Maximum Parsimony – In Diving and the Cosmos

Image credit: Niko Lang and Booyabazooka

I admit it, my early training in physics has made me irritatingly sensitive to the principle of parsimony.

Parsimony, pronounced similarly to “alimony”, can be summed up by the following: the simplest approach to understanding nature should be considered before contemplating a more complicated line of reasoning. In a famous example, it is more probable that planets, including the Earth, orbit around the sun than the visible planets and the sun orbit around the Earth. Of course, in a different time that probability was not obvious to the common man. But then they hadn’t been thinking about parsimony.

Thank-goodness someone (Nicolaus Copernicus) did.

In the search for habitable exoplanets (planets outside of our solar system), the following statement was recently made by astronomer Steve Vogt in response to a storm of skepticism about a potentially habitable planet. “I do believe that the all-circular-orbits solution is the most defensible and credible,” he said. “For all the reasons I explain in detail … it wins on account of dynamic stability, goodness-of-fit, and the principle of parsimony (Occam’s Razor; in Latin, lex parsimoniae).”

http://www.space.com/16673-gliese-581g-habitable-planet-existence.html

William of Occam (also Ockham) was an English theologian of the 14th century. He did not invent the premise behind his razor, but he famously used it to slice through the complicated philosophies of the day and rebut them by an unfaltering demand for simplicity over complexity.

Photo credit: damianskinner.com

Medical students are taught essentially the same principle, albeit using different words: “When you hear hoof-beats, don’t think of zebras.” Wise physicians know that occasionally zebras do show themselves, but they should not be the first thought when a patient presents with unusual symptoms.

If simplicity is to be generally preferred over complexity, then an example in the diving literature comes to mind. This example annoys me to no end, but I’m slowly coming to terms with it. It is the growing popularity of referring to the respiratory effort required to breathe through a scuba regulator or a closed-circuit underwater breathing apparatus (a rebreather) as work (in joules, J) per tidal volume in liters, L.

When work in joules (J) is divided by volume (L), dimensionally the result is pressure (kiloPascals, kPa). To be exact, what is often called work of breathing in diving is actually the average pressure exerted by a person over the entire volume of a breath. The principal of parsimony says that if it is a pressure, if it has units of pressure, then we should call it a pressure (kPa)  and not something more complicated, such as Work of Breathing specified with units of J/L.

The light grey ellipsoidal area within this pressure-volume loop is equal to the work (J) of breathing for that breath.

(Examples in the regulatory diving literature correctly using Work of Breathing with units of joules can be found in early editions of NATO STANAG 1410. EN250:2000 is an example using the units of J/L for work.)

I find in my dealings with non-respiratory physiologists, that the concept of work of breathing is difficult to grasp since mathematically it involves a definite integral of pressure over a change in volume. I have made various attempts to simplify the concept, but I still find knowledgeable medical professionals misunderstanding it. In fact, mathematical integrals seem to be as frightening to most physicians as poorly dissected cadavers would be to laymen. Even engineers who certainly should grasp the intricacies of work and power end up confused.

I’m sure it adds to the confusion when some diving physiologists speak in quotients. For example, since a cubit is a length of 48 cm, and a hectare is 2.47105 acres, you could describe a person’s height as 165,400 cubic cubits/hectare. Dimensionally, that would be correct for a six foot (1.8 m) tall individual. However, most people would prefer the units of feet or meters rather than cubic cubits per hectare. Certainly, the simpler description is far more parsimonious than the former.

The shaded area within this triangle is equal to the “Work” inside the previous P-V loop. By dividing by tidal volume, you obtain the average mouth pressure on the vertical axis.

For the same reason, it makes more sense to speak of a descriptor with units of pressure as simply pressure (kPa) rather than a quotient of work per liter (Joules/L).

If describing a simple parameter like pressure as a quotient is not defensible scientifically, is it defensible psychologically?

Maybe. The U.S. Navy has used terms like “resistive effort” to convey the impression that a volume-averaged pressure is something that can be sensed by a diver. To breathe, divers have to generate a pressure in their chest, and that pressure generation requires effort.

“Effort” is admittedly not a hard-science term: it doesn’t even pretend to be. However, the use of “Work of Breathing” connotes hard science; the concept of work is pure physics. But as I have shown, the way it is increasingly used in diving is not pure physics at all. So its use is misleading in the eyes of a purist, and undoubtedly confusing to a young engineer or physicist.

But to a diver, does it matter? Does it somehow make sense? Do divers care about parsimony?

Well, I have yet to find anyone who does not intuitively understand the notion of the work involved in breathing. If they have asthma, or have tried breathing through a too long snorkel, they sense the work of breathing. So I imagine that the inexactitude of J/L is of no import to divers.

However, I also believe that the over-complication of an arguably simple concept should be just as unappealing to designers of underwater breathing apparatus as it was to William of Occam or, for that matter, the designer of the Cosmos.

 

 

 

 

 

Cold Water Scuba Regulator Testing — U.S. Navy vs. EN 250

Under thick ice in the Ross Sea, near McMurdo, Antarctica.

