Computers Suck

[Snowbear]

It's close if Haldanian model is used (like Buhlman used in GAP) but it's not exact. Such calculations are always close and can be used for some aproximation but I wouldn't say there's correlation besides "close".

Decompression models are NOT an exact science.
Approximation of average depth is a close enough if you keep your profiles reasonable.

 

[MikeFerrara]

It's close if Haldanian model is used (like Buhlman used in GAP) but it's not exact. Such calculations are always close and can be used for some aproximation but I wouldn't say there's correlation besides "close".

It's close with models like VPM also but for no-stop diving I'm ok with a Haldanian approximation of no-stop limits. I just start my "safety stop" a little deeper.

Exact doesn't matter to me. Pick up 4 or 5 different tables and notice the wide difference in NDL's.

Also when doing these comparisons notice what effect a minor change in depth has on the calculated NDL. The table or computer you use will make more of a difference than an inprecise estimate of profile depth. Even if you take the "NDL" as something that really exists it won't matter if you're not riding the edge in the first place. If I'm anywhere near something I view as a limit I'll do some minimum decompression (call it a safety stop if you want).

As far as I'm concerned good dive habits like slow ascents (especially near the surface), time spent shallow after time spent deep, and added conservativism for things like cold, hard work (during or after the dive) or multiple dives over multiple days is where my margin comes from not from precise calculations in an inexact model.

 

[MonkSeal]

It's close with models like VPM also but for no-stop diving I'm ok with a Haldanian approximation of no-stop limits. I just start my "safety stop" a little deeper.

Exact doesn't matter to me. Pick up 4 or 5 different tables and notice the wide difference in NDL's.

Also when doing these comparisons notice what effect a minor change in depth has on the calculated NDL. The table or computer you use will make more of a difference than an inprecise estimate of profile depth. Even if you take the "NDL" as something that really exists it won't matter if you're not riding the edge in the first place. If I'm anywhere near something I view as a limit I'll do some minimum decompression (call it a safety stop if you want).

As far as I'm concerned good dive habits like slow ascents (especially near the surface), time spent shallow after time spent deep, and added conservativism for things like cold, hard work (during or after the dive) or multiple dives over multiple days is where my margin comes from not from precise calculations in an inexact model.

I agree with you. "Exact" calculation doesn't exist in this field because of many factors that can't be calculated.

 

[donacheson]

Actually if you use some decompression software to compare some dives calculated as multilevel dives to the same dive calculated using "average depth" it does in fact correlate very well.

Even for long and or deep dives the calculated decompression or remaining no-stop time will be very close.

Please define "average depth". I can think of at least significantly different two ways to do it.

 

[donacheson]

I agree with you. "Exact" calculation doesn't exist in this field because of many factors that can't be calculated.

The calculation can be as exact as you want to make it. <G>

It's the (mathematical) model which isn't doesn't exactly describe the solution and outgassing and consequences of metabolically inactive gases in the diver's body. (Is "metabolically inactive" the correct term?)

Yes, I know - picky, picky.

 

[MikeFerrara]

Please define "average depth". I can think of at least significantly different two ways to do it.

For the purposes of the comparison excersize with decompression software that I suggested in an earlier post try a time weighted average. What I mean is if you use 2 different depths for equal times it's a streight average (midpoint)...or... If you are at the second depth for 1/3 of the total time the average would move 1/3 of the way from the fist depth to the second depth.

For diving purposes I simplify it a bit.

 

[MikeFerrara]

It's also interesting to look for patterns in the tables.

For instance since I was a PADI instructor for several years I spent lots of time looking at the DSAT (PADI) tables. I couldn't help but remember NDL's but there's more.

Noting that the table designers don't recommend them for multilevel diving.....

look at the table if you have one. Not the front side (table one) but the back side (table three). Look the the RNT's. look under any given pressure group (vertical column). As you move from one depth to another is there a pattern to the way RNT (time you already used) changes between the two depths.

Could you use that to get a hint as to how no-stop time is left as you move from one depth to the other?

Try using a set of tables this way and then punching the dive into some decompression software to see how you did.

It's fun to play with anyway.

 

[donacheson]

For the purposes of the comparison excersize with decompression software that I suggested in an earlier post try a time weighted average. What I mean is if you use 2 different depths for equal times it's a streight average (midpoint)...or... If you are at the second depth for 1/3 of the total time the average would move 1/3 of the way from the fist depth to the second depth. ...

Yes, it seems to me that the time-weighted average is the better measure of average depth.