U.S. Navy UBA 0.7 atm NDLs

[jlewis007]

I am trying to understand the U.S. Navy decompression table for the fixed 0.7 atm oxygen nitrogen closed circuit UBA. When I compare the NoD limits to the standard air table (FO2=0.21), all is well for depths of less than about 80 fsw where the increased oxygen content of the UBA allows increased bottom times. The problem that I have is that for greater depths, the UBA oxygen content is less, the nitrogen partial pressures higher and yet the UBA NoD limits are greater than that for air. Can anyone explain to me this nonsequitor?

 

[Thalassamania]

This might interest you: "Computer Algorithms Used in Computing the Mk 15/16 Constant 0.7 ATA Oxygen Partial Pressure .Decompression Tables." by Thalmann. at RUBICON.

 

[kgault]

I am trying to understand the U.S. Navy decompression table for the fixed 0.7 atm oxygen nitrogen closed circuit UBA. When I compare the NoD limits to the standard air table (FO2=0.21), all is well for depths of less than about 80 fsw where the increased oxygen content of the UBA allows increased bottom times. The problem that I have is that for greater depths, the UBA oxygen content is less, the nitrogen partial pressures higher and yet the UBA NoD limits are greater than that for air. Can anyone explain to me this nonsequitor?

I will assume that you are using the latest version of the Navy Diving Manual (http://www.supsalv.org/pdf/DiveMan_rev6.pdf).

You may wish to ask such questions in the Navy Experimental Diving Unit section of scubaboard in the future as our Executive Officer directs the question asked there to the appropriate contact point (http://www.scubaboard.com/forums/nedu-q/).

Table 17-6 is the 0.7 PO2 in Nitrogen no-stop limits, and Table 9-7 is the no-stop limits for air.

Table 17-6 was developed using the VVAL-18 parameterization to the Thalmann Algorithm prior to
Revision 5 of the Diving Manual.
The development of this algorithm is discussed in:
Rubicon Research Repository: Item 123456789/5490
The suitability of these tables for air diving was discussed in the following technical report (NEDU TR 03-12)
Rubicon Research Repository: Item 123456789/4586

For Revision 6 the air tables were changed from those that had been used for decades (refered to as USN56) to tables developed using VVAL-18M (a modification of VVAL-18 -- Technical Reports will be available from the Rubicon Foundation in the future). Due to concerns about the severity of decompression sickness when conducting a dive trial for the extensions to the no-stop limits deeper than 100 ft with VVAL-18 and 18M, NAVSEA chose to perform an executive cut and past to replace the algorithmically calculated no-stop limits with those from the USN56 tables. This cut and paste of the no-stop limits is what makes the inconsistencies you are noting between the tables. You can note these substituted values as they are all multiples by 5.

Keith

 

[floridanewbe]

welcome aboard

 

[jlewis007]

Many thanks for the reply. In particular to the comment regarding the NAVSEA revision of the VVal-18 calculations for depths greater than 100 ft.