General NDL - gas loading discussion.

[DiveBandit]

Let me first say that I dive a dive computer, and although I only use the tables as a backup, I stay current in there use. Also, I'm just starting this thread to get a discussion going, not to Flame or Criticize anybody.

Ok, I've always been taught (PADI) that your decent, and bottom time were what counted towards the NDL for a given depth because once you started your ascent you were off-gassing.

After transferring my dives from my dive computer to my PC, I was looking at the dive in "Graph mode" and noticed that my gas loading was still increasing as I made my ascent.

I realize that when using the dive computer , the computer is taking this into consideration while figuring out how much time at depth you still have and adjusting the NDL as you go. However, doesn't that mean that if you were using the "tables" and did a solid down to "X" depth, stayed there until within a minute or two of the NDL, then made an straight up ascent, you just went into deco because you were still gas loading on the way up?

 

[Scubakevdm]

What kind of computer is it?

 

[TSandM]

I believe the tables are constructed to consider descent and bottom time up to beginning the ascent. Ascent is to take place at a prescribed rate and not slower. Whatever ongassing is occurring during the ascent is figured into the table construction.

Noting this, though, shows you why it is not necessarily better to ascend more slowly, especially in the early part of the ascent.

 

[Charlie99]

An interesting, although long, thread on "Ascending TOO Slow

The short version of the thread is that at some point a very slow ascent becomes a multilevel dive and must be planned as such. 20 and 30fpm ascents don't add much bottom time, but once you slow down to 10fpm in the deeper portions, you need to start counting that in as bottom time.

 

[DiveBandit]

What kind of computer is it?

Computer is a Sunnto Vyper.

 

[Saturation]

Some quick answers from basic diving physio: You will continue to ongas until you reach the 1st offgassing depth. The offgas depth is a function of depth and time and related to tissue types. Fast tissues will control the ascent and deep stops, stops that may have long tissue types continuing to ongas as the faster tissues offgas. Roughly not until you are shallower than 50% of your max depth will all tissues begin to offgas.

Let me first say that I dive a dive computer, and although I only use the tables as a backup, I stay current in there use. Also, I'm just starting this thread to get a discussion going, not to Flame or Criticize anybody.

Ok, I've always been taught (PADI) that your decent, and bottom time were what counted towards the NDL for a given depth because once you started your ascent you were off-gassing.

After transferring my dives from my dive computer to my PC, I was looking at the dive in "Graph mode" and noticed that my gas loading was still increasing as I made my ascent.

I realize that when using the dive computer , the computer is taking this into consideration while figuring out how much time at depth you still have and adjusting the NDL as you go. However, doesn't that mean that if you were using the "tables" and did a solid down to "X" depth, stayed there until within a minute or two of the NDL, then made an straight up ascent, you just went into deco because you were still gas loading on the way up?

 

[Diver0001]

An interesting, although long, thread on "Ascending TOO Slow

The short version of the thread is that at some point a very slow ascent becomes a multilevel dive and must be planned as such. 20 and 30fpm ascents don't add much bottom time, but once you slow down to 10fpm in the deeper portions, you need to start counting that in as bottom time.

Boy....I'm glad at least one person understood that thread. At the time I had the feeling that it was a complete train wreck.

R..

 

[TSandM]

Oh, that was a great thread. I found that one because, shortly after I got on SB, I pulled up all threads started by the people whose writing impressed me.

 

[Charlie99]

Roughly not until you are shallower than 50% of your max depth will all tissues begin to offgas.

Actually, on most recreational dives, one will have to be much shallower than 1/2 depth to have all compartments offgassing.

At 35', unless you are in decompression, you will be still ongassing the slower tissues/compartments. (60 minute halftime M-value is around 52fsw absolute, which is what one has for inspired ppN2 at 35'.)

OTOH, the deeper of 1/2 depth or 40' is a reasonable place to start deep stops (or slow to 10fpm) for offgassing the fastest tissues/compartments.

 

[Saturation]

Yes, I stand corrected.

Actually, on most recreational dives, one will have to be much shallower than 1/2 depth to have all compartments offgassing.

At 35', unless you are in decompression, you will be still ongassing the slower tissues/compartments. (60 minute halftime M-value is around 52fsw absolute, which is what one has for inspired ppN2 at 35'.)

OTOH, the deeper of 1/2 depth or 40' is a reasonable place to start deep stops (or slow to 10fpm) for offgassing the fastest tissues/compartments.