The best way to dive helium is with a decompression planning computer program. In my experience, GUE's Deco planner is the best one out there, but for dives of 200' or less they all will do. All programs have safety value adjustments and the individual diver needs to experiment with them. For example one diver uses joint pain in his elbow as an indicator that he's pushing it. You can crank them all the way out, but you are risking exposure to the elements and frying your lungs on 02. Or you can decrease the values until you start "feeling it".

While DAN states that the slightest indication, or even an indication of an indication of being bent means a trip to the chamber, divers routinely (some whether they know it or not) will feel the effects of incomplete decompression sooner or later. And most won't hit the panic button as mild type I hits tend to go away on their own. Type II, neurological hits are another matter and do require prompt attention. I think that for the cautious person, DAN's advice is good, but it's your body, and your choice.

One piece of information which is coming out of the WKPP and other extreme divers is that doing deep stops is a good thing. A real good thing. One characteristic of helium is that, due to it's low density, it enters and exits tissue very rapidly compared to nitrogen. This means that rapid ascents, even deep ones, can cause bubble formation. An it's the bubbles that screw you up. So create deep stops and ascend very slowly and deliberately.

One technique is to create a 5-10 min. stop halfway between your bottom depth and the first stop the computer calls for. Then going halfway between this stop and the first one the computer calls for 5-10 minutes. To incorporate these stops into your software of choice you need to enter them manually.

This rapid dissipation of helium can be turned to your advantage by using stages composed of nitrox 50 and pure O2. Both mixes have no helium so the pressure differential is at it's maximum, causing accelerated helium off-gassing. Here are some techniques to do this safely. Also presented is a discussion as to why you should use pure O2 as your final deco gas.

We need to look at some topics in gas diving that are being misapplied due to misunderstanding, and then used to justify deep air diving.

#1 - "air tables" are wrong. We all learned some simple things first, like PADI tables, and then Navy tables. PADI tables are Navy no-decompression tables. Navy tables are nonsense - they bend you by not doing the deep stops and then treat you by extending the shallow stops.

#2 - Gas tables are more correct: unlike the more arcane Haldanean models in the air tables, gas tables were mostly developed using Bulhmann's theories which started deco deeper, but not deep enough.

#3 - In real life, stops start deep, helium is your friend, it makes you fell better after the dive and keeps you from narcosis, it is EASIER to decompress from.

The result is that people were first taught to believe that gas needs deeper deco than air, when in fact air needs deeper deco than is in the "air" tables.

The term "decompress from gas on an air table" is an oxymoron - there is no "correct" "air" table as we know it, the gas tables are what is needed for air.

Having said that, let's look at a diving situation:

I want to do the "Doria", but my captain, Janet Beiser, has to limit my gear baggage since there are others on the boat. I can be a dope and dive air ( if Janet would let me), or a convolute and put air in my back tanks and take gas stages, losing redundancy in an emergency, and being forced to air at the worst possible time ( like what happened in the death of Rob Parker), or sharing air with an out of gas diver, who is now hammered , scared, and on my long hose, or I can make the exact mix for one dive and then blow it into oblivion with air, or,

I can "Do It Right":

I can lose the abject fear of helium and low oxygen mixtures, and make up two sets of doubles with high helium, like 50%, and low oxygen , like 14 percent, and take stages of the exact mix for the depth , probably something like 18/33, and some stages of 50/50 and my oxygen bottle.

I dive the stages and try to save the back gas, but let's say I want to use the back gas. I can blow it back three times and still be ok on the oxygen, and probably pretty good on the helium, but what happens to the deco?

My first dive for 25 minutes is probably a good hour of deco, my second with the diluted mix is more like 52, and the third more like 45 ( relatively 60,50,40 or padded ratios like that). In other words, for a few extra minutes in the water, I get to do it safely. I then do the same with my other set, and/or my stages were dived first, and then I do a couple of back gas dives . I keep the dives to reasonable bottom times, and end up making the deco gas last longer, and as the deco gas gets diluted or lower, the deco using the higher oxygen "reblows" is getting shorter and shorter anyway, assuming I am giving myself a decent interval between dives. Most of the deco time is on oxygen anyway.

POINT here: mixes that are too low in oxygen and too high in helium are not a bad thing - this is ok. The opposite is not. The former means a tad more deco, the later means a lot more risk.

