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Physics of diving question... - DetectorGuy - 12-07-2009
I am studying for a class and I have some questions about the physics of diving. Some of the questions I had have become crystal clear and some of the things I thought I knew have become confusing. I am struggling with the Haldanean model. I realize that in 1906 Haldane came up with the theory of compartments, which is just a mathematical model reflecting that parts of the body absorb and off gas at different rates. The tissue compartments are a theoretical model of the body. Each part of the body loads N2 at different rates, and also off gases at different rates. For instance blood absorbs N2 rather quickly, and bone much more slowly. Instead of doing a model for every tissue in the body, His original model was based on 5 compartments, the Navy tables on 6 compartments, and the PADI RDP on 14. This much I get...
Where I start to get lost is when Haldane assigned a halftime to each compartment. A compartment half time is the time in minutes for a particular compartment to go "half way" from its beginning tissue pressure to equilibrium at a new depth. Haldane's original model had halftimes ranging from 5 minutes to 75 minutes. Modern models range from 3 minutes to over 600 minutes. My first set of questions would be this: What body parts relate to each of the compartments and is it important to know which body parts are fast compartments and which are slow? I am assuming that blood is a fast compartment and bone is slow, but other than that I can not find reference to answer this in any books. I might be over thinking this...
The second set of questions would be: How do I find out which compartment has a 5 minute halftime and which has a 20 minute halftime? Are these minutes just theoretical per compartment or are they written somewhere?
The third question is an example out of the book asking: A 20 minute halftime compartment will have how much tissue pressure after 40 minutes at 80fsw?
Their answer was: 60fsw of pressure (40 minutes = 2 halftimes for a 20 minute halftime. After 1st halftime, pressure goes halfway from 0 to 80fsw, which is 40fsw. After the second halftime, pressure goes halfway from 40fsw to 80fsw, which is 60FSW). I get that part of the theory, but I am still confused as to which part of the body the 20 minute halftime compartment is assigned to.
Then I was wondering about M-values. I read that M-value is the maximum tissue pressure allowed in the compartment when the diver surfaces, so as to prevent exceeding the maximum acceptable gradient. If the diver exceeds the M-value in any compartment, there's an unacceptable risk of decompression sickness. Also that when calculating decompression dives, there are M-values for each compartment at each decompression stop depth. For Recreational diving we only need to be concerned with the surface M-value. The faster compartments have a higher M-value and slower compartments have a lower M-value.
My fourth and final set of questions for the night are relating to M-values: Is it important for a recreational diver to know what the M-value is for a specific compartment, or is this just a theoretical value given to a theoretical compartment by the people that created the model that our computers and tables are based on?
Let me know if you can explain these questions to me in laymans terms, John
Re: Physics of diving question... - Hydro - 12-07-2009
ahahah gen chem stuff... i'll make some inquiries later, but right now im exhausted on all fronts, so i don't wanna activate that part of my brain... as i recall half lives are fun.
ok this post perked me up a little..
1)i don't think you need to know what exact tissue is fast and slow, i think its the idea that while some tissues can adapt faster you will still have more saturated tissues, and over time you will get a wider gradient of saturation.
just because some tissues will adapt faster, you will still have slower compartments that will require surface intervals to get back down enough to have decent non deco diving time. the opposite would be true for faster tissues being affected more when descending. ( being bored at work i like to read pubmed articles on diving and i recall there was something that talked about the relationship of fast descents and getting narc'd vs slow descents but greater possibility of deco problems later?)
2)i think its more important to know there is a difference in compartments... look at the graph on the link and it will show how the different compartments respond much differently to pressure over time. i think its cool how the fastest HL becomes the most saturated compartment and then becomes the least saturated the fastest... so depending on how saturated you are would have a big effect on your ascent rate... so assuming you make a miscalculation and ascent too fast for your offgassing, you could blow an mvalue for 1 of the 3 possibilities 1) blow an mvalue for just the fast HL compartments 2) just the slow compartments 3)blow the mvalue for all compartments in the worst case.
