dive computers and reverse profiles

[dmaziuk]

Correct, I think it may have been Simon Mitchell, but may have been the guys from Duke Hyperbarics and DAN that were saying they're running 60/70 right now. That's a FAR cry from 10/90...

Actually it looks a touch over 10/100 - I was mis-remembering -- and over 100 is NEDU study territory.

 

[sea_ledford]

Sometimes a crew member will even sneak up on you from behind and check your valve, after you're standing, getting ready to go. Last thing I do before jumping is to breathe off my reg and watch my SPG. I made this a habit long ago and do not forget it

I was one of those crew members. And if I started working on charter boats, I would be again! Every person that walked out the gate got their valve checked, including other guides and instructors. Even if a diver was a super DIR "I don't use computers and don't touch my s*@t!" (that would have been from the early 2000s) I would check their valves. Because I KNOW that I KNOW which direction a valve goes on. The rest of the muppets on board? Not so much. We had a couple of unusually large tanks for the big boys that had left handed valves, and those were almost always off and a 1/4 turn on.

If I was told by a diver to not touch their stuff, I would still check, but would let them know if anything moved and let them re-verify. I'm not a complete ashhole.

 

[wetb4igetinthewater]

I was one of those crew members. And if I started working on charter boats, I would be again! Every person that walked out the gate got their valve checked, including other guides and instructors. Even if a diver was a super DIR "I don't use computers and don't touch my s*@t!" (that would have been from the early 2000s) I would check their valves. Because I KNOW that I KNOW which direction a valve goes on. The rest of the muppets on board? Not so much. We had a couple of unusually large tanks for the big boys that had left handed valves, and those were almost always off and a 1/4 turn on.

If I was told by a diver to not touch their stuff, I would still check, but would let them know if anything moved and let them re-verify. I'm not a complete ashhole.

How would you check someone in sidemount?

 

[scubadada]

How would you check someone in sidemount?

No sidemount back in the day

 

[saxman242]

How would you check someone in sidemount?

Buy them dinner first

 

[wetb4igetinthewater]

No sidemount back in the day

I can imagine the scuffle today!

Buy them dinner first

Hahahahaha!!!!

 

[EFX]

As mentioned, the leading researchers in decompression theory have essentially said that the theory behind bubble models is not in line with the current state of the art in decompression. The Spisni study was DIR divers thinking their brains were smarter than computers, they were proven wrong. The researchers have come out saying that even the Shearwater default of 30/70 for gradient factors has a GF-Lo that is far too low for optimal decompression. If you're doing NDL diving it really doesn't matter, but if you're doing dives where the bubble models have you prioritizing deep stops, well, there is a reason no real technical diving is done with Suunto computers.

I agree with everything you said. Your statement that "bubble models is not in line with the current state of the art in decompression" is perhaps a softer way of saying that bubble models have not been proven safe, or if your glass is half full that bubble models haven't been proven unsafe. Irregardless, I believe that Buhlmann with GF's, assuming those GF's are higher than originally used are more efficient for decompression. But efficiency doesn't necessarily equate with safety. Obviously, if you ascend immediately to the surface from a 170 ft 30 minute dive you're going to be very efficient; your pressure gradient will be the greatest than with deco stops but you'll be bubbling worse than a shaken warm beer on a 100 degree day. (Did you know what S.C.U.B.A. really stands for? Some Come Up Bubbling Alot.)

But, there's another reason why technical divers prefer Buhlmann with GF's over bubble models. It's not as easy to visualize what VPM-B +2 looks like when compared to tissue compartment supersaturation/depth charts. What does +2 mean? Even if you know what the critical bubble size is, how does that affect the graph of supersaturation/depth verses the critical supersaturation line?

