Decompression Stop Questions

[waterpirate]

I am unclear on how you got certed to your current level, and posed these questions/ unless you were really bored. I secon the Powel book, buy a good software like vplanner or simmilar, and spend some quality time running scenarios untill you start getting your brain wrapped around these concepts. After that I would recomend some remedial discussion with a instructor for TDI, that you have not met yet. Your previous instructor did you a dis service IMHO.
Eric

 

[boulderjohn]

I recently had an experience that readers might find helpful.

I went to Cozumel by myself this past fall in the hope of doing some deco diving there, and it was much, much more economical for me to tag along with an advanced trimix class on their dives than to pay for a technical diving DM (required) and the DM's gas. I of course agreed to do whatever they were doing. On the last day, our key dive was to 300 feet, using V-Planner for the decompression profile. They inputed the gases and the planned bottom time and got a plan in accordance with the VPM algorithm. They were using the same software on their Liquision computers as backup. I put their plan into my wetnotes, and I inputed the same gases and bottom time into my Shearwater Predator to use as a backup. The Predator's Buhlmann algorithm gave me a much different profile from the V-Planner. The nice thing about the Predator is that it will continually adjust to the dive you are actually doing, and it will not lock you out if you ignore it in favor of a different plan. (Unlike some computers, the Predator understands that it might be used as a backup and does not get all huffy if you ignore it.)

As we ascended, I stayed with the group and their V-Planner profile while my Predator kept telling me to do something different and then adjusting when I didn't. I did stay another minute or two longer at the 30 foot stop, but other than that I stayed with the team throughout the dive.

Amazingly enough, everything we were using (the V-Planner plan, the Liquivision backups, and the Predator backup) had us finishing the dive within 30 seconds of each other.

 

[NAM001]

There is another factor that is in play and being ignored. Many of the answers are once again being given frm the DIR prospective. You do a ratio deco calc and all is fine. For those that are using computers being shallower than the depth called for can cause a problem. Deco time stops or you have to increase depth and reapproach again. if 9m is called for then 9.5 is ok but 8.5 is considered a missed stop and not ok. Time does not proceed untill the misst stop has been cleared/handled. One drawback of computers when complared to RD.

 

[danvolker]

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

After reading most of the interactions in this thread, I think it is clear that your situation makes you a "poster child example" of why the GUE approach with Fundies then, REAL tech, can be a much more functional way to become a skilled tech diver. It is not too late.... sign up for the next Fundies class you can get into. I would search out Bob Sherwood, if I were you

 

[boulderjohn]

There is another factor that is in play and being ignored. Many of the answers are once again being given frm the DIR prospective. You do a ratio deco calc and all is fine. For those that are using computers being shallower than the depth called for can cause a problem. Deco time stops or you have to increase depth and reapproach again. if 9m is called for then 9.5 is ok but 8.5 is considered a missed stop and not ok. Time does not proceed untill the misst stop has been cleared/handled. One drawback of computers when complared to RD.

That is not true with most decompression computers. They constantly adjust to what you are actually doing on the dive, as I described above.

Almost all of my early technical training was DIR with ratio deco. We were taught never to use computers unless they were in gauge mode. We were taught to use only ratio deco for planning, with ascent profiles based on an estimate of average maximum depth. We were taught how to make that estimate. We were taught to trust the computer between our ears rather then the one on a wrist.

Some friends of mine did such a dive, and one of them used a computer in gauge mode. That computer was able to log a precise profile of the dive. That profile showed that they had underestimated their average maximum depth by a surprising amount. It showed that they had taken much longer to reach their first deep stop than they thought they had. It showed they had mistimed a stop. They were surprised to see how different their actual profile was from the one that they had thought they had done. They wondered if that was the reason they got bent.

