Diving with gradient factors for a new recreational diver

[doctormike]

You can acquire a pulmonary shunt later in life though, which also allows a bubble to pass into arterial blood similar to a PFO.

Right. But to acquire a PFO would require a gunshot wound or possibly a stab wound. Hard to get a big enough cohort do to statistics on what happens when those people go diving...

Of course, you could make the argument that it's not really a PFO until there is flow through it, and sometimes that doesn't happen without a significant pressure gradient (like with straining). The actual passageway is congenital.

 

[ginti]

Are you sure diving is for you? You seem REALLY concerned about established recreational diving practices which have a very low rate of DCS already (1: 4 to 8,000 dives roughly). This is far lower than the injury rate for almost any other outdoor activity.

To be honest, when I started diving, my instructors gave me the impression that DCS is the worst possible thing. I wouldn't be too surprised to discover that @HomeBarista had the same impression, and the fault is the one of his instructor(s), not his own...

@HomeBarista , in reality, DCS not only isn't that common, but it is also less lethal than you think; there are different degrees of DCS, starting from joint pain, skin bends, and then more serious ones. I have never read a statistic about it, but I am fairly sure that serious DCS events are extremely rare in non-decompression diving. Really, stop worrying about it.

 

[Akimbo]

To be honest, when I started diving, my instructors gave me the impression that DCS is the worst possible thing.

Agreed, in fact it is third on my list of potential problems:

Avoid drowning, embolism, and getting bent... in that order.

I have never read a statistic about it, but I am fairly sure that serious DCS events are extremely rare in non-decompression diving.

Based on conversations with liveaboard operators, misreading or misunderstanding computers is a leading cause. However, I was surprised that DCS was so common for them, like one mild but treatable case every month or two. Of course they are making 4-5 No-D dives per day.

My "impression" is that decompression algorithms are still less reliable with so many repetitive dives. Inaccuracies in calculated estimates of residual nitrogen (and/or Helium) accumulate and compound. This is one of the many reasons that dives were restricted to two/day in many of the tables used when these tissue calculations were done with slide rules and paper.

This leads me to be less aggressive with days of repetitive dives than one decompression jump in a day. When in doubt, prophylactic pure Oxygen is my friend.

I have been on projects where I made 4 working decompression dives/day. We did initial in-water decompression at 20' on pure Oxygen and completed decompression in a chamber. We basically ran an abbreviated treatment table on each chamber run (pure O2 at 60'/18M) that left us with nitrogen saturation levels well below normal, so we were "cleaner" after each chamber run than when we started our first dive.

 

[tursiops]

Based on conversations with liveaboard operators, misreading or misunderstanding computers is a leading cause. However, I was surprised that DCS was so common for them, like one mild but treatable case every month or two. Of course they are making 4-5 No-D dives per day.

My "impression" is that decompression algorithms are still less reliable with so many repetitive dives.

My impression is that the decompression algorithms are far more reliable than are the divers using them. I've never been on a trip where at least one person didn't say, "What is my computer saying? What is is trying to tell me? Why is it not working anymore?"

 

[ginti]

Based on conversations with liveaboard operators, misreading or misunderstanding computers is a leading cause. However, I was surprised that DCS was so common for them, like one mild but treatable case every month or two. Of course they are making 4-5 No-D dives per day.

Just to reassure the OP, let's do some calculations:
1 month=30 days;
Four dives per day = 120 dives per month;
Assuming ten divers per cruise, the result is one treatable DCS case over 1800 (I use 1800 since it is the average between one and two months)

Since the leading case is misreading/misunderstanding computers -> the "undeserved" hits are probably one over four or five thousand dives (or even more!).

And we are speaking of rather aggressive situations (4/5 dives per day!) where fatigue (and maybe too much beer) most likely plays a role - so in the end, nothing is undeserved here.

My "impression" is that decompression algorithms are still less reliable with so many repetitive dives. Inaccuracies in calculated estimates of residual nitrogen (and/or Helium) accumulate and compound. This is one of the many reasons that dives were restricted to two/day in many of the tables used when these tissue calculations were done with slide rules and paper.

The science behind it is not accurate yet, so for sure you are at least partially right, but I also agree with this statement:

My impression is that the decompression algorithms are fare more reliable than are the divers using them. I've never been on a trip where at least one person didn't say, "What is my computer saying? What is is trying to tell me? Why is not working anymore?"

 

[Celt]

Just stick to a safe assent rate, and don’t look up the symptoms of decompression sickness, forget about it.

 

[Akimbo]

My impression is that the decompression algorithms are far more reliable than are the divers using them.

I can't imagine anyone disagreeing with that, but it represents two different problems, human factors and inadequate input data.

The science behind it is not accurate yet,

Related to tursiops comment, the problem is both lack of understanding (the science) and ability to measure all the variables that influence DCS. There is a significant black hole of factors called "human variability".

Time, depth, and the gas mix are easy to measure and by far the most impactful. We know that exertion and temperature are lesser factors but so is age, overall health, smoking history, and what you consumed in the last 48 hours. For all we know, so is gender, family history, state of mind and the phase of the moon.

This leads us to two options: Figure out ways to measure all the secondary factors and how they impact the algorithms, or find a way to take direct measurements of the gases coming out of solution that causes DCS. I believe that the latter will happen long before the former.

Meanwhile, we live with a system that has proven to be quite adequate and deal with rare exceptions when they occur — basically chamber ride(s) and lots of Oxygen.

Adjusting gradient factors, conservatism settings, not "pushing" NDLs, safety stops, lying to your computer about the mix you are using, and prophylactic Oxygen are all attempts at reducing DCS risk. Fortunately, DCS has been a pretty low risk to recreational Scuba divers long before dive computers were available.

 

[scubaozy]

Its only 7 divers and 14 dives.... and they are changing more than one variable at a time and puzzled that they can't isolate the cause of the differences. This is not really robust science...

Assuming the figure is later supported by a study which actually differentiates the high and low GFs with a multifactorial design:
The bubble wave is probably shifted longer because the 20/85 profile has more time deeper, so there is more slow tissue on-gassing, those tissues offgas slower and you end up with more bubbles at 60+mins into the SI. The 50/75 profile is shifted shallower, there is less slow tissue on-gassing during the middle/lower portions of the deco profile, so there is less of a gas load to get released later in the SI.

I agree, it is not ideal that they changed both parameters but presenter is not drawing any conclusions on the outcome, he also does point out that he could not say which profile would be considered more conservative. Your comment is a tad too harsh. I have seen others like Mark Powell sharing same research and he also stated that outcome is not conclusive.
For the uneducated, life is more simple and right or wrong is more obvious.

 

[rongoodman]

On recreational dives you're simply not going to be down there long enough to load slow tissues.

This is following 39 dives on 32% over the course of nine days on the Palau Aggressor. (Sorry for the poor quality photo).

 

[Dan]

This is following 39 dives on 32% over the course of nine days on the Palau Aggressor. (Sorry for the poor quality).

You don’t have a link to it?

I am curious as I actually did this last month in Palau. I was on Palau Aggressor 2 for from Dec 13 to 19 (21 dives) and Rock Islands Aggressor from Dec 20 to 26 (23 dives). Made a total of 44 dives in 2 weeks.