Gradient Factors and Deep Stops

[boulderjohn]

Finally, as I have told you in the past, based on experience of acting in the role of medical officer for over 10 major technical diving expeditions to remote locations, I believe there is a substantial burden of unreported relatively mild DCS among technical divers that is self treated in the field. It follows that even if we had a numerator for DCS cases in technical decompression dives, there would be problems with accuracy.

When I was a technical diving student diving in New Mexico, we had a number of DCS cases (or at least suspected DCS cases. Two that I know of resolved with surface oxygen. Three that I know of resolved with IWR. One got chamber treatments starting after driving home to Colorado, and the other got chamber treatments after a helicopter evacuation. the last two are the only ones I know that were reported to any kind of authority. (Note: I was only present for a couple of these and am relying on information given to me afterward.)

 

[boulderjohn]

Is the protocol always 20/85, or does it vary with situation?

Although it is not as deep as has been used by some in the past, 20/85 is still a deeper stop profile than those being used by a number of other people these days. Here is a dive with 20/85 and then one with 50/85. The 20/85 profile's first stop is 30 feet deeper than the 50/85. It has 4 more minutes decompressing on back gas before the 70 foot switch, and it adds 2 minutes at the shallow depths to compensate.

MultiDeco 4.14 by Ross Hemingway,
ZHL code by Erik C. Baker.
Decompression model: ZHL16-C + GF
DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 20/85
Dec to 200ft (3) Trimix 18/45 60ft/min descent.
Level 200ft 26:40 (30) Trimix 18/45 1.24 ppO2, 76ft ead, 95ft end
Asc to 130ft (32) Trimix 18/45 -30ft/min ascent.
Stop at 130ft 0:40 (33) Trimix 18/45 0.87 ppO2, 43ft ead, 56ft end
Stop at 120ft 1:00 (34) Trimix 18/45 0.82 ppO2, 38ft ead, 51ft end
Stop at 110ft 1:00 (35) Trimix 18/45 0.76 ppO2, 33ft ead, 45ft end
Stop at 100ft 1:00 (36) Trimix 18/45 0.71 ppO2, 29ft ead, 40ft end
Stop at 90ft 2:00 (38) Trimix 18/45 0.66 ppO2, 24ft ead, 34ft end
Stop at 80ft 3:00 (41) Trimix 18/45 0.60 ppO2, 19ft ead, 29ft end
Stop at 70ft 2:00 (43) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 3:00 (46) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 4:00 (50) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 5:00 (55) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 8:00 (63) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 10:00 (73) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 20:00 (93) Oxygen 1.29 ppO2, 0ft ead
Surface (93) Oxygen -20ft/min ascent.
OTU's this dive: 122
CNS Total: 52.6%
Gas density: 5.1g/l
168.7 cu ft Trimix 18/45
30.0 cu ft Nitrox 50
25.1 cu ft Oxygen
223.8 cu ft TOTAL



MultiDeco 4.14 by Ross Hemingway,
ZHL code by Erik C. Baker.
Decompression model: ZHL16-C + GF
DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 50/85
Dec to 200ft (3) Trimix 18/45 60ft/min descent.
Level 200ft 26:40 (30) Trimix 18/45 1.24 ppO2, 76ft ead, 95ft end
Asc to 100ft (33) Trimix 18/45 -30ft/min ascent.
Stop at 100ft 0:40 (34) Trimix 18/45 0.71 ppO2, 29ft ead, 40ft end
Stop at 90ft 1:00 (35) Trimix 18/45 0.66 ppO2, 24ft ead, 34ft end
Stop at 80ft 3:00 (38) Trimix 18/45 0.60 ppO2, 19ft ead, 29ft end
Stop at 70ft 2:00 (40) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 2:00 (42) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 4:00 (46) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 5:00 (51) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 8:00 (59) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 10:00 (69) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 18:00 (87) Oxygen 1.29 ppO2, 0ft ead
Surface (87) Oxygen -20ft/min ascent.
OTU's this dive: 115
CNS Total: 50.3%
Gas density: 5.1g/l
161.4 cu ft Trimix 18/45
28.4 cu ft Nitrox 50
23.6 cu ft Oxygen
213.4 cu ft TOTAL​

 

[Akimbo]

... It is true that the number of DCS cases in all recreational diving has been reported as falling by a number of authorities. An example is our own data: see this post. However, this tells you very little about what is going on in technical diving...

I concur, and probably take an even more cynical view of the stats. (As you know) Gross numbers are pretty meaningless unless you are also comparing the dive profile and decompression profile. It is statistically difficult enough for the world's navies to cobble meaningful numbers together with strict reporting procedures and a larger number of similar square-wave dive profiles. Field data collected from recreational tech divers will always be of marginal value given the meandering profile that most naturally follow. Scientific tech divers "might" make enough statistically comparable dives.

