BSAC88 Decompression Tables

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Gareth J

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This is for @boulderjohn, and any other of you who are interested in decompression tables and theory.

BSAC 88 tables are copyright, so I will not attempt to publish or link to pictures of the actual tables. If you are that studious or keen, you can order them from the BSAC shop, or find them on E-bay!

Overview
The tables where developed by the BSAC with Dr Tom Hennessy in 1988. Hence the name!
Intended to be easy to read and use. They include decompression data for dives that exceed No-Stop limits, as well as no-stop dives.
The original set include 4 altitude levels, 1, 2, 3, & 4.

There are three table sets currently produced by the BSAC.

BSAC88 Level 1-4.
These are the original set tables. Unsurprisingly, for air!
Although they are quoted as Altitude tables. They are actual ambient pressure tables.
Level 1 pressures greater than 984 millibar
Level 2 899-984 millibar
Level 3 795-899 millibar
Level 4 701-795 millibar (3200m at 1040mbar[sea level])

BSAC Nitrox Tables
These where developed for the BSAC Nitrox courses. They cover 4 gases, air 21%, 27%, 32% and 36%
They are all level 1 tables.

BSAC Ox-Stop Tables
Introduced for the BSAC accelerated decompression courses. Using standard mixes and decompression gases.

There was some discussion if the tables would be dropped from the training program. However, tables are an excellent teaching aid, so they have remained in the courses.

ALL tables have the same format, you can move between the tables with ease. They are basically a booklet.

Table Format
Table format is the same for each set of tables. I will explain the Level 1 air table.

The Level 1 tables consist of
  • 7 Decompression tables A to G
  • Surface interval table
The 7 different tables, relate to the retained Nitrogen saturation level at the start of each dive.
Table A, desaturated diver, to table G heavily saturated diver (i.e. exited after completing decompression stops)
The format of each table is the same.

Four Columns;
  • Depth, in 3m increments
  • Ascent time, in minutes
  • No-stop Dive times
  • Decompression Dive times
The Decompression Stop dive times are highlighted in a different colour.
Every 6 rows or so the table is separated to make it easier to read, At this row, two (or three) extra rows are inserted containing decompression information, and Surfacing code. Decompression stops (for level 1) are at 6m and 9m.
AIR LEVEL 1 - (greater than 984 millibar
TABLE​
DEPTH_ASCENT_________DIVE TIME
______________________No-stop Dives___________Stop dives
3_____ (1)________166__infinity_________|
6______(1)________36_166 _593_infinity__|
|
18______1_________- __17__37__44__51_I__68__78__84__88__92__95__98__101
DECOMPRESSION STOPS at 6m________|__1___3___6___9___12__15__18__21
SURFACE CODE____B__C__D___E____F__|__G___G__G___G___G___G__G___G

21_______1_________-__13__28__32__37_|__ 51 .............

So for the above, a 17m dive (use 18m, because the preceding set would be 15m, (so next deepest,) 40 minutes, exit on surface code E/1.
For an 18m dive of 90 minutes requires 12 minutes at 6m exiting on a surface code G/1

The surface interval table gives you the code for the next dive based on your surface interval time. There is NO CALCULATION, on any of the tables.

Rules and Definitions
The BSAC definitions are different to other tables. As an example,
  • descent rate = 30m maximum
  • ascent rate = 15m to 6m and 1min from 6m to the surface
  • dive time = time leaving the surface to reaching the ascent check depth (6m), or the first stop
This isn't comprehensive. It is an over view of the BSAC 88 tables.

THERE MAY BE ERRORS IN THE EXAMPLE FIGURES USED ABOVE - do not use for diving

Moving between the tables.
The surface code for any level 1 dive can be moved to the Nitrox or Ox-Stop tables (and vice verse).
So the surface code from an air dive (level 1) can be moved directly to the Nitrox tables and applied to the surface interval table to determine the current nitrox 36 table e.g. table B/1(36%).

Moving between altitude tables is done using the altitude transfer table and the Atmospheric pressure chart. But that's a whole new issue!

I hope this is a understandable overview of the BSAC tables.

Sorry for the table format, very difficult to achieve a decent table. :(
 

Storker

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AFAIU, the BSAC tables - like USN and our own - are a bit more liberal on the first dive than e.g. PADI's, but significantly more conservative on the second dive and dissuades doing more than two dives per day. Am I right?
 

Gareth J

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Under the Safer diving statement in the table.

2. It is recommended that no more than 3 dives be performed in any 24 hour period and any dive series involving consecutive days diving to 30m+should be limited to four days, after which a 24 hour break should be taken,

3. It is advisable to limit any diving within a 24 hour period to dives requiring a total of 20minutes in water stops.
 

Edward3c

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For an 18m dive of 90 minutes requires 12 minutes at 6m exiting on a surface code G/1
Just to clarify,

The total in-water time for the above dive would be:
90 minutes from leaving surface to arriving at 6m deco stop.
12 minutes at 6m undertaking the mandatory deco stop.
1 minute to move from 6m to the surface.
In-water time 103 minutes.
 

boulderjohn

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Hm. I once did a comparison of the USN tables, BSAC-88, PADI RDP and our own tables. The difference in 2nd dive NDLs was... significant.
This is a significant issue for people planning to dive with groups doing multiple dives. If someone is using tables that call for significantly longer surface intervals, they may not be able to keep up with the schedule of the other divers in the group, and if they do keep up with that schedule, they may actually be violating their tables, meaning they are actually following the algorithm(s) used by the rest of the group rather than the tables they claim to be following.

Here is a quick history to explain why for those who are not aware of it.

Surface intervals, whether with a table or computer, are based upon the rate at which a chosen theoretical tissue off-gases during the surface interval. More than a half century ago, the US Navy selected a theoretical tissue that loses half of its nitrogen in 120 minutes and then continues in 120 minute halftimes, meaning it is cleared ("washed out") in 6 * 120 = 720 (12 hours). That was the standard for a long time, and almost all tables used decades ago were based on that system. This led to extremely long surface intervals between dives.

A frustrated diver who was also a scientist got PADI to conduct a massive research project to see (among other things) if that much time was needed on typical recreational dives. That research indicated that for the great majority of recreational level dives, a theoretical tissue with a 40 minute halftime was sufficient. PADI decided, though, to use a 60 minute tissue. They also shortened the first dive times a bit to be more conservative, and they doubled the number of pressure groups to diminish the significant rounding (which is always done toward the conservative number) involved with the PADI tables. This resulted in much shorter surface intervals on recreational dives, and it became quite popular for that reason.

Today, computer algorithms are generally compatible with the PADI tables in terms of surface intervals, and dive boats around the world have schedules compatible with that surface interval as well.

If you are trying to dive with a group using tables as your guide, if your tables are based on the US Navy tables or another algorithm that allows more time on a first dive and then uses a tissue with a longer halftime to determine the surface interval, you may find yourself out of synch with the rest of your group.
 
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