3) similar response to 1, i think as long as you are aware there is a different compartmental mvalue what part doesnt matter unless you only want to risk a certain compartment of your body? things like ascent rate would be more aimed at your fast compartments in a no deco stop situation... and no fly time would be related to your slowest compartments. surface interval between dives would be aimed at keeping your mid compartments down.
4)i dont think recreational diving gets that technical in dive planning, its not just theory, there are a bunch of studies with saturation data (i don't have a good reply to this.)
(i'm open to disagreement here, since im no expert)
Re: Physics of diving question... - arcFlash - 12-08-2009
John, I don't have answers but I may have another question. Say I know this stuff on a dive and I know I have to violate something to surface before I'm OOA. Are you asking if I know the compartment/body part and the mvalues (generally) that I may be able to get bent on something like and arm as apposed to a spine if I ascend at a different curve?
Just for example.
Quick to 30ft then very slow - bend arm
very slow to 30, then quick - bend spine
Re: Physics of diving question... - DetectorGuy - 12-08-2009
I don't have the answers either, but I want to fully understand the fundamentals of dive physics and physiology with 100% certainty, so that when the little trinket questions get asked I will be able to snap the answers out like they are second nature. I think I was over thinking the whole Haldanean theory.
Re: Physics of diving question... - SmithDiving - 12-08-2009
John, first of all I don't think you need to know which compartment saturates first - almost nobody knows this. You say you would like to have an understanding of the fundamentals of dive physics and physiology - almost nobody has this understanding as it's all theortical. What you should understand is this (some of this I know you understand, but just so everyone can understand) - Each tissue compartment in the body (doesn't really matter if you identify 5 or 28) fills up or off gasses a specific gas (nitrogen is what we generally deal with) at a different rate, this is described as the compartment's half-time (the number of minutes required to fill or deplete the compartment by 50%). You are correct that softer tissues (fat cells) generally in & off gas quicker, and hard tissue (cartiledge & bone) in & off gas at a slower rate. As I understand it, a compartment's M-Value is a US Navy term referring to "Maximum Value" for a compartment. In other words, the maximum amount of time required for a compartment to become "full" of a specific gas. The accepted M-value for a compartment occurs after 6 half-times when a compartment is considered to be completely filled (98%+), so a 5 minute compartment would reach it's M-value at :30 (5x6). Up until a few years ago (pre-RGBM) all "reliable" dive tables for recreation and technical diving were based on this theory - The first tissue type to reach it's M-value for a specific depth controlled the length of the dive for that depth. The specific calculations for in-gassing and off-gassing for specific tissue types are very complicated (mostly because they are non-linear) and are something that very few people understand - or need to for that matter. Remember, this is all theoretical and your dive table or computer are just making calculations based on a generic diver in generic conditions.
Re: Physics of diving question... - JointVentureDive - 12-08-2009
DETECTOR GUY.... THANK YOU.... FINALLY, SOME REAL SCUBA RELATED DISCUSSION. THANK GOD. I was about to think we only discussed coffee beans here and occassionally look at some of John J. great video's. (WHICH I REALLY LIKED) Would love to have more scuba related discussion in here.
Is this class you are taking Advanced Nitrox and Deco proceedures?
I agree with most of what Hydro said, and Everything Smith Diving said. I might just add one or two things.
You are over thinking this. I don't think the instructor would want you to know the answers to this. Infact, they probably would think you are being anal. But. that being said, and off my chest, .... Why would you want to even get close to the tissue compartment over saturation risk or want to know the specifics of this. You always take the worst possible case of saturation, half it and that is your maximum length of your dive when depth and time is factored.
In otherwords, never exceed it and that is going to be the maximum length of time with deco time included and assumed that you will make on that dive. You can simply rely on your computer or actual panned dive if you are using your Navy dive tables. They work. The only time they don't work is when someone does the math wrong and its pretty hard to do that, seeing its already done. But I do know some people like to slide they're fingers down, when they are supposed to move them over. Gosh, that sounds riske, but you know what I mean.
The computer is so darn accuate on these things. And even if I did the math on a planned dive, and my computer said its time to go, but my math said it I could be at depth longer etc, I would rely on my computer, get out, because in the end, why would you argue with your computer. Trust it. EVERYONE SHOULD DIVE WITH COMPUTERS and I personally believe every certified diver should have one.