[--- open can of worms here]
Some (all?) of the researchers of the "beloved" NEDU study reached a conclusion that went beyond the efficacy of what they were studying: the effect of supersaturation of deep stops versus shallow stops. In regards to the safety of shallow stops over deep ones they proved their point, but there was no mention of bubble size and its effects on decompression. You can read the study yourself (most of you already have). Here is what I wrote on the NEDU thread:

Here are the numbers from the NEDU study (depth = 170 ft, BT = 30 min.):

Shallow stop deco stops (depth/time): 40/9, 30/20, 20/52, 10/93. DT = 174 (neglecting ascent time, DT = decompression time).
Deep stop deco stops (depth/time): 70/12, 60/17, 50/15, 40/18, 30/23, 20/17, 10/72. DT = 174.

Running the dive plan on my Perdix with VPM-B +2 gave this profile for the same depth and BT:
Deco stops (depth/time): 100/1, 90/2, 80/3, 70/4, 60/5, 50/5, 40/7, 30/14, 20/20, 10/34. DT = 95.

Running the dive plan on my Perdix with GF's of 35/82 gave this profile for the same depth and BT:
80/1, 70/2, 60/3, 50/5, 40/6, 30/11, 20/21, 10/43. DT = 95 min.

As you can see the deepest stops for VPM-B are very short compared to the Navy's deep stop schedule (1,2,3,4 min vs. 12,17,15,18 min.). This is in keeping with the idea of limiting slow tissue on-gassing and the resultant super-saturation of those tissues which led to the DCS hits as reported in the Navy study, while limiting bubble growth beyond the critical size.

My question is: why would you be so quick to agree that bubble models in general and VPM in particular are inferior to dissolved gas models in general and Buhlmann GF's in particular, when the NEDU study is testing something quite different from what [non-Navy] bubble models are doing?
[--- close can of worms here]

 

[tbone1004]

My question is: why would you be so quick to agree that bubble models in general and VPM in particular are inferior to dissolved gas models in general and Buhlmann GF's in particular, when the NEDU study is testing something quite different from what [non-Navy] bubble models are doing?
[--- close can of worms here]

I want to like bubble models, and I think most of the engineery type divers do. It's almost too obvious. The problem is that the studies have all shown that the curves don't play out and too many people have been bent like pretzels from those profiles. Not that they can't or won't get bent on ZHL, and the studies were anything but conclusive to give a formal recommendation for ideal gradient factors, but the results were at least to me conclusive enough when combined with the other studies being done and recommendations from the leading professionals in the field that we want to have a steeper ascent curve to minimize risk of what we understand to be factors of decompression stress. Sorry for that long sentence, but not a great way to chop it up.
Since the bubble models are going to inherently create a flatter ascent profile, and that goes against the current recommendations of leading decompression researchers, the models themselves are essentially disproven because what they are meant to do goes against the current understanding. In contrast, things like the ZHL models are more in line with our current understanding and are a better model to work with.

 

[EFX]

Since the bubble models are going to inherently create a flatter ascent profile, and that goes against the current recommendations of leading decompression researchers, the models themselves are essentially disproven because what they are meant to do goes against the current understanding. In contrast, things like the ZHL models are more in line with our current understanding and are a better model to work with.

Disproven? Hardly! Your contention that ZHL produces flatter ascent profiles is hardly worth mentioning, at least for the example profiles I gave above. The VPM +2 profile only calls for an extra 3 minutes at the deep end adding stops at 90 and 100 ft while saving you 9 minutes at the 10 foot stop. Granted, +2 is not very conservative. VPM-B +5 (the most conservative setting) gave 5 more deep minutes over ZHL GF 35/82 with the same added deep stops of VPM-B +2.

 

[sea_ledford]

How would you check someone in sidemount?

Never saw sidemont at that point. Had a few CCRs (didn't touch their stuff), I don't even remember any doubles, no need really.

If I were doing it today, I would leave the more technically oriented setups alone. If they can't sort out their own stuff, it'll be a good learning experience for them! Speaking as someone who has jumped in the water with doubles turned off... where was the damn crew to turn my tanks on for me!!!?