I would guess that we are accumulating a reasonable number of dive profiles off computers from divers that were bent, but not nearly enough from all those were not. It would be really helpful if dive computer manufacturers had a simple option in their dowload software to upload profiles to a central database... but I have yet to see one. DAN made some noise about it several years ago but I haven't seen much about it since.

There is very little wonder that almost no commercial mixed gas diving is done anymore that isn't full saturation. It is ironic that the most sophisticated and expensive form of deep diving has the safest record (on a man-hour/Meter basis). There is no coincidence that the decompression profile is also the simplest -- basically calculated for the slowest tissues only:

 

[Miyaru]

Is the protocol always 20/85, or does it vary with situation?

In addition to John's post on a gradient factor of 20/85:

It's a personal setting, one you have to determine for yourself.
Dr. Neil Pollock explains GF in this video and this article.

The goal is:

• to have a short decompression time
• to use as little decompression gas as possible
• to prevent DCS

20/85 is considered rather aggressive when you start to determine your own GF. A safer starting point is 30/70 (now default for Shearwater), increase in steps of maximum 5% after evaluating how you feel after a dive. The slightest itch, pain or other discomfort is likely an indication that you just passed your own safe factor.
What works for somebody else, is not necessarily the best GF setting for you.

 

[tursiops]

In addition to John's post on a gradient factor of 20/85:

It's a personal setting, one you have to determine for yourself.
Dr. Neil Pollock explains GF in this video and this article.

The goal is:

• to have a short decompression time
• to use as little decompression gas as possible
• to prevent DCS

20/85 is considered rather aggressive when you start to determine your own GF. A safer starting point is 30/70 (now default for Shearwater), increase in steps of maximum 5% after evaluating how you feel after a dive. The slightest itch, pain or other discomfort is likely an indication that you just passed your own safe factor.
What works for somebody else, is not necessarily the best GF setting for you.

Yes, I understand that and agree. I was asking what the GUE protocol is. Is it fixed at 20/85?

 

[Miyaru]

Yes, I understand that and agree. I was asking what the GUE protocol is. Is it fixed at 20/85?

GUE standards version 8, page 11:
Decompression Parameters: Decompression diving conducted during GUE classes must use GUE’s DecoPlanner as the reference standard, using the Bühlmann algorithm with a gradient factor of 20/85.

So not following current recommendations.

 

[PfcAJ]

20/85 is considered rather aggressive when you start to determine your own GF. A safer starting point is 30/70 (now default for Shearwater), increase in steps of maximum 5% after evaluating how you feel after a dive. The slightest itch, pain or other discomfort is likely an indication that you just passed your own safe factor.
What works for somebody else, is not necessarily the best GF setting for you.

Imo you need a bunch of dives to figure out what works. A succesful dive at a certain gradient factor really doesn’t mean anything. Also, an unsuccessful decompression at a certain gf doesn’t mean anything.

Additionally, a particular gradient factor isn’t iso-risk across all decompression times.

The plural of anecdote isn’t data.

 

[rossh]

It would be really helpful if dive computer manufacturers had a simple option in their dowload software to upload profiles to a central database... but I have yet to see one. DAN made some noise about it several years ago but I haven't seen much about it since.

Done already.... http://database.hhssoftware.com/ contains 150,000 dive records, from 2008 to now, collected through the X1 and DR5 dive computer logs. Tech and rec divers the world over, VPM and ZHL, GF, OC and CCR.

This is the only public data available of widespread tech and rec dive practices.

The results - both good and bad - were sent and collected through the DAN PDE program, and they have received 7,500 individual PDE submissions from this database.

What were the results I hear you ask??? DAN won't say and has published virtually nothing over the years, but I think Peter Buzzacott has some words to say on this in the next TekDiveUSA.

P.S. I know the results already - its an extremely low injury rate all round.

Cheers.

 

[Akimbo]

Done already.... http://database.hhssoftware.com/ contains 150,000 dive records

Unfortunately, given the number of possible dive and decompression profiles 150K dives isn't enough to conclude much of anything. Especially when there is no way to tell how representative they are to the total population.

 

[KevinNM]

GUE standards version 8, page 11:
Decompression Parameters: Decompression diving conducted during GUE classes must use GUE’s DecoPlanner as the reference standard, using the Bühlmann algorithm with a gradient factor of 20/85.

So not following current recommendations.

Classes are not supposed to be doing aggressive dives. Outside of class the team decides what profile to run. And for that matter, how they will dive.