 

[waterpirate]

After reading most of the interactions in this thread, I think it is clear that your situation makes you a "poster child example" of why the GUE approach with Fundies then, REAL tech, can be a much more functional way to become a skilled tech diver. It is not too late.... sign up for the next Fundies class you can get into. I would search out Bob Sherwood, if I were you

Surely you jest? There is so much in play/missing here I doubt a rec/provisional would be issued. Finding Bob to start over with a mulligan might work.
Eric

 

[Kevrumbo]

Craig Chamberlain's original questions were not trolls or catfish posts --maybe naive in general. Give this guy a break! Try to elicit some useful dialog. . .

Easier for y'all to be dismissively uninformative than trying to explain something constructive for something as VITAL as this topic. . . Glib replies like Dan Volker's "just take Fundies" or obtuse Boulderjohn anecdotes and the rest of the patronizing replies above are the real disservice here.

 

[danvolker]

Craig Chamberlain's original questions were not trolls or catfish posts --maybe naive in general. Give this guy a break! Try to elicit some useful dialog. . .

Easier for y'all to be dismissively uninformative than trying to explain something constructive for something as VITAL as this topic. . . Glib replies like Dan Volker's "just take Fundies" or obtuse Boulderjohn anecdotes and the rest of the patronizing replies above are the real disservice here.

I beg to differ..... I was not being glib..... I believe there are divers like this OP with an honest desire to become safe tech divers ( as safe as we can be doing something like this), and that there is something like a "minefield" of tech instruction choices for a guy like this to try and chose from....his own class was a great case-in-point.... What came across to you as glib, was my honestly believing that a guy like this could get exactly what he really wanted from the start, via the Fundies route. Tech instruction has always had a percentage of shops offering the equivalent of what underhanded Used Car Salesmen have pushed for years.....Well, this may be unfair to the used car salesmen, but the idea should get across. Sure, mentoring is awesome IF you can find the right tech expert. And yes, most agencies have some great instructors that teach tech (most )....But a budding tech student with normal dive background will not be liekly to know good from bad.....The GUE Fundies route eliminates the danger of this.....and even if he just took Fundies, after this, his knowledge and skill would be such that from then on he "could" discern good tech instructor from bad, and nonsense tech ideas and proceedures from good ones.

---------- Post added January 26th, 2013 at 08:21 PM ----------

Surely you jest? There is so much in play/missing here I doubt a rec/provisional would be issued. Finding Bob to start over with a mulligan might work.
Eric

I don't know what you are getting at....If this guy went through fundies with Bob, he would finally get all the dive education he somehow missed before. Even if he could not pass on the first go around, he would "know what he did not know", and could keep working on this till he did know. One thing you could be sure of, he would not get a pass, without knowing enough to be an entirely different diver than he is now.

 

[kierentec]

Craig Chamberlain's original questions were not trolls or catfish posts --maybe naive in general. Give this guy a break! Try to elicit some useful dialog. . .

Easier for y'all to be dismissively uninformative than trying to explain something constructive for something as VITAL as this topic. . . Glib replies like Dan Volker's "just take Fundies" or obtuse Boulderjohn anecdotes and the rest of the patronizing replies above are the real disservice here.

I think the overall consensus is that the OP is in need of far more than what any discussion here will be able to provide. he has somehow completely missed the basics of deco theory and dive planning, and needs remedial training. I think you're right, it is easier for everyone to be dismissive than give an online lesson on deco theory and dive planning, but I don't feel that is the disservice here. This guy has been shorted pretty much the entire classroom portions of all of his courses from AN through ER, and I would bet his in water training was far from thorough as well.

The real disservice would be if everyone acted as if he was up to speed and his questions were typical for someone with his rating. I am relieved that he has seen the gap in his knowledge and has had the mind to seek answers to his questions, but it should not have come to that for these basic principals. Yes the topic is vital, but that is why it's so alarming that it has not been covered in his training.

 

[ajduplessis]

Everyone here assumes that the lack of understanding is due to shortfalls during training. I am not saying it is the case but the reverse could also hold true.