For a shallower dive, lets say 130-160, I can take my doubles with something like a 16/40 and blow that up a couple of times for back gas diving in that range with the boat's compressor. The deco pickup over a higher oxygen mix is not enough to warrant the air, especially at the more insidious depths, like 150, that have enough impairment to cause an accident, but not enough to "ring your bell" and make you aware of the impairment.

Keep in mind I am talking about trips where you have limited gear space and want to maximize your gas .

Technical diving is fun, but it is getting a bad name due to the accidents.The accidents are due to impairment from narcosis. The accidents need to stop. I repeatedly do dives that were not even thought possible by my original dive partner , Bill Gavin, and I do them safely, and I do them all the time, and they are fun to do. If I can do 300 for three hours and then go out to dinner with my pals, then you can dive the Doria or anything else without the self-imposed risk. I know what I am talking about-learn this stuff right and stop the nonsense.

If you need information to keep yourselves alive, ask me or any of the WKPP guys, and examine your own misconceptions - there is no such thing as an "air table", and the real risk is death. - G

Ben,

You raise some very interesting questions regarding blood viscosity and the behavior of blood within the microcirculation. The reality may be a lot worse than any of us may have imagined. Fortunately, for vertebrates, the apparent viscosity of whole blood decreases in tubes with diameters less than 0.1cm. This observation is known as the Fahraeus-Lindqvist effect and implies that the shear stress at the capillary wall decreases as the tube diameter decreases. On the contrary suspensions of rigid particles display no such behavior. Mammalian capillaries range in diameter from 10 micrometers down to 3 micrometers so that blood cells have to flow down them in single file. For those capillaries whose diameters are less than that of a cell (7 - 8 micrometers) Red blood cells must deform to pass through the vessel. High pressures must be generated locally within the thin layer of fluid around the cell in order to deform it and therefore depend considerably on the elastic properties of the cell. Anything which impairs the elastic characteristics of the blood cell may have a profound mechanical effect on both the cell and the blood vessel.

Abnormal cells which are hardened or almost spherical cannot be deformed so easily and therefore cannot pass through the smallest vessels. In experiments performed to determine the size of the smallest vessel through which a red blood cell can transit it was found that anything smaller than 3 micrometers caused damage to the cells. Cells which are artificially hardened or made to swell into a spherical shape (osmosis), impairing flexibility, cannot pass through pores less than 8 micrometers in diameter without suffering haemolysis (cell destruction).

In view of the above scenario it would seem that any action which would artificially "stiffen" blood cells could have a potentially negative impact in terms of both the mechanical damage to the cells and capillaries themselves as well as decreased perfusion of the surrounding tissues. Where critical tissues are concerned, such as those subserving neurological processes (your brain for instance) the effects of diminished mass transport could be profound. Since the capillary micro-circulation plays an essential role in gas transfer during compression and decompression one might be further concerned as to the effects of red blood cell rigidity associated with hyperbaric exposures to air.

Blood is forced to flow through capillaries as little as 3 micrometers across in the splenic pulp. Normal cells can do this with little haemolysis; however abnormal spherical cells cannot, such as what occurs in the disease known as hereditary spherocytosis (sickle cell anemia is another disease which distorts blood cells). The spleen filters out blood borne debris as well as housing a large population of B cells, the immune soldiers which manufacture antibodies in response to an alert by the T cells. There was some speculation recently in the DAN journal regarding "Divers Disease", a flu-like illness which sometimes accompanies hyperbaric exposures. I wouldn't be at all surprised if the cause of this problem turns out to be the aftereffects of red blood cell rigidity and the consequent build up of haemolized cells in the spleen and lymph nodes.

Maybe its just a coincidence, but deep air diving carries obvious and subtle neurological effects and it poses very problematical decompression issues for long exposures (empirically and anecdotal observations). I personally have experienced fatigue and other flu-like symptoms following exposures to air diving and attribute this to some form of immune system response.

Eventually, physiologists and hematologists will understand the specific underlying effects of excessive hyperbaric air. For the mean time I think its a safe bet to avoid deep air at all costs.

Regards,

Bill Mee

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How To Decompress From Trimix

I notice in the latest horror story that 32% was used up to 80/20 (strokemix) for the deco. I have already pointed out the stupidity of 80/20 in another post, so will not beat that dead horse. I will , however, drag out another old dead horse that I thought we had already given a sufficient beating, and that is the low ox is no ox bendomatic schedule.

