Buhlmann Calculation Help

[lamont]

Here's my calculations for a 54 min dive to 60 fsw in the 27 minute half-time compartment:

N2 start = 26.07 fsw
N2 inspired = 73.47 fsw

After 54 mins we've covered 3/4 of the 'distance':

26.07 + 3/4 (73.47 - 26.07) = 61.62 fsw

Using the following Bulhmann values for the 27 min compartment:

AN2 = 21.71 fsw
BN2 = 0.8126

I get this for our ceiling:

( 61.62 - 21.71 ) * 0.8126 = 32.43 fsw

Which gives me a pressure depth of:

32.43 / 0.79 = 41.05 fsw

Which means that this should be a deco dive (ceiling at appx 8 feet). I get an NDL at this depth of a little over 30 mins. Am i doing something wrong? Shouldn't this computation produce and NDL closer to 60 mins?

 

[Charlie99]

1. You forgot to account for water vapor and CO2 in the inspired gas. If you use the conservative 1.6fsw number, then starting N2 is 24.8fsw, rather than 26fsw. Inspired N2 is 72.2fsw or a bit less if you use the 0.627m/2fsw value Buhlmann chose.

2. The gas breathed doesn't affect the ceiling depth. The tissue tensions vs ambient pressure is what counts, so your very last calculation of converting 32.43 to 41fsw is in error. So, (ignoring H2O vapor correction) your ceiling depth is 32.43fsw -- i.e. you are just within NDL.

 

[lamont]

Thanks. I knew about H2O and CO2 and had included that in other calculations, but they didn't help at all in solving the factor of 2 issue I was having in the NDL (in fact, I think they make it slightly worse). Your second point is where I was really going wrong.