NEDU Study

[Remy B.]

The closes I can get to what you mention that supposedly the Nave Manual said is 50/85, much closer of what Dr. Mitchel personally use.

Decompression model: ZHL16-B + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = GF 50/85

Dec to 40m (2) Air 18m/min descent.
Level 40m 37:47 (40) Air 1.04 ppO2, 40m ead
Asc to 30m (41) Air -8m/min ascent.
Asc to 18m (42) Air -8m/min ascent.
Stop at 18m 0:15 (43) Air 0.59 ppO2, 18m ead
Stop at 15m 3:00 (46) Air 0.52 ppO2, 15m ead
Stop at 12m 6:00 (52) Air 0.46 ppO2, 12m ead
Stop at 9m 8:00 (60) Air 0.40 ppO2, 9m ead
Stop at 6m 65:00 (125) Air 0.34 ppO2, 6m ead
Surface (126) Air -6m/min ascent.

OTU's this dive: 44
CNS Total: 15.2%

 

[Remy B.]

To put a comparison of what Rossh VPM advocates and Dr. Mitchell suggest 50/80 and the Nedu study outcome was, and what I believe it is a balance between the two 40/70

But at this depth it is likely that all give positive outcomes as the deeper stops are not so far apart.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = Nominal

Dec to 40m (2) Air 18m/min descent.
Level 40m 37:47 (40) Air 1.04 ppO2, 40m ead
Asc to 30m (41) Air -8m/min ascent.
Asc to 21m (42) Air -8m/min ascent.
Stop at 21m 0:38 (43) Air 0.65 ppO2, 21m ead
Stop at 18m 3:00 (46) Air 0.59 ppO2, 18m ead
Stop at 15m 5:00 (51) Air 0.52 ppO2, 15m ead
Stop at 12m 6:00 (57) Air 0.46 ppO2, 12m ead
Stop at 9m 9:00 (66) Air 0.40 ppO2, 9m ead
Stop at 6m 55:00 (121) Air 0.34 ppO2, 6m ead
Surface (122) Air -6m/min ascent.

Off gassing starts at 29.6m

Decompression model: ZHL16-B + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = GF 50/80

Dec to 40m (2) Air 18m/min descent.
Level 40m 37:47 (40) Air 1.04 ppO2, 40m ead
Asc to 30m (41) Air -8m/min ascent.
Asc to 18m (42) Air -8m/min ascent.
Stop at 18m 0:15 (43) Air 0.59 ppO2, 18m ead
Stop at 15m 3:00 (46) Air 0.52 ppO2, 15m ead
Stop at 12m 7:00 (53) Air 0.46 ppO2, 12m ead
Stop at 9m 9:00 (62) Air 0.40 ppO2, 9m ead
Stop at 6m 77:00 (139) Air 0.34 ppO2, 6m ead
Surface (140) Air -6m/min ascent.

OTU's this dive: 44
CNS Total: 15.2%

Decompression model: ZHL16-B + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0m
Conservatism = GF 40/70

Dec to 40m (2) Air 18m/min descent.
Level 40m 37:47 (40) Air 1.04 ppO2, 40m ead
Asc to 30m (41) Air -8m/min ascent.
Asc to 18m (42) Air -8m/min ascent.
Stop at 18m 2:15 (45) Air 0.59 ppO2, 18m ead
Stop at 15m 5:00 (50) Air 0.52 ppO2, 15m ead
Stop at 12m 7:00 (57) Air 0.46 ppO2, 12m ead
Stop at 9m 12:00 (69) Air 0.40 ppO2, 9m ead
Stop at 6m 108:00 (177) Air 0.34 ppO2, 6m ead
Surface (178) Air -6m/min ascent.

OTU's this dive: 45
CNS Total: 15.7%

 

[leadduck]

I ran it with subsurface and set the GF to match the USN plan. With GF 105/90, 40min at 39m, you get 6min@9m + 65min@6m, quite close to the USN plan 6min@9m + 66min@6m.
Your plan with 40/70 contains stops at 18m, 15m, 12m, which are not in the USNavy plan. Your 40/70 stop time at 6m is 108min, as opposed to 66min in the USN plan. Why do you think this is closer? Why do you think GF 105/90 is impossible?

 

[Remy B.]

In Multi-Deco it is not possible

It seems hard to believe that the Navy will skip all those deeper stops, but who knows, I wonder what Dr. David D. and Simon M, have to said regarding what you mention that the Navy manual said in their latest version.

