Can you do too much deco?

[RedSeaDiver2]

Good questions, and your main point is correct--we really don't know.

Buhlmann did many, many years of research to come up with his tables, and his tables are in the range of others who did many, many years of study, going back more than 100 years. If you just followed pure Buhlmann, you should be OK; at least, that's what his research says.

But when deep stops became the rage about 20 years ago, RGBM and (especially) VPM became the most popular algorithms, and they had first deco stops that were far deeper than Buhlmann. In order to mimic those deep stops, gradient factors were created for Buhlmann. People were using Buhlmann with GF lows of 20 to match the more radical of the deep stop algorithms.

Then the first attempts to study deep stops came out and they were not favorable. It appeared that those stops were too deep. So now people are still doing first stops deeper than Buhlmann envisioned, but not as deep as was popular in the deep stop heyday.

What research supports the modern diver's use of GFs to define a first stop depth? If there is any, I don't know about it.

One of the problems is that we don't have a good way of testing. The NEDU test had divers get DCS. That is not going to be used as a metric today. The Spisni study found excess inflammation in the deeper stop divers--and that was in comparison with a relatively deep Buhlmann, and under test conditions that should have favored the deep stop divers. That kind of metric probably won't be used. Bubble imaging is the most popular method of studying today, but that is not a really good measure because although high bubble scores are commonly found in people who get DCS, they are also commonly found in people who don't get DCS. There seems to be some correlation, but the exact nature is unproven.

The final reason we aren't seeing studies is a simple one--what we have now, for both decompression dives and recreational dives, seems to be working reasonably well. DCS rates are very low. This lessens the urgency of creating such a study.

I suspect that when people were using Buhlmann with GF lows of 20 they were probably still using GF Highs of 80 - which as we all know was never going to work.

While NEDU do great work - unfortunately their research isn't done using typical divers like us but is instead done on ultra-fit navy divers, and an acceptable rate of DCS there is 2% of dives resulting in DCS - obviously not something that we could live with.

 

[tursiops]

While NEDU do great work - unfortunately their research isn't done using typical divers like us but is instead done on ultra-fit navy divers, and an acceptable rate of DCS there is 2% of dives resulting in DCS - obviously not something that we could live with.

That study was 11 years ago (attached).
A fair amount has been learned since then.
Time to catch up on the more recent work by the NEDU first author (Doolette), and by others.

Also attached are two more, more recent, articles.

 

[RedSeaDiver2]

I feel like I am talking to a wall.

GF-High sets conservativism, pick what you are comfortable with. I'm not arguing for any particular value. Changing it has the expected effect.

But: GF-Low << GF-High is a bad idea no matter how conservative or not you are trying to be.

If you are comfortable with a GF-High of 80, good for you. GF-Low should be > 50 probably even higher. GF-Low = 80 is Buhlmann as designed with conservatism. GF-Low ~= 65 (per Doolette see below) might be better.

If you are comfortable with a GF-High of 60, good for you. GF-Low should be > 50. GF-Low = 60 is Buhlmann as designed with conservatism. GF-Low ~= 50 (per Doolette see below) might be better.

Interestingly I just ran the same bottom time with GF of 40/50 which gives a low GF which is 80% of the high, the deepest stop was 10 feet/3 metres shallower and overall dive time went out by 8 minutes due to longer shallow decompression.

 

[Wibble]

I suspect that when people were using Buhlmann with GF lows of 20 they were probably still using GF Highs of 80 - which as we all know was never going to work.

While NEDU do great work - unfortunately their research isn't done using typical divers like us but is instead done on ultra-fit navy divers, and an acceptable rate of DCS there is 2% of dives resulting in DCS - obviously not something that we could live with.

Think you'll find it was to do with matching Buhlmann to VPM/bubble models.

Because the GF-LO of 20 added enormously to the deco load, the dive lengths were crazy, so they started to do the high GF-HI's, believing the bubble model concepts of "doing the deco deep".

 

[boulderjohn]

I suspect that when people were using Buhlmann with GF lows of 20 they were probably still using GF Highs of 80 - which as we all know was never going to work.

While NEDU do great work - unfortunately their research isn't done using typical divers like us but is instead done on ultra-fit navy divers, and an acceptable rate of DCS there is 2% of dives resulting in DCS - obviously not something that we could live with.

20/80 was indeed a highly recommended setting in the midst of the deep stop craze, and it is still very much in use by divers who have either not been following more recent developments or who do not agree with them. Saying it was never going to work is contradicted by the fact that it worked the vast majority of times. As I indicated earlier, the 80 part of the setting should compensate for the 20 part of it.

Perhaps I did not understand your second paragraph, but the way I read it, you seem to be saying that all those super smart NEDU people (and other researchers over the years, including Buhlmann) do not know as much about the kind of diving you do, so it is safe for you to make up your own algorithm. Is that what you are saying?

 

[RedSeaDiver2]

It's true. Deeper stops are the equivalent of longer bottom times, and you have to compensate for that during the shallower stops.

The problem with that is that many of the early proponents of deep stops believed that deep stops were so very, very good for you that you not only did not have to compensate for them, you could actually spend less time on the shallow stops. Doing deep stops, they believed, lowered your total decompression time.

These divers were not using computers that automatically add shallow time for the extra deep time. They were using pre-planned written schedules, and they were doing that planning in accordance with that theory.

I think that the points you raise are where a lot of problems came in with deep stops, and they were making the deep stops too deep.

 

[RedSeaDiver2]

Here are three diveplans in Subsurface, similar to what the OP describes.
Tissue saturation heatmaps are at the bottom of each.
GFs of 30/60, 50/60, and 80/80 are all shown (not necessarily in order)
Which plan is "best?"
The answer is, it depends--sometimes.
30/60 and 50/60 are nearly indistinguishable.
Should we really keep debating 30/60 vs. 50/60 in the context of the OP's dive plans?

80/80 gets you out faster... it should be obvious which one that is.

OK I didn't know that Subsurface created tissue saturation heatmaps - I will take a closer look at it - many thanks!

 

[RedSeaDiver2]

Back to the OP, "can you do too much deco?"
Is 30/60 "extra cautious about DCS" with a "pretty good safety margin?"
#TriggerAlert



I'll take it as some indication that the very first answer, from a prominent advocate of ditching excessive "deep stops," basically said you're fine mate, enjoy your dives.

Essentially that is what we are aiming for - extra cautious about DCS with a pretty good safety margin - that is the sensible way to operate out here.
When Simon replied and said that it is fine then I wasn't really expecting too much more discussion - boy was I wrong!

 

[RedSeaDiver2]

Perhaps I did not understand your second paragraph, but the way I read it, you seem to be saying that all those super smart NEDU people (and other researchers over the years, including Buhlmann) do not know as much about the kind of diving you do, so it is safe for you to make up your own algorithm. Is that what you are saying?

Not make our own algorithm, but certainly dive more conservative GF's because we are neither as fit as navy divers, nor can we accept a DCS rate of 2%

 

[tursiops]

Not make our own algorithm, but certainly dive more conservative GF's because we are neither as fit as navy divers, nor can we accept a DCS rate of 2%

You are incorrectly conflating Buhlmann/GFs with an operational Navy 2% DCS rate.