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This is great "NEWS". Will writes that depth does not matter after you switch to pure oxygen (i.e. partial pressure of nitrogen is zero). This means that from now on, when I reach 20 ft and go to pure oxygen, I can put the regulator in my mouth and go straight to the surface (because depth does not come into the equation). Thanks Will, I might try that next time. NOT !!! I know!, I know!, a deco ceiling does come into the equation. As with all Buhlmann type deco programs, we use assumptions which are not in the equations. In this case the point is to avoid bubble formation. When bubbles form (even micro bubbles) they are harder to eliminate than dissolved nitrogen. We come back to the question of just what is taught in a TDI class, which I have not taken. Will, are playing some type of lawyer game where you don't use all the facts, common sense and the total reasons and ideas behind doing deco (are you just looking at the Buhlmann equations and not what decompression is all about)? I know from classes I've taken (several IANTD classes and one with JJ) that any discussion of deco contains a lot more than just what Buhlmann equations set forth. I don't stay at 20 ft to shorten my deco, but rather to avoid possible bubble formation. tim > ---------- > From: will@tr*.co*[SMTP:will@tradeware.com] > Sent: Friday, April 10, 1998 11:40 PM > To: TOM.MOUNT@wo*.at*.ne* > Cc: brownies@ne*.ne*; armantrout@wo*.at*.ne*; > techdiver@aquanaut.com; cavers@ww*.ge*.co* > Subject: Buhlmann, O2 (Re: Equine Mousse (was Re: Lovely)) > > > On Fri, 10 Apr 1998, Tom Mount wrote: > > > William > > I just did that in the examples I post and although it is just a > small > > difference it is slightly shorter so the answer is true. > > Because I can see where this thread is going, let me cut > to the chase and do the maths showing why depth > won't matter (bear with me, it's not too painful :) > > Here's the core Buhlmann ZHL16 formula, I used slightly > different names for the terms, because Buhlmann's are > really verbose (and german), but the formula is the same. > > This is the way you calculate a single gas, N2 or He. Trimix > is a bit trickier, but it basically works the same > way: > > ptol = (prev + (pp - prev) * (1.0 - 2^(-time/ht)) -a ) * b > > where: > > "ptol" is the minimum tolerable pressure, in BAR for a given tissue. > "prev" is the saturation level of that tissue at the previous sample > interval, and is what you get if you strip out the "a" and "b" > references. > "pp" is the inspired partial pressure of N2 > "time" is the time elapsed since the last sample interval > "ht" is the halftime of the tissue in question (supplied by Buhlmann) > "a" and "b" are constant coefficents supplied by Buhlmann. There's > a different set for each ofthe 16 compartments. > "^" is notation for exponentiation (two to the power of the stuff > after it). > > The core of this is pretty much a standard halftime decay formula. > The > key here is the inputs - partial pressure of N2 and elapsed time. > Notice there's nothing about oxygen here. > > The way you run the algorithm is: > [1] Calculate the partial pressure of the nitrogen in the > mix by subtracting the amount of o2. > [2] Get the elapsed time since you last did a calculation. > [3] Run the above formula for each of the 16 tissue > compartments. > [4] The compartment with the least tolerable ceiling > is your "controlling tissue" - the deco ceiling. > [5] Keep doing steps 1-4 until all tissues can tolerate > less than 1 BAR (less if you are diving at altitude). > Do this every few seconds or so, stopping the > ascent just below the ceiling until it's OK to ascend > further. > > They key to the whole thing is that absolute depth isn't > used anywhere - all you need at any given time to calculate > your ceiling (and saturation) is the current partial pressure > of N2 in the mix you are breathing, and the amount of time since the > last sample. Oxygen itself isn't considered anywhere. > > So you can see that once you go onto 100% O2, your "pp" > (inspired partial pressure of N2) variable goes to > zero (or close to it, if you are including > some water vapor). If you are at 100 feet or 10, "pp" will still > be zero, which means the rate of offgassing will not > change. This means that your ultimate deco profile > will not change. > > Of course, as soon as you have a FN2 above zero > (a Nitrox), then your PPN2 will vary with depth, > and so where you do your hang will matter, a litte. > > For instance, for the 100 for 100 dive on air, with > deco on 90%, doing both the 10 and 20 gives a runtime of > 158 minutes. Combining the stops gives 159 minutes. > > Notice that combining the stops actually *adds* time, > which makes sense, because the PN2 is higher at 20ft, > so the off-gassing rate will be slower. > > There are instantaneous versions of this algorithm, > where you don't need to iterate, and different > sets of a/b coefficents for tables, dive computers, > and pure theory, but it's still the same thing > when it comes to exclusive use of partial pressures. > > Not having depth involved in the input > is very convenient, because it makes for easy > calculation for diving at altitude, and makes > it very simple to switch deco gasses. > > But any way you slice it, combining your > 10/20 stops at 20 while on pure O2 will > not reduce your deco time. > > There are always minor variations in > implementation details between packages that > will swing deco times a bit in either direction, > but if ProPlanner is ZHL16 based, I can't > figure out how they are getting these results. > > -Will > > > If you read the two examples of the bulhman model I posted when you > do the > > entire stop at 20 feet and do not go to 10 feet the deco time is > shortened. > > . > > Below is the example this is a pure bulhman model on a square dive > ran these > > on pro planner as it is the quickest to print out. > > > > 1.air to 100 feet 100 minutes deco on air runtime is 243 minutes > > 2 air to 100 feet 100 minutes deco on oxygen at 20 and 10 feet RT > 165 > > 3.same dive deco on oxygen at 20 feet only RT is 162 minutes > > This is 3 minutes less than if deco is at 20 and 10 feet, thus > oxygen at 20 > > allows a shorter deco than doing a 20 and 10, thus answer is true > > another example > > 160 (air for simplicity) 30 minutes deco on air RT105 min. > > Same dive deco on oxygen at 20 and 10 feet RT 66 minutes > > Same dive do oxygen stop at 20 feet only (no 10 foot stop) RT 64 min > > Deco on oxygen at 20 feet only is 2 minutes less than if stops were > made at > > 20 and 10 > > Another example trimix 21 25 160 30 minutes > > RT deco on bottom mix 159 minutes > > RT deco on oxygen at 20 and 10 74 minutes > > RT deco on oxygen at 20 feet only 72 minutes > > again we see a shorter deco by using oxygen at 20 feet only > > or > > RT deco on EAN 70 starting at 40 feet 67 minutes > > It is a small difference but you deco faster on oxygen at 20 feet > than you > > do on oxygen at 20 and 10 feet. It is a fact > > Tom Mount > > > > > > > > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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