 

[leadduck]

what I wanted to say: it seems like going from 30/80 to 50/70 is just a first step towards a much shallower decompression. In the new USN manual they write (p. 9-7) "The new air tables are designed to provide safe decompression even for divers who work hard on the bottom or are exceptionally cold during decompression. It is not necessary to select the next deeper or next longer decompression schedule under these conditions as in the past.". That for GF 105/90.

 

[Dr Simon Mitchell]

Hi guys,

They are all interesting observations. You need to be careful interpreting US Navy operational requirements (as reflected in their tables) with the safety goals and aspirations of recreational divers. They may not be equivalent. For example, there will be a level of risk that is acceptable to the USN which may not be acceptable to recreational divers. I would have to ask David (he is currently on his way out here and I will see him at Oztek) but some of these USN schedules are generated from probabilistic models in which the model calculates the shortest decompression schedule for a targeted risk of DCS. If that were the case here and leadduck is right about the GF comparison, then it simply implies that the model thinks the quickest way of decompressing from this dive for a specified level of risk is to completely omit deeper stops and do something that looks a lot like raw Buhlmann.

Can I be clear that my personal use of GFs around 50/70 or 50/80 is not intended to be a recommendation, though I know it is inevitable some will interpret it that way. My choices reflect what I consider to be a sensible interpretation of the current evidence about optimal decompression which can be summarised as `we have probably be over-emphasizing deep stops, and longer shallow stops seem to decrease bubble formation`. Thus, I have backed away from deep stops as they were practiced by our technical diving craft group from the early 2000s until recently. I have said many times that I don`t know how far to back away, and GF 50 lo is intuitively acceptable to me for the time being. Based on available evidence I think I have improved my decompression by doing this, but I have no idea whether it is optimal.

Simon M

 

[rossh]

In Multi-Deco it is not possible

It seems hard to believe that the Navy will skip all those deeper stops, but who knows, I wonder what Dr. David D. and Simon M, have to said regarding what you mention that the Navy manual said in their latest version.

Yes, they are in stark contrast to what we do and the way its heading.

The published USN manual tables - these are not the original VVAL18 tables. They are Modified by the Navy (here) - that's the M designation in VVAL18M. That M modification basically slashed the times in half and speed em up. This brings them back closer to where times used to be in earlier USN tables (workman 56) - which were some of the fastest tables of all.

As David explained, "VVal-18M was the result of a desktop exercise by Dr Ed Flynn (CAPT, USN, retired), working in the office of the Supervisor of Diving and Salvage. By adjusting some parameters of VVal-18 (developed by Ed Thalmann) he came up with a parameter set that produced similar oxygen decompression times to VVal-18, but much abbreviated air decompression times. "

These M tables will not align with ZHL very well. But they clearly are more shallow and faster than most things we would try.

.

 

[Remy B.]

Hi guys,



Can I be clear that my personal use of GFs around 50/70 or 50/80 is not intended to be a recommendation, though I know it is inevitable some will interpret it that way. My choices reflect what I consider to be a sensible interpretation of the current evidence about optimal decompression which can be summarised as `we have probably be over-emphasizing deep stops, and longer shallow stops seem to decrease bubble formation`. Thus, I have backed away from deep stops as they were practiced by our technical diving craft group from the early 2000s until recently. I have said many times that I don`t know how far to back away, and GF 50 lo is intuitively acceptable to me for the time being. Based on available evidence I think I have improved my decompression by doing this, but I have no idea whether it is optimal.

Simon M

[ Edited my posts ]

To clarify, I was incorrect on my comments of recommendation, it is a personal choice of Dr. Mitchell, and to the ones reading but not fully understanding, why GF-50/70 or 50/80, if I'm not mistaken and recall right Dr. Mitchell changes to more conservative GF as he do multi-days Deco dives, one number is not a magic bullet.

 

[Freewillow]

[ Edited my posts ]

To clarify, I was incorrect on my comments of recommendation, it is a personal choice of Dr. Mitchell, and to the ones reading but not fully understanding, why GF-50/70 or 50/80, if I'm not mistaken and recall right Dr. Mitchell changes to more conservative GF as he do multi-days Deco dives, one number is not a magic bullet.

I am not sure that a lot of people will judge it more conservatist to move from 30/80 to 50/80, even if I agree with the process PROVIDED that the leading gas is Nitrogen. Trimis is another matter

 

[Patoux01]

Trimix is another matter

Shearwater's take on the helium penalty

Might be interesting to you...