Calculating pressure groups and RNT

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jaybombs25

Contributor
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393
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Location
St. Kitts
# of dives
500 - 999
How would you use excel to calculate pressure groups and RNT in terms of formulas. I'm making a personal divelog using excel and I'm only missing this one formula to complete it. Everything I have tried hasn't worked however. It is possible to do it for single dives it just becomes a lot harder to do it for multiple dives. Any advice?

Thanks
 
Well, I suppose you could copy the data from the RDP into a spread sheet and do a regression analysis on it.

the K
 
Yea, I tried to work out a formula when doing the Computerized Table Tutor, and the problem is - while the numbers are based on formulas... they were big time altered and adjusted to have "nice numbers" 60' for 55 minutes just looks a lot better than 60 for 57. And with everyone diving computers - I don't know I'd bother build in a pressure group into your log... you will probably find that you've busted the tables on virtually every dive anyway due to the multi-level profiles.
 
Thanks thats exactly what I was thinking, so unless the dives are straight square profiles 90% of the time just using max depth and BT you will bust the tables. So I guess I won't even bother with this since as you said most people dive computers anyway it isnt absolutely necessary to include in a log
 
The pressure groups on the RDP correspond to dissolved N2 partial pressure in the 60 minute compartment.

I did a spreadsheet, and then fiddled with the numbers a bit to match up, within +/-1 minute to the various entries in the RDP. I came to the conclusion that they used H2O vapor pressure of 1.6fsw. (H2O pressure is subtracted from ambient pressure first, and then the remaining pressure is multiplied by 0.79 to get inspired N2 pressure).

I furthermore came to the conclusion that pressure group Z corresponds to 98% of the way from a starting saturation N2 pressure of 24.81fsw-absolute and the DSAT algorithm 60 minute M-value. Each pressure group below that is 3% less -- ie Y is 95%, W is 92%, .... down to C is 26%. A and B are unique, and correspond to 12% and 21% of M value.

There is just one basic equation used for the body of the table. It is the no-stop equation of http://www.rebreather.ca/Library/No-Stop_Time.pdf , but with the lower % of M value for that particular pressure group subtituted for the full M-value.

To get the NDL numbers, however, you have to run the equation of each compartment halftime and limit, and NDL is reach whenever any one of the compartment limits is reached. The pressure groups, however, are solely based upon the 60 minute compartment.

Charlie Allen
 
The problem is that the "dive tables" the OP is using are probably not based on formulation.

I'd suggest using the Pivot or Crosstab Table function of Excel to do what you want. In short, you create a "reference" worksheet to pull data from to perform a calculation. Duplicate the dive table on a worksheet using rows and columns, define it as a pivot table, use it as a source for calculation references. A good Excel "how to" book at any large bookstore will have tutorials to show you how to do it.

Good Luck!
 
you would need up to 12 equations to simulate each tissue time, then pick worst case.
Anyway, padi's electronic rdp just uses table and similar rounding to doing it manually. You can just do a table lookup in excel to reproduce it. You cannot really interpolate since the equations use logic (ie worst case . . .)
I do have weinke book, but tons of math for simple answers!!!
good luck
 
you would need up to 12 equations to simulate each tissue time, then pick worst case.
Only if you are calculating NDLs. If you just want to track the repetitive groups, then you only need to track one and only one compartment --- the 60 minute compartment for the PADI RDP/Wheel; or if you are using a USN-derived table (NAUI, YMCA, SSI, etc.) the 120 minute compartment.
 

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