When scuba diving under 3-m thick polar ice with no easy access to the surface, the last thing you want to worry about is a failure of your scuba regulator, the system that provides air on demand from the aluminum or steel bottle on your back.

However, cold water regulators do fail occasionally by free-flowing, uncontrollably releasing massive amounts of the diver’s precious air supply. When they fail, the second stage regulators, the part held in a scuba diver’s mouth, is often found to be full of ice.

The U.S. Navy uses scuba in polar regions where water temperature is typically -2° C (28° F).  That water temperature is beyond cold; it is frigid. Accordingly, the Navy Experimental Diving Unit developed in 1995 a machine-based regulator testing protocol that most would consider extreme. However, that protocol has reliably reflected field diving experience in both Arctic and Antarctic diving regions, for example, in Ny-Ålesund, Svalbard, or under the Ross Sea ice near McMurdo Station.

There are currently both philosophical and quantitative differences between European standards and the U.S. Navy standard for cold water regulator testing. Regulators submitted for a European CE mark for cold water diving must pass the testing requirements specified in European Normative Standard EN 250 January 2000 and EN 250 Annex A1 of May 2006. In EN 250 the water temperature requirement for cold water testing ranges from 2° C to 4° C. Oftentimes, regulators that pass the EN 250 standard do not even come close to passing U.S. Navy testing.

An iced up, highly modified Sherwood SRB3600 Maximus second stage regulator

The Navy’s primary interest is in avoiding regulator free-flow under polar ice. The breathing effort, which is a focal point of the EN 250 standard, is of lesser importance. For instance, the 1991 Sherwood SRB3600 Maximus regulators long used by the U.S. Antarctic program have been highly modified and “detuned” to prevent free-flows. You cannot buy them off-the-shelf. Detuning means they are not as easy to breathe as stock regulators, but they also don’t lose control of air flow to the diver; at least not very often. Here is a photo of one that did lose control.

NEDU performs a survival test on regulators, and any that pass the harshest test are then tested for ease of breathing. The so-called “freeze-up” evaluation breathes the regulator on a breathing machine with warmed  (74 ±10°F; 23.3 ±5.6°C) and humidified air (simulating a diver’s exhaled breath) at 198 feet sea water (~6 bar) in 29 ± 1°F (-1.7 ± 0.6°C) water. Testing is at a moderately high ventilation rate of 62.5 L/min maintained for 30 minutes. (In my experience a typical dive duration for a dry-suit equipped diver in Antarctica is 30-40 min.)

To represent polar sea water, the test water is salted to a salinity of 35-40 parts per thousand.  The possible development of a “freeze up” of the regulator 2nd stage, indicated by a sustained flow of bubbles from the exhaust port, is determined visually.

In contrast, the European standards call for slightly, but critically, warmer temperatures, and do not specify a duration for testing at an elevated respiratory flow rate. I have watched regulators performing normally under EN 250 test conditions (4° C), but free-flowing in water temperatures approaching 0° C. Those tests were run entirely by a non-U.S. Navy test facility, by non-U.S. personnel, using a U.K. produced breathing machine, with all testing being conducted in a European country. The differences in testing temperatures made a remarkable difference.

Haakon Hop of the Norwegian Polar Institute in Ny-Ålesund, Svalbard.

The NEDU testing results have been validated during field testing by scientific diving professionals under Arctic and Antarctic ice. The same regulators that excel in the NEDU protocol, also excel in the field. Conversely, those that fail NEDU testing fare poorly under the polar ice. For instance, a Norwegian biologist and his team exclusively use Poseidon regulators for their studies of sea life inhabiting the bottom of Arctic ice.  (The hard hat in the photo is to protect cold skulls from jagged ice under the ice-pack.) Poseidon produces some of the few U.S. Navy approved cold-water regulators.

As is usual for a science diver in the U.S. Antarctic Program, a friend of mine had fully redundant regulators for his dive deep under Antarctic ice. He was fully prepared for one to fail. As he experienced both those regulator systems failing within seconds of each other, with massive free-flow, he might have been thinking of the words of Roberto “Bob” Palozzi spoken during an Arctic Diving Workshop run by the Smithsonian Scientific Diving program. Those words were: “It’s better to finish your dive before you finish your gas…”

In both NEDU’s and the Smithsonian’s experience, any regulator can fail under polar ice. However, those which have successfully passed U.S. Navy testing are very unlikely to do so.

 

A previous blog posting on the subject of Antarctic diving may also be of interest.

 

Margin of Safety

A diver’s breathing equipment, helmet, gas bottle, umbilicals and buoyancy compensator lie stretched out on the grey concrete floor.  The diving gear has a look of sadness about it. Perhaps that equipment will tell a story of why its owner is dead, but usually it does not.

Storm clouds from 30,000 ft. Photo by Wendell Hull.

In another part of the world the NTSB catalogs the fragments of an airplane shredded by the elements and thrown in a heap back to earth. The only good thing to come from an aircraft accident is that usually there are enough clues from wreckage, radio recordings, radar returns and weather reports to piece together a story of the end of life for pilot and passengers.

It’s always the question of “Why?” that drives any investigation.

