Total time to off-gas.

[Woody88]

Hey.. Dr Deco. I was wondering if you know what the total time to off-gas the slowest compartments is? Also, how much faster is it for helium vs. nitrogen. I have had a hard time finding any good data on this. Thanks for any help.

 

[DandyDon]

FYI...

Hello readers:

I will be away on vacation from Saturday, December 4th, until Monday, December 12th.

I will not have Internet access and therefore will not be available to answer any posting.

Dr Deco :doctor:

 

[BigJetDriver]

Hey.. Dr Deco. I was wondering if you know what the total time to off-gas the slowest compartments is? Also, how much faster is it for helium vs. nitrogen. I have had a hard time finding any good data on this. Thanks for any help.

Woody,

It must be remembered that "compartment" is a concept, not a physical thing, and various models use different "slowest compartments". As for helium versus nitrogen, that speed is relative, depending on the type of tissue saturated. To say, for instance, that nitrogen is 2/3'rds slower over-all, can be misleading.

I am not trying to be obtuse. Can you be a bit more specific in your questions, please?

 

[Diver0001]

Hey.. Dr Deco. I was wondering if you know what the total time to off-gas the slowest compartments is? Also, how much faster is it for helium vs. nitrogen. I have had a hard time finding any good data on this. Thanks for any help.

The slowest compartment I remember hearing about being used in Bhulmann algorithms is 640 minutes. It takes 6 cycles to clear, which is (640*6)/60 or 64 hours.

R..

 

[WarmWaterDiver]

That's if the slowest compartment is "full", correct? If it's less than "full" the time to clear is shortened proportionately?

And I agree, 640 minutes is what's used in the 12th tissue compartment model TUSA & Apeks current offerings that use a Buhlmann base algorithm.

 

[MonkSeal]

If it's less than "full" the time to clear is shortened proportionately?

It's not a direct proportion. It's rather exponential function that's used to describe the model.

 

[Dr Deco]

Hello readers:

Compartments

In the old Haldane model, these were actual tissues. The common usage today is to refer to these as "compartments" and view them more as mathematical entities used to calculate your “position” in a "map" of dissolved inert gases in the body.

The time constants in all models [except the NASA model] are fixed and independent of physical activity [and thus blood flow]. If one is very active at depth, more dissolved nitrogen will be taken in by the tissues. :sprite10:
One might exceed the surfacing gas loads, and DCS will be the result.

Long Half Times

The very long half times in saturation schedules probably reflect the fact that dissolved nitrogen, or helium, has come out of solution and now is in small tissue bubbles. The exchange rate for free [gaseous] helium in a bubble is much slower than that for the dissolved state of helium.

Dr Deco :doctor:

 

[WarmWaterDiver]

It's not a direct proportion. It's rather exponential function that's used to describe the model.

Monkseal,

I understand the mathematics of half-times - by their purest definition, they never reach zero, which is what the question was about, and Diver0001's equation about. There is no exponential function in Diver0001's equation to reach zero. By pure application of half-times, 6 half-time or half-life cycles starting with 640 minutes results in 10 minutes still remaining. This is (rounded) 1.6% of the original value of 640 minutes. This can be demonstrated a series of simple calculations.

640 / 2 = 320 (first half-time or half-life)
320 / 2 = 160 (second half-time of half-life)
160 / 2 = 80 (third half-time or half-life)
80 / 2 = 40 (fourth half-time or half-life)
40 / 2 = 20 (fifth half-time or half-life)
20 / 2 = 10 (sixth half-time or half-life)

Or exponentially,

640 / (2^6) = 10

The only number divided by 2 which will equal zero is zero itself. Using the exponential approach solely, a compartment that is ever once not at zero can never return to zero.

The distinction becomes when mathematically the asypmtotic approach to zero is defined as close enough to zero to call zero. Diver0001's equation and verbiage is clear on that.

What value do you use Monkseal, and what terms is it couched in (number of cycles, % of original value, etc.)?

 

[rawls]

That's if the slowest compartment is "full", correct? If it's less than "full" the time to clear is shortened proportionately?

And I agree, 640 minutes is what's used in the 12th tissue compartment model TUSA & Apeks current offerings that use a Buhlmann base algorithm.

I need to get back into the books:huh: