Decompression Macro
In an attempt to learn more about decompression theory, I decided to use the information from DIY DECO and abyss software and write my own macro to perform the decompression algorithms.
The macro is based upon the Bühlmann ZH-
I chose to have the decompression stops at a minimum of 6m similar to the BSAC ’88 tables, and then in 3m intervals for the deeper stops. The macro can only handle the first dive of the day, and is reinitialised for the next dive. Any tissue loadings from the first dive are lost.
The macro may contain errors, and should not be used for dive planning of any sort. If you are interested in seeing the macro, it can be download here. However, the author accepts no responsibility for errors contained in this file, or injuries or illness arising from the use of this macro in any way. In downloading this file you agree to these terms.
Also, there are much better freeware and shareware decompression programs already available on the Internet.
After writing this macro, in an attempt to debug or test the program I compared the results with the BSAC 1988 tables, the DECO2000 tables, and the PADI tables.
The macro shows about 3 or 4 times the amount of decompression required by the BSAC tables, and about 4 times less no stop time. However, the no stop limits compare well to the PADI tables. The decompression durations are within a few minutes of the DECO2000 tables.
It is felt that these differences may be due to different models being used for implementing the plans. The PADI and DECO2000 tables may be based on the Bühlmann model, and the BSAC tables based are based on a triple compartmental model developed by Dr. Tom Hennessy, which allows interaction between its compartments. This explains the differences between the deco times. There is some more discussion on these points on the BSAC forum web page.
To highlight the differences between the tables I have plotted two graphs showing no stop times for 4 different table types. The first graph shows the no stop time as a function of depth for each table type. It can be seen that the four different tables give similar values.
I then calculated the no stop times for an identical second dive, assuming 1 hour surface interval. I understand that this is not good dive planning, and the second dive should be shallower than the first. The results are displayed in the following figure.
It can be seen that the repeat dive results are very different. The BSAC tables show a much lower no stop time for the second dive.
Some tables, in order to make them more marketable, use a varying DCI risk model. This is done so that dive times in zones that are popular are not ultra conservative, but aim to prevent DCI in zones where the risk is unknown. At the time of the BSAC tables’ creation, the approach was to curtail the depth/ time/ repeat dive combinations, and to avoid the dive zones where the DCI risk is unknown, and have a low known risk of DCI for the popular dive times and depths.
Improvements to the Macro
Since version 1 I have added the capability to include a surface interval, and plan a second dive. Version 2 can be down loaded here. The same disclaimer applies.
Version 3 can be down loaded here. This version uses forms to enter and read the data. The same disclaimer applies.
Extra Stuff
I have created a gas consumption calculator for the HP Jornada 540 (SH3 processor). You can down load it here. The same disclaimer applies.