Tables, Advice sought.

[Dave Crampton]

I was recently reading an online account of what happened to some unfortunate sole who had a DCI whilst on a live-a-board in Oz. He explained what happed and how great the Hyperbaric center was and how the Hyperbaric doctor kept refering to the DCIEM tables, and saying well you say you were diving within your tabels but these say different. I recalled that on a course earlier this year the BSAC tables were compared to Bhulmann's tables and some differences were pointed out, mostly towards the limit of the tables diving. I was reveiwing today how we use Bhulmann's tables (as I usually use BSAC 88s, for my No Deco Limit) and that to plan repetitive dives with Bhulmann's tables is a slightly different technique to BSAC's. I relerned how to use the tables from my text book which gave an example using the US navy tables. I then recalled this American's account and decided to compare the tables, on one (hyperthetical) series of dives. The example in the text I have, gives:

Dive one to 21m for 35mins
Then a surface interval of 2hr 45m, followed by a
Dive two to 18m.
How long can one dive for on the second dive?
Now assume a Sea level dive.

For the US Navy tables (unmodified)
Dive one gives, NGD = G
After SI:- G becomes D
D @ 18m, remove 24 mins from No stop max time,
Dive two No stop time of 36mins.

For DCIEM
Dive one gives, Current Tissue code E
After the SI, E becomes Rep Factor = 1.3
1.3 @ 18m, remove 31mins from No stop max time,
Dive two No stop time of 19mins.

For BSAC
Dive one gives, Current Tissue code F
After the SI, CTC = C, from table C:
Dive two No stop time of 15mins.

For Bhulmann
Dive one gives, CTC = E
...
+-----------------------------+
|Now this is where I trip up!
|Reading off the Rep Letter Group Table, Letter E only goes upto 0:45?
|Does this mean that after 45 mins I am clear? Surely not!
|
|Ok, then reading off the table everything over 0.45 is RNT A, remove |14 mins for dive two to 18m:
|Dive two No stop time of 37mins.
+-----------------------------+
Could anyone clear my confusion, please. The example in the text uses US Navy Tables and I can't seem to apply this to the Bhulmann Tables: Repetitive Letter Group Table.

Also I am in a quandry as I thought the DCIEM tables were supposed to be the most conservative? or was this bent American's account just refering to US Navy/Padi Tables, Which I don't have to hand.... Oh wait a minute:

... rumaging...

Ah?? ...its in Feet!! (Calculator!!)

Dive one to 21m/69ft gives 'Pressure Group' = Q
From table three, Max No stop dive is 49mins,
To acheive this I have to wait at least 2h 31m, which fits in with the 2h 45mins SI original planned upon. (ed. Wow I did it!!)
Dive two to 60ft for 49 mins.

Anyone care to offer advice on how I use the Bhulmann table correctly for repetitive dive planning?

On another point, how do these table compare, for a series of dives over varying depths and for varying times? Is any one table considered least conservative/most conservative?

Thanks,

Dave C

[miketsp]

I am not familiar with all the other tables you mention but I did it with my (recent) CMAS table just for comparison.

21m (NDL=40mins) 35mins group F
14mins residual after 2hr 45min if next dive 18m
18m NDL is 65mins so subtract 14min
leaves 51mins for 2nd dive. - Very close to your calc of 49mins.

However if you are going to compare tables you have to be careful to use the correct definitions of bottom time to go into the tables.
The bottom time for CMAS is from start of descent to start of ascent so total dive time would be longer than shown here by the ascent time + stop time(if any).

I know that some other agencies have other definitions.

[sweep]

Exit RG is E
Re-entry RG is indeed an A
Max no stop time at 18mtrs is 51 mins, less RNT of 14 mins, gives you a ABT of 37 mins

[Dave Crampton]

Originally Posted by sweep

Exit RG is E
Re-entry RG is indeed an A
Max no stop time at 18mtrs is 51 mins, less RNT of 14 mins, gives you a ABT of 37 mins

Cheers guys,

So going by Bhulmann, At what point do I become completely desaturated? the three hours to fly or 12 or 24 hours?

Dave C

[Andy Phillips]

eeeeeeeeeeeeeeeeeeeeeeeeeeekkkkkkkkkkkkkkkkkkkkkkk


Andrew

[Mark Chase]

Originally Posted by imported_andyp

eeeeeeeeeeeeeeeeeeeeeeeeeeekkkkkkkkkkkkkkkkkkkkkkk


Andrew

Oh my God they got you too......


Sorry only those people who remember the Nite of the Body Snatchers will understand that

ATB

Mark Chase

[Dave Crampton]

I read my text last night and it did mention in the example that following along the row E in Bhulmann's 'Residual Nitrogen Time in Minutes' one would track along to the point where 0.45/2.00 was read off. Now in the Tables I have printed it only shows a single row of figures for each Nitrogen group instead of the usual time period like most other tables, 0.45 : 1.20 for example. I even downloaded an independant set of Bhulman tables, and they have the same second table with one row (up time) instead of the usual Between these two times). Now Bhulmann's tables appear conservative at first as they have stops of at least 1 minutes @ 3m regardless of the dive but the tables then seem a little lax in the 'Residual Nitrogen Time in Minutes' table, as they reduce to lower Nitorgen groups much quicker than any other table I have. Is this taken care of in the Residual Nitrogen look up table giving time off the max no stop time?

What does Bhulmann suggest as a completely off gassed time?

Dave C

[And]

Hi

Given the nature of using half times theoretically you will never be 'offgassed'. But the usual assumption is 6 half times, of the slowest tissue.

Andy

[MattS]

Buhlmann uses symetrical off gas rates, some other tables use asymetrical rates and so off gas slower than they on gas, which makes subsequent schedules more conservative.

Off the top of my head, complete desaturation on Buhlmann is judged to be around 96 hours - I may have that wrong. As And says, take the longest half time and multiply by 6.

[Dave Crampton]

Thanks guys,

I've been looking for more info on Tables, Decompression, etc.. on the net. I haven't been able to find the paper published by J.S.Haldane, and think it might be only available in my local Uni's Med Lib. Does anyone know of a good source on the net for reading material about Deco & the authors etc.. apart from Baker/s publications on Decompression.org, Nigel Hewitt web pages, Divernet, etal..

I'm too cheep at the moment to order The Deco Matrix by Bob Cole.

Cheers in advance.

Dave C

EDIT: Found a bit about NOAA:

The daily and multi-day limits fopr OTU were established by Dr. Bill Hamilton and colleagues in the NOAA Repetitive Excursions (REPEX) Procedures Report.

The NOAA limits for CNS oxygen toxicity (normal, single exposure) are
plotted in Figure 2. These limits are based on research by Drs. F.K. Butler
and Edward D. Thalmann, U.S. Navy Experimental Diving Unit, and by
Drs. Christian J. Lambertsen and Russell E. Peterson, Institute for
Environmental Medicine, University of Pennsylvania.