Buhlmann, Computers, and Helium and what it means in scuba diving December 16, 2005
Posted by Andy Carroll in : Decompression , trackbackThis is a bit of a geeky article on deco and gasses and scuba diving decompression models. I wrote it a back in July 2004, when I was initially getting into deeper diving using mixed gasses and starting to do scuba dives which required decompression. I thought it might be useful to some people doing the same and I just fancied getting a bit geeky, as you do.
Disclaimer: Like you, I am continually learning about deco and technical diving. These posts are written primarily to ensure that I have understood what I have learnt and hopefully to encourage others to explore this fascinating subject. I do not guarantee this is correct and I do not recommend you do anything without knowing why you are doing it. I am not responsible if you read anything I write and change your plans and get bent. I am not responsible if your buddy or anyone you know gets bent, and I am not responsible should your dog be run over by a car or because you find a fingernail in your Big Mac. You make your own choices.
Just wanted to make that clear.
What I am discussing today is the decompression model that most scuba diving computers are based on and why we, as technical divers, are beginning to question these models.
Haldane
Let me introduce you to a clever man from Scotland called John Haldane. He was around at the time when the caisson workers were suffering decompression illness and he started experimenting on goats to find a solution.
Yes, the deco computer you wear on your wrist and the tables in your laptop have their roots in goat anatomy
Haldanes research led him to the conclusion that gasses (in this case air) breathed by the caisson workers at pressure diffused into their tissues and when they ascended from the caisson, these gasses diffused out of the tissues and formed bubbles which caused DCI.
He knew that certain tissues worked differently from others and his model suggested the human body be viewed as a group of tissues, all arranged in parallel, ie they all diffused gasses at the same time. He also realised that the caisson workers were able to work at shallow depths of 10mtrs or so without getting bent, no matter how long they worked. We call this tolerable overpressure ie the tissues are able to withstand a certain amount of pressure without resulting in DCI symptoms. One point to note is that we are talking severe pain and disablity or type II DCS, not tiredness and headache, what we might call type I DCS.
Workman and Buhlmann
This idea of an unlimited level of tolerance was refined by Haldane in his model and was further developed by Robert Workman who created the �M Values� the maximum tolerance of each tissue group, dependent on the depth and tissue ie he started splitting the body into more than one tissue group. Also at the same time as Workman was working on this so was Albert Buhlmann, a very clever fellow focussing on altitude diving, as he was based in Switzerland. Buhlmann published a book which gave instructions on how to calculate decompression and this is perhaps why his model is the most popular as it is readily available and is extremely flexible in that it can be calculated for repetitive diving, altitude diving, flying after diving etc etc.
Ongassing and Offgassing
We know that tissues with good perfusion absorb inert gas quicker but also diffuse quicker. When a tissue is exposed to a higher partial pressure of inert gas than is inside the tissue then it absorbs the inert gas until it reaches the same pressure. The same thing happens with offgassing, when the external partial pressure drops then the gas is diffused from the tissue. This is the main assumption in the Buhlmann model. The Tissues are said to ongas and off gas in halftimes, ie after a certain time, the pressure is halved and then after that same time it is halved again. This forms a decay curve common to many biologists and scientists. If we ascend too fast ie faster than the M value the gas cannot diffuse at its proper rate and forms bubbles in the tissue which cause DCI.
ZH-L16 Model, ‘Bend and Mend’
Buhlmann created the ZH-L16 algorithm which split the body into 16 tissue compartments, each of them having a half time, from minutes to many hours. He used M values for each of the compartments to determine the theoretical maximum tolerance or tolerable overpressure as I mentioned earlier. One point to note is that the whole focus on this calculation is the partial pressure in the tissue compartment versus the partial pressue of the inert gas breathed, in this case nitrogen.
How Buhlmann works is that it calculates a maximum depth you can ascend to with each compartment based on the depth and time and also referring to the halftimes and the tolerable overpressure. This would mean that on shallower dives the faster tissues are the �controlling� tissue and on longer deeper dives then the middle compartments would be the controllers. This is logical as the slower tissues will not load up on the shorter shallower dives. One main point about this is that we are using tolerable overpressure ie the amount of abuse a tissue can take before bubbles are formed. This is in essence why we sometimes refer to these profiles as �bend and mend� simply because you bend those faster tissues at the start of the ascent and then fix them later during the shallow stops.
Testing and Versions
Buhlmann did loads of testing with his model, but only with nitrogen as the inert gas. He also padded the model to allow for things like rapid ascents, heavy workload, and poor physical conditioning and a whole bunch of other things. This modified algorithm is called ZH-L16B. He further padded the algorithm again for use with computers as their real time calculations removed some of the conservatism of the table model. This is called ZH-L16C. There was another model even more padded which is used in Aladdins computers, the ZH-L8 ADT model. Other computers will adapt the algorithm depending on how conservative they want to be, and can do this in a variety of methods, by planning dives deeper or longer, or by altering the compartment overpressure value (Gradient Factors is an example of this)
Helium, Giving the Wrong Impression?
And so, how does Buhlmann cope with helium? Well, his calculations are simply modified to include the different partial pressure of the helium as well as the nitrogen. He derived his half times for the compartments on the ones for nitrogen and largely guessed the M-Values for the tissue compartments with regard to helium. He died before he could test his theories properly.
The model we are left with seems to treat helium far too conservatively and gives the impression that decompressions from helium are longer than with nitrogen. Helium is said to be a �faster� gas than nitrogen to the tune of around 2.65 times. This means in straightforward terms that the helium will diffuse quicker into the tissues than nitrogen but also diffuse out of them quicker which Buhlmanns model does not seem to accommodate and is perhaps were the logic of �getting off the helium as early as possible� came about.
Knowing what we now know about both helium and how bubbles are formed during deco we are now seeing a growth in the popularity of deep stops on trimix dives as well as air dives. Some computers have incorporated them into the algotithm, sometimes called microbubble stops, such as in the Aladin computers and the VR3, although the VR3 is reported to have a more effective strategy than the Aladin. This simply slows the ascent to allow the faster tissues to offgas without punishing them and also for the faster helium to offgas properly, although it should be pointed out that these models are still based on the algorithm which we now believe treats helium too conservatively or incorrectly, depending on your point of view. For those of us without computers the deep stops benefit us in that we can reduce the shallow stops and create a more efficient deco curve IMHO.
One other point is that Doppler studies have shown that bubbles form after most dives. Buhlmanns model assumes all gas is eliminated in the liquid phase (diffused in tissues) rather than the gas phase (bubble form). These phases are quite different in how the diffusion process works. Newer deco models are attempting to model this gas phase as well as the liquid phase such as VPM and RGBM. Currently these are models which seem to more realistically model the decompression process.
If there are some more knowledgable scuba divers out there who can correct me on any mistakes, then please feel free to comment.
Comments»
Pre Buhlmann there were guidelines for delayed decompression if you had been unfortunate enough to come to the surface not observing the proper acsent speed.
Is anyone aware of the recommendations for delayed decompression when one uses the Buhlmann ZH-L16 tables?