Can you do too much deco?

[Wibble]

...That's an argument to prefer for example 40/70 over 50/80, but not a reason to pick 20/80.

Why not 50:70? The point about the GF-LO is to get shallower and stop on-gassing in the slow compartments which is what happens when you've a lower GF-LO.

Using 50% as GF-LO is not going anywhere near the 100% M-value (your bend & mend comment earlier), but it does reduce the deeper stop times which therefore reduces the overall decompression times.

GF-HI affects the shallow stops, with a lesser affect on deeper stops (i.e. the curve).

At this point we'd normally refer to the ambient pressure (X) vs compartment inert gas pressure (Y) graphs.

Bottom line: the lower the GF-LO is, the slower the decompression will be (because the ambient tissue tension is lower). It appears to be accepted that this isn't a good thing.

 

[rob.mwpropane]

I am a really low man on the totem pole in this conversation, but I believe it was one of the renound Dr's that stated that they dive 60/80 unless they're the only Dr on board or too far from medical help then they'll switch to 50/70 to be a little more conservative.

 

[boulderjohn]

I don't hold to that theory and wouldn't suggest to move time from shallow to deep, like 20/80 or VPM does. I think that exertion at depth increases the decompression obligation in every tissue, fast and slow. That's an argument to prefer for example 40/70 over 50/80, but not a reason to pick 20/80. The other trick with calculating an virtually extended bottom time does just that; every tissue gets more gas, every stop becomes longer.

As I said earlier, increased perfusion at depth will increase nitrogen loading, and it would be wise to compensate for it. Why would doing deeper first stops (GF low = 40) be the solution? You are still on-gassing those tissues. Wouldn't it make sense to extend shallow stops (GF high) or do as Richard Pyle's group does and simply increase perfusion in the final stops?

 

[sea_ledford]

Also keep in mind we are talking about two ~200 foot dives every three days, with only one dive per day. There are probably technical instructors in popular areas that do more than that in both depth and quantity all the time.

I think the risks are relatively low even following standard GF practices. The big unknown factor is how much exertion is getting done at depth.

 

[bubblemonkey2]

Where is this 80/80 and 100/100 coming from? All anyone was advocating for was raising the GF-low to 50, making it 50/60 instead of 30/60. Not once has someone proposed that OP should raise his GF-high.

The ongassing through the slow ascent of the deeper stops results is slower tissues having to offgas more at close to the GF-high later in the deco profile. For each minute fast tissue offgassing you add at 30% GF, you add 10ish minutes of slow tissue offgassing at close to GF-high (60% in this case). It is a fundamentally bad trade off.

Low GF-Highs are better, all tissues are further from limits, but at a cost in time. Low GF-Lows are worse because they make the GF-High problem worse later in the deco and add a high cost in time.

Pretty good and succinct voicing of the tradeoff idea, as long as we accept that it's bad to on-gas a bit more and spend more time in deco. Reasonable assumptions... but the deco obligation is completed in either case.

Which leads to... why not 80/80? How high is too high? We actually don't really know, and it probably varies by the day and diver. I'm sure we would all like to know.

Apparently some people think a GF_low of 50 is too high. Why do they think that? Is anyone reporting DCS at GF_low of 50? Is it is less at GF_low of 30? Or more? Is there any study that tested that without violating deco obligations?

More time in the water (exposure/operational) and more cumulative on-gassing/off-gassing sounds like two reasons to go for the higher GFs. But we're picking 50/80 arbitrarily instead of 100/100, or some other numbers.

What if someone were to tell you that your GF_low of 50 was too low and that you were wasting time on-gassing? Would you say that you are choosing GF_low of 50 to protect your "fast tissues?" Isn't this exactly what the GF_low 30 people are saying?

 

[boulderjohn]

Which leads to... why not 80/80? How high is too high? We actually don't really know, and it probably varies by the day and diver. I'm sure we would all like to know.

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.

 

[leadduck]

As I said earlier, increased perfusion at depth will increase nitrogen loading, and it would be wise to compensate for it. Why would doing deeper first stops (GF low = 40) be the solution? You are still on-gassing those tissues. Wouldn't it make sense to extend shallow stops (GF high) or do as Richard Pyle's group does and simply increase perfusion in the final stops?

got it, agree. If you use 50/80 initially, you could go down to 50/70 for working dives and that makes sense to me, too.
Fits also to the concept with virtually extending bottom time:
For example, say we have a 20min@40m (130ft) dive with air only and get a decompression schedule with GF50/80 of 1min@12m, 3min@9m, 5min@6m and 9min@3m (total 18min)
Now we assume +20% faster gas loading at depth due to exertion, being virtually equal to +4min longer bottom time, which gives us (with 50/80 again) 2min@12m, 4min@9m, 7min@6m, 16min@3m (total 29min).
If I want to set my computer to a GF that gives me this schedule for the original 20min dive, then I have to use GF 50/67. So, indeed, GFlow doesn't have to go down, reducing GFhigh is sufficient for this example.

 

[boulderjohn]

Now we assume +20% faster gas loading at depth due to exertion

I have never seen any kind of estimate for increased gas loading due to exertion, but 20% seems awfully high. Divers are almost always at least swimming gently at depth, so some exercise has always been part of the algorithm. I don;t know the current situation, but at least a decade ago ScubaPro had computers that tried to use heart rate is an indication of increased on-gassing (I guess that is what it was for), but I don't think it made anything close to that much difference.

 

[inquis]

Which leads to... why not 80/80?

Buhlmann found the faster tissues could handle greater tension over ambient (as evidenced by the M-value line slope greater than 1), and indeed, an 80/80 leverages that (along with a 20% margin at the surface). Doolette favors treating the fast & slow tissues the same which is why he runs a GFlow of about 0.83*GFHigh; This results in a slope of about 1 for the GF-modified M-line. This means that a 1 fsw reduction in tissue pressure allows a 1 fsw reduction in depth. I personally like that consequence as well; and conveniently, I then only have to pick a final surface loading (GFhigh).

 

[boulderjohn]

Buhlmann found the faster tissues could handle greater tension over ambient (as evidenced by the M-value line slope greater than 1), and indeed, an 80/80 leverages that (along with a 20% margin at the surface). Doolette favors treating the fast & slow tissues the same which is why he runs a GFlow of about 0.83*GFHigh; This results in a slope of about 1 for the GF-modified M-line. This means that a 1 fsw reduction in tissue pressure allows a 1 fsw reduction in depth. I personally like that consequence as well; and conveniently, I then only have to pick a final surface loading (GFhigh).

For those interested, here is the reference:

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

gue.com

Most of the people with whom I dive are in this ballpark.