Tissue Compartments and Halftimes

[ScubaDork]

Can someone please explain this to me. I have read my DM manual, read the Encyclopedia, and the decompression manual, and am still having a problem with this concept. I'm sure its not difficult, and everyone says just read the book. I have, and reread it and reread it and just dont get it. I get parts of it, that different tissues absorb at different rates, that part isnt confusing me, its figuring out how a compartment gets filled and what the halftime actually represents. I just need it set to examples that I can understand.

[drdiver]

Can someone please explain this to me. I have read my DM manual, read the Encyclopedia, and the decompression manual, and am still having a problem with this concept. I'm sure its not difficult, and everyone says just read the book. I have, and reread it and reread it and just dont get it. I get parts of it, that different tissues absorb at different rates, that part isnt confusing me, its figuring out how a compartment gets filled and what the halftime actually represents. I just need it set to examples that I can understand.

Rick, here is a thread that may illuminate it more for you--it addresses the issue from several different angles.


Tissue Compartments (http://www.scubaboard.com/showthread.php?t=27222)

They are a theoretical construct to model the variable loading of the body with nitrogen.

[DyvrDan]

Can someone please explain this to me. I have read my DM manual, read the Encyclopedia, and the decompression manual, and am still having a problem with this concept. I'm sure its not difficult, and everyone says just read the book. I have, and reread it and reread it and just dont get it. I get parts of it, that different tissues absorb at different rates, that part isnt confusing me, its figuring out how a compartment gets filled and what the halftime actually represents. I just need it set to examples that I can understand.
Rick-

Not the best format (this would be much easier in a room with a dry erase board), but let's give it a try:

Let's use a 30 minute compartment as an example. A 30 minute compartment asbsorbs nitrogen such that it reaches 50% of its saturation capacity in 30 minutes (hence the term "halftime"). So after 30 mins (one halftime), that compartment is has reached 50% (that's half of 100%). After 60 minutes (two halftimes), 75% (the 50% from before plus 50% of the remaining 50%). After 90 minutes (3 halftimes), 87.5% (the 75% from before plus 50% of the remaining 25%. After 120 minutes (4 halftimes), 93.75% (the 87.5% from before plus 50% of the remaining 12.5%) after 150 minutes (5 halftimes), 96.875% (the 93.75% from before plus 50% of the remaining 6.25%), after 180 minutes (6 halftimes), 98.4275% (96.875% plus 50% of the remaining 3.125%). Since we are always halving the difference between where we are and 100%, we'll never get to 100% but after 6 halftimes we're more than 98%, so we consider that saturated.

These theoretical compartments also release nitrogen at these same rates.

Hope that helps.

-Dan

[ScubaDork]

Dan,
Thanks, now I understand that part, now where does depth mix into this equation? and how many compartments are there, I get differing opinions on this, and I am sure the DM test is gonna just want one answer LOL

[DyvrDan]

Dan,
Thanks, now I understand that part, now where does depth mix into this equation? and how many compartments are there, I get differing opinions on this, and I am sure the DM test is gonna just want one answer LOL

To see how this info figures into depth calculations, let's look at the following questions:


A 10 minute halftime compartment will have how much tissue pressure 10 minutes after taken from the surface to 60' in seawater?<br>The answer is 30 FSW of pressure (in one halftime the compartment goes half the distance from beginning to new pressure).<p><br><p>
A 40 minute halftime compartment will have how much tissue pressure after 80 minutes at 80 FSW?<br>Eighty minutes is 2 halftimes (of a 40 minute compartment). In 2 halftimes, a compartment goes to 75%. Seventy-five percent of 80 FSW is 60 FSW, which is the answer to the question. <p><br><p>
How long would it take a 5 minute compoartment to saturate to a given depth?<br>The answer is 30 minutes--6 halftimes (6 x 5 minutes).

Make sense?

Different tables have a different number of compartments. The RDP has 14 compartments ranging from 5 to 480 minute halftimes.

-Dan

BTW, this info should be covered in the DM lecture, "Decompression Theory and the RDP". Your instructor should be willing to go over this stuff with you.

[AggieDad]

DyverDan
I know this is a several month old thread, but I am sitting in my hotel room in China now with my Encycloepdia of Recreational Diving sitting next to me, after reading the section on "halftimes, etc" three times, your explanation cleared it up nicely. Thanks, especially from a mathmatically challenged person.

Dan Drew
aka AggieDad