Navy vs Buhlmann tables

[loganmackay]

I must apologise first, as this has probably been covered many times.

When comparing the navy and Buhlmann tables, I am interested in the differences, especially when calculating the RNT.

With navy tables when calculating RNT for an intermediate depth, it is recommended to take the deeper depth, with Buhlmann tables, you take the next shallower depth.

I imagine that that this offers a greater safer margin with Buhlamann tables, but what is this difference based on?

 

[DA Aquamaster]

Actually, it would provide slightly less saftey margin compared to using the next deeper depth for RNT with the Buehlman tables. Depending on the profiles of the two repetetive dives the Buehlman tables may be more or less conservative than the US Navy tables.

The original US Navy Tables used a Haldane based model but with a couple of differences. The US Navy tables use 5 tissue compartments with half lifes of 10, 20, 40, 80 and 120 minutes. They also differ from the original haldane tables in using critical ratios that vary from compartment to compartment rather than the single critical tissue nitrogen tension to ambient pressure ratio of 1.58 to 1. The original US navy tables were updated in 1985 to incorporate a new mathematical model based on their large experiential data base of bends occurrences.

Buehlman's tables are also based on a haldane model but he uses a slower ascent rate and 12 compartments with half lifes that vary from 4 to 635 minutes that are better adapted to the slow buildup of nitorgen in slower tissues on longer dives and on repetetive dives. These types of dive profiles will be where the greatest differences between the Buehlman and US Navy tables occur with respect to RNT amounts.

The basic idea behind the Buehlman tables is that a shorter initial dive will reduce bubble formation and improve offgassing between dives which will often allow a longer second dive than other tables.

So even though the Buehlman and US Navy tables use repetetive groups and the same basic approach to figuring RNT, the two are not really comparable.

 

[Walter]

I've heard of Buehlman, but never seen "Buehlman tables." Where would one find them?

 

[DA Aquamaster]

They are pretty common in Europe, but I have never seen a Buehlman table outside of a book in the US. I don't think there are any US training agencies that promote them.

There have however been several dive computers sold in the US that utilize Buehlman models.

 

[loganmackay]

I am confused why taking the low depth would give less margin of saftey.

Taking the lower depth when RNT gives the highest penalty. Then you apply this to table 1 and use deeper depth.

To me this looks like you have the largest RNT and the lowest MBT

 

[loganmackay]

I am confused why taking the low depth would give less margin of saftey.

Taking the lower depth when calculating RNT gives the highest penalty. Then you apply this to table 1 and use deeper depth.

To me this looks like you have the largest RNT and the lowest MBT

 

[chrisch]

There have however been several dive computers sold in the US that utilize Buehlman models.

Nearly ALL computers available today use Buhlmann algorithms. Suunto use a modified program that has some RGBM in it and the new Mares/Dacor has an RGBM model.

Buhlmann published loads of stuff and many have copied or adapted it with their own M values etc. - there is no single set of tables as such.

Chris.

 

[Walter]

I am confused why taking the low depth would give less margin of saftey.

Depends on what you mean by "low depth." Stay with "deeper" vs "shallower" as you did in the first post and it stays clear.

NDL's for deeper divers are less than for shallower dives. If your planned depth isn't on the tables, it is more conservative to plan for the next deeper depth that is on the tables. That results in less bottom time than if you'd planned for the next shallower depth. As I said before, I've never seen Buehlman tables, but I doubt you'd plan RNT at 70 feet and then apply that to 80 feet. That's unnecessarily complicated and would result in lots of errors. You may have misread your manual.

 

[loganmackay]

As I said before, I've never seen Buehlman tables, but I doubt you'd plan RNT at 70 feet and then apply that to 80 feet. That's unnecessarily complicated and would result in lots of errors. You may have misread your manual.

That is exactly the way the table works and yes it does appear unneccessarily complicated, which is why I'm asking the question..

Quote from table: " Enter Table 3 at the row letter from the previous stage. Read along this row until the column intersection point with the planned depth for the next dive is reached. If the required depth falls between the two tabulated values, select the lower value (this will carry the greater penalty)."

You would then return to the first table and take the deepest depth

 

[Charlie99]

Nearly ALL computers available today use Buhlmann algorithms. Suunto use a modified program that has some RGBM in it and the new Mares/Dacor has an RGBM model.

Buhlmann published loads of stuff and many have copied or adapted it with their own M values etc. - there is no single set of tables as such.

Chris.

Chris, you seem to be confusing the specific limits developed by Buhlmann with the generic dissolved gas / neo-haldanian model.