Your next possible dive will be determined by the "theoretical surface interval time." Never exceed you Max M value on your lowest compartment (or fastest saturated compartment). or Simply put don't violate the Surface Interval Time. You don't need to know the specifics. Others have already died for us and the studies behind the theory is pretty sound as I understand it. oH, AND GOD FORBID YOU DIE, THEY ARE GOING TO SAY YOU DROWNED ANYWAY. "Not his tissue compartment was over saturated with Nitrogen and he exceed the limits."
This past Nov.'2009, IN FLORIDA, 4 REBREATHER DIVERS DIED in 3 different instances. Obviously they didn't drown but I'm sure thats what it will be written up as, instead of equipment failure as is most of the cases, or operator error in selecting the wrong gas mix for the kind of dive they are doing which is probably what happened to the 2 Rebreather divers that died together.
I think in the very first books we all started with be PADI, SDI/TDI NAUI,or SSI etc, they clearly said something to the effect that it is.... INFACT, ALL THEORETICAL and that it is not an exact science. The Navy dive tables and Dr. Haldane theory was just that, and it actual saturation compartments are different in everyone. SIZE, SHAPE, and Gas's used as well as consumption rate of the gas, and the Nitrogen loading, and lets not forget max depth and time at depth will effect us all differently. Skinny, vs. fat, etc. short stature, vs. Tall and lets not forget actual body health and condition. Alcohol, asprins, all are variables in satuation rates and will change from month to month and physical metobolic rate.and a womens time of the month would also be a factor.
Now as far as saturation, yes there are many studies and facts behind the tables they have done with with Many navy divers and other ginea pigs, but the bottom line here for recreational divers is use your tables, and rely on your computers. The bottom line for tech divers is use the tables and rely on your computers. If your mixing gases, YOU BETTER RELY ON YOUR MATH... All the work has been done for you. Plan you dive, check it twice and enjoy the dive. Follow your computers commands. ... I'm sorry Dave, I can't do that....
And to comment on Arcflash., If you are OOA, your only option (assumes no buddy and no back up and you are OOA) is to SURFACE.. I'd rather take the chance of getting a decompression hit, and get help, rather then to learn how to breath water through my gills. LOL.
I do have nice gills' but they can't make O2 yet. In fact they tend to take peoples air away so you'd really be in big trouble if you were diving watching me.
BUT THEN AGAIN, I'M A GREAT BUDDY, AND I WOULD NEVER LEAVE MY BUDDY WITH AN OOA. We would have been reviewing this all along, AKA BUDDY CHECKS...and we would be on the surface with plenty of air left in our tanks as per the plan. Unlike an account I read about in a certain mine pit in Northern Minnesota about buddies.
LAST BUT NOT LEAST
Another sore spot in my ...(well I won't say) is why do we have to pay for a full tank of air up here if we still have 6 -to 800 pounds of air left. In Florida some shops are charging by the actual gas required to fill the tank to say 3000 pounds. Avg. fill is a couple bucks cheaper this way in Florida. One shop owner told me that they are starting to do this, because, it gives the shop owner, and his insurance company, a partial out of a law suit if the air is found to be bad air, because, other air was mixed with the new air etc. (does that make sense) way to much litigation.
Nicki
Re: Physics of diving question... - DetectorGuy - 12-08-2009
Thanks to all of you for clearing this up. I had been reading and reading until my mind was numb. When I got to that point things did not seem to make sense. I thought I was missing something. I am glad to hear all of your responses. I never took any physics classes in school and I pretty much just goofed around all the time.
The class is dive master. I am not taking it to lead tourists on dives in the pristine waters of Square Lake or to eventually become an instructor. With the help of some peer pressure from a friend of mine (if he doesn't mind me calling myself one of his peers). I am taking the class only to build my knowledge base about diving. The class hasn't started yet but he gave me the crew pack and I thought I would get a head start with independently studying prior to the first class to make it easier him and me too.