We discovered at WKPP that when we used one ox, like anything 30-40 percent from anywhere like 90 to 120 up to the oxygen tanks, we got bent. We found that inserting the 50/50 from 70 feet through 30 made all the difference, with or without the low ox.

For bounce dive planning, like what most people do, you are better off picking the 50/50 and skipping the 32-35 if you are choosing.

Better yet is a trimix deco gas from whatever up to the nitrox.

The tables we got hammered on where Hamiltons. We told him, he fixed it, that was years ago, but now I see this stupidity has resurfaced.

The problem is that the guys who really do this are never used as a source of info for it.

Rule Number Two is "Don't Listen To Strokes".

George Irvine

A (BAKER'S) DOZEN REASONS

1) This gas was introduced in an effort to overcome the inability of unqualified student "tech" divers to control their buoyancy in open water, and is as such is yet one more concession to doing things in a convoluted fashion to offset a self- inflicted set of problems brought on by the "doing it wrong" thinking that pervades diving today.

2) A heavy sea is not a problem for a deco stop if it is not posing a lung-loading problem. Look at your depth gauge in a heavy sea and "see" for yourself what the changes are - insignificant, and if they are not, you should either not have been diving or incurring a decompression liability of this magnitude in the first place. In the event of a change in conditions during the dive, see below where the 80/20 becomes a liability rather than an asset.

3) In the interest of using a standardized set of gases for which you can permanently mark your bottles , it is a poor concession to inability to sacrifice the benefits of pure O2 to accommodate a real or perceived lack of skill - learn to dive before taking up tech diving.

4) In this same interest you will find that when you graduate to real diving, as in caves, you will not want to accelerate your PPO2 at lower depths while still being faced with a long decompression at shallower depths, and making bizarre mixes to do this is a dangerous mistake (just like the fantasy of holding an accelerated PPO2 on a rebreather throughout a deco). I am anticipating the thinking that the 80/20 crowd would then go to an additional oxygen in cave without accounting for total exposure, and subject themselves to the risk of tox in the final deco steps. Tox you do not get out of - bends you do.

5) The 80/20 mix is in fact totally useless and contraindicated as a deco gas. At thirty feet it is only a 1.52 PPO2 ( the real 1.6 PPO2 gas would be 84/16) and as such does not either provide the right oxygen window, nor does it does it work as well as pure oxygen without an inert gas at any depth. The gas mixing in your lungs has already lowered the effective PPO2 enough to prevent spiking at 20 feet anyway with the use of pure oxygen - in other words, we are dealing with a simplistic misunderstanding here, or "old wives tale" that is typical in diving.

6) If 100% oxygen is a perceived buoyancy control risk at 20 feet, then why is the same PPO2 ( intended) not a risk at 30 feet? This shows the total lack of reasonable logic involved in the decision to use this gas, as well as a lack of understanding of the whole picture ( see the rest of this discussion).

7) Along those lines, all we hear is howling about "oxygen cleaning" above 40% mixtures, and dive shop proprietors on here complaining about scuba tanks with oxygen in them being filled in their shops. With a pure oxygen system, the tank only ever gets filled with oxygen from oxygen tanks, not from every dive shop compressor it sees. Again , this shows the total inconsistency of agency thinking, and reveals that the true reason for this gas is to pretend to lower liability for teaching incompetents to dive, which is bull, and to attempt to accrue some inventive accomplishments to the dive agency pundits who themselves prove that they do no real diving by making this recommendation in the first place. This is like the colored regs, the stages on either side, the quick-release buckle, and the poodle jacket: nonsense of the most obvious nature developed through one-dimensional thinking by those whose universe of understanding is not only severely limited, but blinded by the hubris of not being the "inventor" of the techniques that work.

8) Any perceived decompression benefit of using a higher PPO2 at 30 feet with 80/20 is then given back by the lowered PPO2 at 20 feet, not to mention the fact that the presence of the inert gas in the breathing mixture defeats the purpose of using oxygen in the first place ( see the Physiology and Medicine of Diving) . The PPO2 of 80/20 at 20 feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 - useless for deco. To make matters worse, you can not get out from your 30 foot stop in an emergency ( not doing the other stops) on the 80/20 mix without really risking a type 2 hit.

