Snoopy
Guest
Hi
I am cross posting this from another diver's forum ....
I am cross posting this from another diver's forum ....
Half Life = "M" ?
- Thread starter Snoopy
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Tissue half times are not M values. I dont know why they are called M values rather than something else. I have a great explanation for what an M value is- unfortunately the reference is at home, and this is one that needs to be spot-on to make sense. I will post (or IM) it to you this evening, provided someone else doesnt do it first.
nickjb
Contributor
While model on which the RDP (PADI tables) is based has many compartments with various half times, the longest half time used by the RDP is 60 minutes. The longest half time used by most other tables is 120 minutes.
Walter,
That is not correct. The RDP uses a 60 min half time compartment as a controling compartment during surface interval. I think the longest half time compartment is like 480 min but I'll check.
That is not correct. The RDP uses a 60 min half time compartment as a controling compartment during surface interval. I think the longest half time compartment is like 480 min but I'll check.
Snoopy
Half times are not the same as M-values. The M-value is the max presure gradient that that is thought to not result in problematic bubbling. I may not have worded that well but I don't have any references in front of me at the moment.
I believe in the beginning there was a "magic" number like 1.53 that was determined through experimentation I believe. Now days it is recognized that different compartments can tolorate different gradients.
So...lets say we only have one compartment. We go to 100 ft. The compartment takes on gas at an expontial rate described by the half time. Now lets say the tissue reaches ambient pressure (for simplicity). The gradient is now Zero. Now we start our ascent. As the ambient presure decreases we have an increasing gradient accross the compartment (faster offgasing). If that gradient equals the M-value before we reach the surface we must stop (we would actually stop before we reach the M-value) and let the compartment off-gas before beginning our ascent.
I hope I didn't botch that too bad.
Half times are not the same as M-values. The M-value is the max presure gradient that that is thought to not result in problematic bubbling. I may not have worded that well but I don't have any references in front of me at the moment.
I believe in the beginning there was a "magic" number like 1.53 that was determined through experimentation I believe. Now days it is recognized that different compartments can tolorate different gradients.
So...lets say we only have one compartment. We go to 100 ft. The compartment takes on gas at an expontial rate described by the half time. Now lets say the tissue reaches ambient pressure (for simplicity). The gradient is now Zero. Now we start our ascent. As the ambient presure decreases we have an increasing gradient accross the compartment (faster offgasing). If that gradient equals the M-value before we reach the surface we must stop (we would actually stop before we reach the M-value) and let the compartment off-gas before beginning our ascent.
I hope I didn't botch that too bad.
Good job, Mike. I wasn't going to try that without my teaching notes in front of me!!
Sorry Mike,
If the RDP used a 480 minute compartment, it would not clear until 48 hours. As it is, it clears after 6 hours. Clearing in 6 hours means the longest compartment is 60 minutes.
If the RDP used a 480 minute compartment, it would not clear until 48 hours. As it is, it clears after 6 hours. Clearing in 6 hours means the longest compartment is 60 minutes.
We can't go off letting Snoopy use neo-Haldanian crap now, can we? He will be doing all his stops at 10' and calculating his deco stops for the maximum overpressure gradients...he'll look like a can of soda..
As I stated the 60 min compartment is use as a controling compartment on the surface. That is why it clears in 6 hours. This is refered to as a 60 min wash out. Various models handle surface wash out differently.
The RDP has 14 compartments ranging from 5 min to 480 min. The choice of a 60 min wash out was thought to be more appropriate for no stop diving as apposed to the 120 used by the Navy tables. However Dr. Rogers lowered the M-values to match doppler data. Therefore even though the model allows longer repetative dive times the maximum allowable nitrogen load is lower. These limits are refered to as spencer limits (named after the physiologist who first proposed them).
The point is the slowest compartment is 480 minutes.
The above is almost word for word from the the PADI Divemaster course lecture outline
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