I learned some very interesting things already... argon... I posted in here a while ago called "how popular is argon" Since reading the manual I have learned that deep divers that use Heliox or trimix, tend to use Argon for suit inflation because helium for suit inflation would not work as it conducts heat much faster than oxygen or nitrogen. Also using helium in the suit would cause decompression problems because it diffuses rapidly. During decompression it can dissolve into the body through the skin faster than other dissolved gasses (like nitrogen) in the tissues can dissolve out through the lungs. This is called isobaric counterdiffusion, and can occur when a diver breathes a slowly diffusing gas while surrounded by a rapidly diffusing gas. It can also occur if you breathe a rapidly diffusing gas while decompressing a slowly diffusing gas.
This is where the slight contradiction in using argon for suit inflation comes into play... Argon is very dense so Isobaric counterdiffusion is not a problem, but there has not been a ton of scientific testing of decompressing surrounded by a heavy gas while breathing a lighter gas. There are no theoretical concerns because no one important cared enough to study its effects. Still divers use argon in their suits for inflation, and have for years, with no widespread problems reported. This would be a good Thesis for some college student. If anyone comes up with a theory for this, I want to call it "The sober Irishmans theory".
Another thing I learned is that the USA is the main commercial supplier of helium. Some say that heluim sources will be depleted within this century. What will shore up the helium needs then you say? Possibly Neon. Neon is a rare gas on earth accounting for less than .002% of the atmosphere. Although it can be extracted from air using a freezing process to create "helioneon" (25%He, 75%Ne). It is thought to be the fourth most abundant element in the universe. Now it the space shuttle could just bring back a cargo bay load every trip back we would be set.
I thought all of that was interesting.
Re: Physics of diving question... - JointVentureDive - 12-08-2009
Congratulations on going for the Dive Master. Its not that hard, and I can tell how studious you are, you will not have a problem passing the test. The only real hard part was pulling a man who was 2 plus times my weight back and forth across the pool about 10 times.
ARGON
The Main reason, they use argon, is because its nice and warm and cheaper. Forget the college thesis, (ask the boss. CCR DIVER ask Bob Olson. ) I've never met him, but I would bet he uses Argon for his deep dives.
Bob if you read this, let us all know if it is true. Also Bob, when are we going to see some of your pictures or get some of your stories about the wrecks you've discovered. or if they are already posted, kindly point reference them to me.
nicki
Re: Physics of diving question... - rcojr - 12-08-2009
John
The best read that I have seen on this subject is Deeper into Diving by John Lippman it is 610 pages. It is an in depth review of decompression procedures and of the physical and physiological aspects of deeper diving. ISBN 0 9590306 3 8
Argon is one of those subjects that gets debated back and forth almost to death. Some people say it is like wearing a warm sweater
and some say they don't feel any difference. I am one of the later I just use air for suit inflation. I know people that have tried helium in their suit and they said they were cold on the surface
Nicki
The wreck hunting gurus are Jerry Eliason, Kraig Smith, Randy Bebee, and Ken Merryman.
I am just lucky enough that they call me to come and dive their new finds.
I dive with Ken Merryman and our purpose on most of the dives is to document them for nomination to the National Registor of Historic Places. This is the best protection we can give them at this point.
If you want to see them go to and on the left side of the screen you will see shipwreck guides. Check out North Shore and Apostle Island wrecks. I am the guy in the lime green DUI TLS 350.
The picture on my avitar is the wing bridge on the Gunuilda taken last Sept. on my 26th dive there.
And who have you been talking to? ;D
Bob
Re: Physics of diving question... - JointVentureDive - 12-08-2009
thanks CCR (Bob. )
I didn't mean to leave the other wreck hunting gurus, Jerry Eliason, Kraig Smith, Randy Bebee, or Ken Merryman out. I just see your name on the board more often, and I got lazy in my post, so I thought I would just mention your name. But, in any case the fact that you answered proved my point that you would know the answer. I just thought for sure you would be argon... as opposed to air.
I am going to the post to see the wrecks and study the writeups particulary of the wrecks you guys have discoverd. Within a week, I will have all of them memorized and be able to speak with some level of confidence on each of them.
Congrats and thank you to all of you great divers, not only for what you do to preserve and document for historical purposes the wrecks but to inspire other divers and non divers to keep looking and stay interested in preservation of these historical findings.
Nicki
Nicki