9) This is a dangerous method to achieve a greater total volume of gas for the bad breathers (another obvious reason the gas is in vogue), who should not be incurring these decos, and even that benefit of having more gas is lost since it is breathed at 30 feet, and then has to last for the other stops. The fact is that gas is effectively saved by using the lower deco gas up to this point, relying on the pressure gradient to both achieve the deco and provide a break from high the previous gas's higher PPO2 prior to going to pure oxygen where the spike could be a problem on an extreme exposure without an adequate low PPO2 break ( again this shows that the 80% user is a neophyte diver with no real experience or understanding of the true risks of these dives) .

10) The 20-30% longer 30 foot time on the lower PPO2 is not only overcome on the pure oxygen at the next stops, the breaks do not come into play until the initial good dose of pure oxygen has been absorbed, since you are not spiking from a high previous dose without a break that is effectively achieved on the previous gas. These things need to be understood and taught by the agencies, not some superficial convolution that is designed to obfuscate the problem rather than openly acknowledge and deal with it in a responsible fashion.

11) In an emergency situation, getting onto the pure O2 for 20 minutes or so (for long dives something approximating the bottom time or a any decent interval) would give you a real good shot at getting out of the water having missed the rest of your deco and living through it with pain hits only. You have to think these things all the way though, not go for the transparent superficial thinking of those who merely are trying to "make their mark" with some "great" idea they can call their own. The acid test is , as always, is the caliber of the divers who adopt these practices.

12) If there is some problem with your deco or you otherwise develop symptoms and need oxygen either on the surface or back in the water, it is silly to have not had it there all along. 80/20 is a joke for that purpose, unless you have asthma, in which case any accelerated oxygen mix would be a nightmare. This is again part of the "thinking it all the way through" philosophy which is obviously missing from the 80/20 argument.

13) Only a card-carrying stroke would do something like this, and showing up with 80/20 is no different than wearing a sign on your back saying "I am a stroke, and have the papers to prove it". It announces to all the world that you have no clue, kind of like wearing clip-on suspenders or having dog dirt on your shoes.

George Irvine

Director, WKPP

"Do It Right" (or don't do it at all)

Bill Mee Wrote-

Thank you for exhaustively laying the reasons why we or anyone else should not use 80/20. The only thing missing from this discussion is the Q.E.D. at the end.

Reason #8, reiterated here for discussion purposes is perhaps the soundest reason, among many very cogent ones, as to why this practice should be avoided:

" Any perceived decompression benefit of using a higher PPO2 at 30 feet with 80/20 is then given back by the lowered PPO2 at 20 feet, not to mention the fact that the presence of the inert gas in the breathing mixture defeats the purpose of using oxygen in the first place ( see "The Physiology and Medicine of Diving") . The PPO2 of 80/20 at 20 feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 useless for deco. To make matters worse, you can not get out from your 30 foot stop in an emergency ( not doing the other stops) on the 80/20 mix without really risking a type 2 hit. "

The rush to embrace this practice, recommended by technical diving diving opinion leaders, was widespread and in retrospect, irrational and poorly thought out, like so many of the "trial balloons" in this field of endeavor. It seemed to many, at first glance, to be a simple means of increasing one's supply of deco gas while eliminating its bothersome volume and mass. In fact, the perceived benefits transform into liabilities when subjected to a thoughtful analysis. When you view decompression as a two pronged challenge: to progressively widen the oxygen window and increase the diffusion gradient to maximize passive transport of dissolved inert gas, it becomes clear that the 80/20 solution falls short on both requirements at a critical point in the decompression profile.

Section 11 emphasizes a very compelling argument for those who are concerned with managing dive related crises. When diving in the open ocean divers and boat operators should always be prepared to "scram" the deco at any time. This could be for any number of reasons, not the least of which might be a sudden change in the sea conditions or unscheduled events such as dive accidents or impending ship collisions. Just follow the Whitefish Point thread for an excellent example of why a deco may require being aborted (or never started in the first place).

Most unfortunately the "80/20 problem" bears a strikingly resemblance to several other ad hoc technical contrivances mentioned in this same article (section 7) i.e. dual BC's, colored regulators, bilateral stage bottle positioning, poodle jacketed second stages and harness quick releases. All of these ideas, while seemingly reasonable, become tainted when subjected to thoughtful review.

Good show Director Irvine.

Bill Mee