .8ATA, complete the dive at .8ATA (same ratio as if you were doing the dive at 1ATA and ending it at 1ATA), and our bodies have already acclimated to the altitude by equalizing itself to the pressures as we have most likely been at the altitude for several hours before even setting foot in the water. At least for you and the rest of my Colorado buddies, y'all are typically at the altitude for 12 hours or more before getting in the water (arriving Thursday night and getting into the water mid morning on Friday). I have on several occasions, driven up (from 3600ft to 7000ft and back down to 5400ft, jumped in the water within an hour or two of arriving, not compensated at all for altitude and have never had an issue.

You have to realize that altitude is just like deco. If you ascend in altitude, your body starts off-gassing to equalize itself with the pressure and vise-versa for the descent in altitude to sea-level. At some point, your body is equalized and it just doesn't matter.

So, yes, my personal feeling is that altitude adjustment is hogwash, RD or no RD. I'd be more concerned about atmospheric pressure changes based on weather patterns than I would based on 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.

George teaches RD 1:1 in Tech 1 and 2. Exponential curve for Tech 1 and S curve for Tech 2.

Related to the topic of deep stops, we were also told something in our RD class that we had not heard before and was not in the written course materials. Andrew said that in calculating RD profiles, bottom time does not end until the first deep stop (75%) is reached.

I don't know GUE's position on this.

That too was the first time I have heard it described that way and honestly it may have been in relation to the incident late last year that I already described above. Honestly, I haven't been treating my profiles this way, but the incident last year really reinforced getting off the bottom and that 75% stops are more likely about arresting your ascent without blowing your stops.

RossH published a discussion of this a long time ago. I read it, it made sense to me. I tried it and have stuck with the 50-60ft up (varies a bit depending on dive length). Unfortunately I can't seem to find his critique. The gist of it was that 75% (or 80% of ATAs) is a linear approximation of a curve. It works for the deepest dives (300ft) but the shallower you are the more absurd it gets. Stopping at 75ft (or even 70ft) from a 100ft dive is just nonsense no matter how long the dive. Even your fastest tissues are not offgassing, there's not enough gradient. Offgassing gradient consistently starts about 2ATAs up hence our choice to slow down 50ft up and start stops 60ft up.

The RD rules are a start to building your own experience base to know how much and where to deco out. Its intended to be an adaptive "algorithm" unlike a fixed computer generated mathematical scheme.

Algorithm. n. A set of instructions for calculating a function. The "function" in this case being how much deco to do and where to do it.



Yes this is a recent pad based on the observation that ALOT of even fairly experienced "DIR" divers are way too slow off the bottom. It can very easily happen and having the default to ignore this extra time wasn't conservative. So the new default is to stop the bottom time clock at the 1st deep stop.

That's exactly how RD was presented to us. Basically that RD was a starting point and that you and your team would most likely make adjustments overtime based on how you felt before, during, and after dives and your experience.

I'll admit, that I was aggressive on a dive last month and cut deco a bit short. Not a niggle, but I was paying attention to how I felt very closely. It's nice to know for when things go wrong, but I'm not going to make a habit out of cutting deco like that.

 

[rjack321]

George teaches RD 1:1 in Tech 1 and 2. Exponential curve for Tech 1 and S curve for Tech 2.

Around here we don't do 150max, 130ft average, 30+mins of BT on 25/25 with O2 for deco at sea level - at any level. I suspect I have been involved in more UTD Tech1 classes than any other ordinary (non instructor) diver and the dive with DCS I recall was way beyond UTD T1. And off the chart aggressive regardless of altitude.

That's exactly how RD was presented to us. Basically that RD was a starting point and that you and your team would most likely make adjustments overtime based on how you felt before, during, and after dives and your experience.

Example:
For O2 time, we choose from 5mins, 10mins, 15, 10/5bg/5, 10/5bg/10 where "bg" = backgas break. There's no "12.5mins on O2" parsing and dividing of the 20ft time. Typically rounding up but not always if, for instance, the 40-30ft segment was unusually long due to the profile we might count that time as extra backgas deco time and be willing to round down from the precise RD O2 time number. And if we brought the stupid bottle along we might as well use it for 5mins.

 

[amascuba]

Around here we don't do 150max, 130ft average, 30+mins of BT on 25/25 with O2 for deco at sea level - at any level. I suspect I have been involved in more UTD Tech1 classes than any other ordinary (non instructor) diver and the dive with DCS I recall was way beyond UTD T1. And off the chart aggressive regardless of altitude.

I'm not going to make assumptions to John's class, but in ours, we didn't make any profiles like that. We've averaged 130', but max depth is typically 140ish and usually one a couple minutes.

Here would be a pretty typical T1 profile for us:

We've also done 130' for 40 minutes, but unfortunately, I do not have those profiles as I just got a irda adapter for my bottom timer, which only holds 25 hours of data before it starts over writing itself.

Example:
For O2 time, we choose from 5mins, 10mins, 15, 10/5bg/5, 10/5bg/10 where "bg" = backgas break. There's no "12.5mins on O2" parsing and dividing of the 20ft time. Typically rounding up but not always if, for instance, the 40-30ft segment was unusually long due to the profile we might count that time as extra backgas deco time and be willing to round down from the precise RD O2 time number. And if we brought the stupid bottle along we might as well use it for 5mins.

Agreed.

Here's the profile that I was pretty aggressive on. I was deco captain, so it's my **** up.

And here's the next day's dive:

For the O2 segment on that dive, we did 10 on, 5 off, 5 on.