Dr Deco
Contributor
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- I just don't log dives
Dear Readers:
I was asked by a SCUBA BOARD reader to comment on the idea he encountered in a discussion about tables and their construction. The basic idea presented to him was that all tables employ “fudge factors” and none are really “pure.” After replying to him, I thought that others might also be interested in my answer to this query.
It is absolutely true that there are no dive tables that are ab initio. That means starting from nothing but theory. All tables start with some data and simple concepts and are then are modified with actual human dive data [i.e., it is “folded in,” as they say].
[1] John Scot Haldane started his table design program for the Royal Navy in about 1905 using field data from actual divers. For example, he knew that divers could surface directly from about 30 fsw no matter how long they were submerged. He worked out concepts with goats and then adjusted his tables in a stepwise fashion descending deeper than 30 fsw. He went to 209 fsw, the deepest that divers went in the early 1900s. These were originally worked out in principle with goats [volunteers, of course
] and then tested on human [volunteers, of course] divers. These tables were to single depths with intermediate “stops” on the ascent to the surface. Thus, all tables [old and new] are made with data from human divers. No guesswork.
[2] For other parts of the tables allowing the deco stops, he introduced the idea of tissue compartments, since he knew that blood did not flow to all parts of the body in an equal fashion. In reality, there are not any compartments in the sense used in table making. All tables incorporate them as a reason behind how they are derived, but they are not there in the manner stated.
[3] There are also micro bubbles [nuclei] that grow that are not considered, although the RGBM does incorporate them to a large degree. Haldane knew that bubbles could not form with low supersaturations and need some type of nucleating material; he simply did not consider nuclei. This is basically how we view it today, one hundred years later.
[4] All tables initially start with NDLs determined with actual human divers. These can be different with different, designers, countries and navies. Some tables might be for warm or cold water or heavy or light workloads. Some might be conservative if treatment options are few. There are no tables that do not have human divers making the initial NDLs. From these NDLs, tables are calculated. The simpler the table [e.g., for SCUBA divers] the less modification that needs to be done. Deep diving commercial tables require much “tweaking.”
After getting a set of NDLs, the tables are modified to conform with known dive data when multiple depths and multiple dives are concerned. Some modifications are made if mutiple dives are made or multiday diving is considered. All table designers do this but do not call them “fudge factors.”The tested tables for SCUBA divers trace their original pedigree back to the US Navy and/or [more recently] the PADI Recreational Dive Planner. [The RGBM has some of its own data also.]
It is also necessary to modify the NDLs if safety factors are considered. All tables are safe in this regard EXCEPT when divers use excessive work loads. This might include swimming against a strong current, for example. It might also include work on the boat such are climbing boat ladders with full gear or hauling air tanks around on deck. These concepts have been discussed on the Dr Deco forum for more than a decade. The concept comes most recently from my research work at NASA.
[5] There are unknown variables amongst divers. Human divers performing identical pressure and work profiles in a laboratory will yield different results. There are many postulated reasons but no proof.
I was asked by a SCUBA BOARD reader to comment on the idea he encountered in a discussion about tables and their construction. The basic idea presented to him was that all tables employ “fudge factors” and none are really “pure.” After replying to him, I thought that others might also be interested in my answer to this query.
It is absolutely true that there are no dive tables that are ab initio. That means starting from nothing but theory. All tables start with some data and simple concepts and are then are modified with actual human dive data [i.e., it is “folded in,” as they say].
[1] John Scot Haldane started his table design program for the Royal Navy in about 1905 using field data from actual divers. For example, he knew that divers could surface directly from about 30 fsw no matter how long they were submerged. He worked out concepts with goats and then adjusted his tables in a stepwise fashion descending deeper than 30 fsw. He went to 209 fsw, the deepest that divers went in the early 1900s. These were originally worked out in principle with goats [volunteers, of course
] and then tested on human [volunteers, of course] divers. These tables were to single depths with intermediate “stops” on the ascent to the surface. Thus, all tables [old and new] are made with data from human divers. No guesswork.[2] For other parts of the tables allowing the deco stops, he introduced the idea of tissue compartments, since he knew that blood did not flow to all parts of the body in an equal fashion. In reality, there are not any compartments in the sense used in table making. All tables incorporate them as a reason behind how they are derived, but they are not there in the manner stated.
[3] There are also micro bubbles [nuclei] that grow that are not considered, although the RGBM does incorporate them to a large degree. Haldane knew that bubbles could not form with low supersaturations and need some type of nucleating material; he simply did not consider nuclei. This is basically how we view it today, one hundred years later.
[4] All tables initially start with NDLs determined with actual human divers. These can be different with different, designers, countries and navies. Some tables might be for warm or cold water or heavy or light workloads. Some might be conservative if treatment options are few. There are no tables that do not have human divers making the initial NDLs. From these NDLs, tables are calculated. The simpler the table [e.g., for SCUBA divers] the less modification that needs to be done. Deep diving commercial tables require much “tweaking.”
After getting a set of NDLs, the tables are modified to conform with known dive data when multiple depths and multiple dives are concerned. Some modifications are made if mutiple dives are made or multiday diving is considered. All table designers do this but do not call them “fudge factors.”The tested tables for SCUBA divers trace their original pedigree back to the US Navy and/or [more recently] the PADI Recreational Dive Planner. [The RGBM has some of its own data also.]
It is also necessary to modify the NDLs if safety factors are considered. All tables are safe in this regard EXCEPT when divers use excessive work loads. This might include swimming against a strong current, for example. It might also include work on the boat such are climbing boat ladders with full gear or hauling air tanks around on deck. These concepts have been discussed on the Dr Deco forum for more than a decade. The concept comes most recently from my research work at NASA.
[5] There are unknown variables amongst divers. Human divers performing identical pressure and work profiles in a laboratory will yield different results. There are many postulated reasons but no proof.
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