Decompression Theory Question???

[RRedd]

I have been doing my DiveMaster Course i have come to a question about the tissue compartments. Can you give me the formula to figure out the Decompression questions like " if you do a dive to 50 feet salt watern for twenty minutes then you get a surface interval for 35 minutes what is the controlling compartment for the next dive to 50 fsw for 35 minutes"................?/?????? These questions get me , but i understand the concept of the compartments but not the math. Please help if possible??????

 

[MookieMoose]

More information is needed in order to answer the question. Without knowing more about the model, such as how many tissue compartments is considers, and what their half-times are, does the model assume symetrical loading/unloading, etc. it is impossible to answer.

Cam

 

[loosebits]

Here you go: http://www.thaiwreckdiver.com/technical_diving_articles/understanding_m_valuesx.pdf

 

[UWSojourner]

There is a more basic discussion on tissue compartment gas loading here:


http://thedecostop.com/forums/showthread.php?t=19228

 

[rawls]

Is one of the answers...This answer cannot be determined with the information provided? That will be the correct answer. The controlling compartment is the compartment that determines when the dive is over and is expressed in M-values. These values are determined using very complicated formulas that are WAY beyond the scope of a divemaster certification.

Just go to the link loosebits gave you, scroll therough it, and you will see what I'm talking about

 

[Gene_Hobbs]

CALCULATION OF DECOMPRESSION SCHEDULES FOR NITROGEN-OXYGEN AND HELIUM-OXYGEN DIVES.
Workman, 1965.
RRR ID: 3367, NEDU: AD0620879

This one was written to make the calculations in the Workman report easier:

Systematic Guide to Decompression Schedule Calculations.
Braithwaite, 1972
RRR ID: 3945, NEDU: AD075102

 

[Dr Deco]

Hello lil lucky:

Gas Loadings

These are the equations needed for a Haldane-type algorithm. Here P is pressure, and t is time. For uptake:

P tissue = P initial + (P final – P initial) (1 – exp [t bottom {0.693 / t ½]})

For elimination:

P tissue = P final+ (P initial – P final) (exp [t bottom {0.693 / t ½]})



Controlling Compartments

For a dive to only 50 fsw, the US Navy tables indicate that you can have a bottom time of about 110 minutes before decompression at 10 fsw is necessary. For the dives that you indicated, the gas loads are rather minimal.

For the 50/20, the most dissolved nitrogen is in the fastest compartment, and the least inert is in the slowest compartment. The algorithm states that the amount of dissolved inert gas that can be sustained is greatest in the fast compartments and least in the slowest compartment. The pressure (amount) of dissolved nitrogen (supersaturation) is termed the “surfacing pressure.” It is these surfacing pressures that are determined empirically in laboratory with humans (never pigs, goats, or rats for human dive tables).

In this case, the greatest “surfacing ratio” [bottom tissue partial pressure divided by the allowed “surfacing pressure”] is the 20-minute compartment. Since no decompression requirements exist, this compartment would not actually be termed the “controlling” compartment.

If you stay at 50 feet for 110 minutes, you will take on enough dissolved nitrogen, and the 40-minute compartment will become the controlling compartment.

The equations shown above can be set up in an EXCEL program and the bottom time and depths can be varied at will. It is interesting. Remember however, that micronuclei and exercise are not a part of this algorithm.

Dr Deco :doctor:

 

[Diver0001]

I have been doing my DiveMaster Course i have come to a question about the tissue compartments. Can you give me the formula to figure out the Decompression questions like " if you do a dive to 50 feet salt watern for twenty minutes then you get a surface interval for 35 minutes what is the controlling compartment for the next dive to 50 fsw for 35 minutes"................?/?????? These questions get me , but i understand the concept of the compartments but not the math. Please help if possible??????

are you sure you understand the difference between M values and 1/2 times....?

That's the main thing you need to be able to keep straight. If you have any confusion about that, then lets get that part clear first. After that the rest should fall into place.

R..

 

[Diver0001]

Hello lil lucky:

Gas Loadings

These are the equations needed for a Haldane-type algorithm. Here P is pressure, and t is time. For uptake:

P tissue = P initial + (P final – P initial) (1 – exp [t bottom {0.693 / t ½]})

For elimination:

P tissue = P final+ (P initial – P final) (exp [t bottom {0.693 / t ½]})



Controlling Compartments

For a dive to only 50 fsw, the US Navy tables indicate that you can have a bottom time of about 110 minutes before decompression at 10 fsw is necessary. For the dives that you indicated, the gas loads are rather minimal.

For the 50/20, the most dissolved nitrogen is in the fastest compartment, and the least inert is in the slowest compartment. The algorithm states that the amount of dissolved inert gas that can be sustained is greatest in the fast compartments and least in the slowest compartment. The pressure (amount) of dissolved nitrogen (supersaturation) is termed the “surfacing pressure.” It is these surfacing pressures that are determined empirically in laboratory with humans (never pigs, goats, or rats for human dive tables).

In this case, the greatest “surfacing ratio” [bottom tissue partial pressure divided by the allowed “surfacing pressure”] is the 20-minute compartment. Since no decompression requirements exist, this compartment would not actually be termed the “controlling” compartment.

If you stay at 50 feet for 110 minutes, you will take on enough dissolved nitrogen, and the 40-minute compartment will become the controlling compartment.

The equations shown above can be set up in an EXCEL program and the bottom time and depths can be varied at will. It is interesting. Remember however, that micronuclei and exercise are not a part of this algorithm.

Dr Deco :doctor:

Your posts always make things look like child's play. When I grow up I want to be like Dr. Deco!

Quick question. What's the constant (0.693).

And another thing that I've been wondering about now that I'm at it. Why, if there are only 14 compartments in the DSAT model, are there 26 pressure groups? Wouldn't it make more sense if each pressure group corresponded to the controlling compartment?

R..

 

[Charlie99]

log natural of 2.

The equations more naturally work as log naturals and power of e.

If you work with doublings or powers of 2, then you have to convert the exponential time constants to halftimes by the ln(2) factor.

=============

The number of compartments has nothing to do with the number of pressure groups.

The 26 pressure groups of the DSAT model are all based upon just one compartment, the 60 minute halftime (which is also the 86.5 minute time constant compartment ).

If I did the reverse engineering of the PADI table correctly, then PG Z is at 95% of the 60 minute compartment M value, then each letter group down from there moves 3% of the distance to the sea level saturation point. This hold true until PG C, which is 26% of M-value. B and A are unique in that B is about 21% of M value and A is 12% (again, 12% means 12% of the distance from the distance from sea level saturation level of N2 of 24.81fsw absolute and the full M value for 60 minute compartment, 51.44fsw N2 absolute). I assumed 1.6fsw for H2O + CO2 pressure to get 24.81fsw as the sea level starting point.

The controlling compartment for a deep dive (actually any dive deeper than 40') will be a compartment faster than the 60 minute compartment upon which the pressure groups are based. That's why the NDLs for deeper than 40' are at pressure groups less than Z. The faster compartment will have reached its M value at NDL time, but the 60 minute compartment is at less than its limit.