Depth in multiple dives

[shurite7]

Google Smithsonian Institute Reverse Profiles. There is some good information regarding your questions.

Attached is a deep to shallow set of dives, three total, and another set in the reverse order. I used a NAUI table to show why one is unable to conduct deeper dives if shallower dives are conducted first.

 

[Bob DBF]

As was said earlier, the US Navy tables were made with the assumption that the divers would be doing one dive a day. Their choice of the controlling compartment for repetitive dives (120 minute) was a decision made without a lot of study, becaues it usually didn't matter.

The Navy has been been doing ongoing research since it initiated the NEDU in 1915. They did this to insure the safety of Navy divers while they are doing their job. Because they did not do the one study PADI did to optimize the tables for recreational divers does not mean that they randomly, without a lot of study, make dive tables for their divers.

The DCIEM decompression theory is based on the Kid-Stubbs model, which was made in 1962 according to the dive table of USNavy and considering multi-level and repetitive dives.

DCIEM decompression theory

It would seem that without the Navy Dive Tables, the DCIEM would not be able to optimize it for repetitive dives and recreational diving.


The Navy Dive Tables

Later extensive research at the U.S. Navy Experimental Diving Unit (NEDU) by Dwyer, Hawkins, Workman, Yarborough, and others from the 1930s-1960s arrived at more precise ratios using empirical data, focusing on the pressure of the inert gas in air directly involved in DCS, nitrogen. Workman’s model introduced M-Values (or greatest partial pressure of nitrogen a “tissue” compartment can tolerate without the onset of DCS at a given absolute pressure) for each of 6 designated hypothetical tissue compartments with 5, 10, 20, 30, 40, 80, and 120-minute nitrogen half-times.

Haldane calculated the 5, 10, 20, 40, and 75 half-times used in his model based on their representation of what he theorized happens in the body. Haldane hypothesized men became fully saturated with nitrogen in 5 hours and that additional nitrogen loading after 4 half-times “would scarcely be appreciable”. The final 75 minute “tissue” compartment therefore corresponds to the fact that 4 x 75 minutes = 300 minutes, or 5 hours. The Navy Tables use Haldane’s calculations as the basis for their theoretical compartments, changing the 75-minute compartment to 80 minutes and adding a 120-minute compartment. The Navy assumed 6 half-times for saturation. The changes reflected problems associated with earlier US Navy tables that had used Haldane’s original five compartments in their calculation.

Because the PADI RPD is superior to Navy Dive Tables for recreational divers, only means that they have a different objective for their diving, not that they do not do extensive research before coming to a conclusion.



Bob

 

[boulderjohn]

The Navy has been been doing ongoing research since it initiated the NEDU in 1915. They did this to insure the safety of Navy divers while they are doing their job. Because they did not do the one study PADI did to optimize the tables for recreational divers does not mean that they randomly, without a lot of study, make dive tables for their divers.

The 120 minute compartment did not exist in the first half of the 20th century. According to my understanding from previous reading, the Navy added it to the list of compartments used in planning at the same time that it made it the controlling compartment for surface intervals. According to this summary, that decision was based on 120 test dives.

 

[Lake Hickory Scuba]

Is this a limitation of diving with tables or do computers give reduced ndls for reversed profiles too?

Great question.

 

[Bob DBF]

The 120 minute compartment did not exist in the first half of the 20th century. According to my understanding from previous reading, the Navy added it to the list of compartments used in planning at the same time that it made it the controlling compartment for surface intervals. According to this summary, that decision was based on 120 test dives.

Reread the third quote in my post. The Navy was using Haldane Tables before the change to the 120 minute compartment. Navy Divers were being bent on a regular basis which caused the NEDU to postulate the 120 minute compartment. The study was short, however it showed results in decreasing the occurrence of the bends, and they acted on it to make the Navy Tables. A longer study may have been scientificly better, but during that time a lot of Navy divers would have been bent diving the old tables for no purpose. One has to realise that the purpose of the NEDU is to make Navy divers as safe as practical for the dives they have to do.

What is not considered is the ongoing checking of the data. Navy Diving Logs are not a personal diary, they are a standardised log of the parameters for use by the NEDU scientists to have the data they need to verify current dive procedures are effective and postulate improvements to be tested. Had the study been a fluke, the data from the dive logs would have shown that over time. That has not been the case, however the Navy Tables have been revised a number of times since that first change from Haldane based US Navy tables to NEDU US Navy tables.


Bob

 

[Magnus Lundstedt]

If you do two dives, where depth on dive 1 is X and dive 2 is 2X you ongas on both dives.
If you do two dives, where depth on dive 1 is 2X and dive 2 is 1X you offgas during the second dive..

 

[tbone1004]

If you do two dives, where depth on dive 1 is X and dive 2 is 2X you ongas on both dives.
If you do two dives, where depth on dive 1 is 2X and dive 2 is 1X you offgas during the second dive..

not true. You are ongasing in most of your compartments for the second dive. Some of the slow ones may be offgasing but the majority of them are ongasing

 

[Magnus Lundstedt]

not true. You are ongasing in most of your compartments for the second dive. Some of the slow ones may be offgasing but the majority of them are ongasing

Yes, but very simplified its the reason for why to do deepest dive first.

 

[tbone1004]

Yes, but very simplified its the reason for why to do deepest dive first.

except DAN et al have published that there is no reason not to do reverse profiles physiologically.
I have a real example of two real dives in a day where using a standard decompression algorithms there was no change in NDL for 80 and 100ft dives to NDL with a 1 hour sit.
If your simplification was accurate, then there would be a difference in the NDL times for the dives.
We've discussed that some dive tables don't like them because of how they were optimized, but that doesn't have a bearing on reality