Perhaps it is the knowing of how death comes, so unexpectedly to surprised souls, that makes it just a little bit easier to make the mental and emotional connection between an interesting moment and a deadly moment. If that is true, and I believe it is, then the telling of such macabre stories can be justified. It is not a telling through morbid interest, but a sincere belief that by examining death closely enough we can somehow force it to keep its distance.

That may be foolish thinking, but humankind seems to have a hunger for it, that esoteric knowledge, so perhaps it is a truism. Perhaps we sense instinctively that the knowing of something makes it less fearsome.

Being a student of diving and diving accidents, I know full well how unexpected events can make you question what is real and what is not, what is normal and what is abnormal. Without practiced calm and reasoning, unexpected events can induce panic, and underwater, panic often leads to death. That is also true for aviation.

The best preventative for panic is a realistic assessment of risk. Risks are additive. For instance, flying in the clouds is accompanied by a slight degree of risk, but with a properly maintained airplane, with a judicious use of backup instruments and power supplies, and with recent and effective training, that risk can be managed. In fact, I delight in flying in clouds; it is never boring, and I know that I am far safer than if I had been driving on two lane roads where the potential for death passes scant feet away every few seconds.

Flying at night is another risk. If something were to go terribly wrong, finding a safe place to land becomes a gamble. On the other hand, seeing and avoiding aircraft at night is simple because of the brilliant strobe lighting which festoons most aircraft. For me, the beauty, peace and calm air of night flight makes it well worth the slight risk.

Garmin NEXRAD Weather display.

Technology has made weather flying safer and, I have to admit, more enjoyable. The combination of GPS driven maps and NEXRAD weather has made it almost impossible to blunder into truly bad weather. During the daytime, my so-called eyeball radar helps to confirm visually what NEXRAD is painting in front of me. If it looks threatening, it probably is.

Unlike aircraft weather radar, virtually every pilot can afford to have NEXRAD weather in the cockpit. And unlike aviation radar, NEXRAD can see behind storms to show the view 100 miles downrange, or more. Having often flown in stormy weather without benefit of NEXRAD,  I truly rejoice in the benefits of that technology.

WX 900 Stormscope

I routinely fly with not only NEXRAD, but also a “Storm Scope” that shows me in real time where lightning is ionizing the sky. Those ozone-laced areas are off-limits to wise aviators. But sometimes even a Storm Scope is not enough to keep the willies, or as some call it, your spidey sense, from striking. (Presumably spiders are not particularly cerebral, but they are pretty adept at surviving, at least as a genus and species.)

I was recently flying around stormy weather, carefully avoiding the worst of it, and maneuvered into a position that would provide a straight shot home with yellow tints showing on the weather screen, suggesting at most light to moderate precipitation. I had flown that sort of weather many times; it usually held just enough rain to wet the windshield.

However, my internal risk computer made note of the following factors: we were in the clouds so if weather worsened I wouldn’t see it. Night was approaching which markedly darkened the wet skies we were beginning to enter.  The clouds and darkness conspired to make useless my eyeball radar. In addition, the Storm Scope was unusually ambiguous at that moment. I thought it was confirming a safe passage home, but I could not be 100% certain.

On top of that, the FAA recently warned that NEXRAD signals can be considerably more delayed than indicated on the weather display. The device might say the data is 2 min old, but the actual delay could be 10 minutes or more. In other words, the displayed image could be hiding the truth.

Aircraft weather radar.

Planes have been lost because of untimely NEXRAD data. For that reason there is a philosophical difference between NEXRAD and true radar. On board weather radar is said to be a tactical weather penetration aid, and NEXRAD is a strategic avoidance asset. My gut told me that at that moment in airspace and time the boundaries between those two uses, tactical and strategic, were getting fuzzy.

It is times like that when an awareness of the slim margin between a safe flight or dive, and a deadly flight or dive, becomes a survival tool. In this case, I and many other experienced pilots have made the call to turn around and land. Unfortunately, the record and the landscape is littered with the wreckage of those who chose otherwise.

They forgot just how thin the margin of safety can be.

The flight (green line) from Cobb County Regional (KRYY) to Panama City (KECP) was interrupted by a stop at Montgomery AL.

 

 

 

 

 

 

 

 

How Long Will Your Rebreather Scrubber Canister Last?

A U.S. Navy Mark 15 closed circuit rebreather

Are you a child of Lake Wobegon, where according to Garrison Keillor “all the women are strong, all the men are good looking, and all of the children are above average?” If you are, you may be headed for trouble with your rebreather scrubber canister.

Or expressed another way, do you know how long your scrubber canister will last?

Believe me when I tell you, it depends.

Below I explain why the above answer is necessarily evasive, and why the true answer is frustratingly elusive. Canister duration depends on things with which you, as a rebreather diver, are all too aware, and things which you may not have thought about before; namely probability and statistics.

Figure 1. CO2 concentration in canister effluent vs. time. Click for a larger image.

All of what follows is based on canister duration data for a particular rebreather of U.S. Navy interest. Data from other rebreathers are similar qualitatively, but the actual numbers may vary.

In Figure 1, the concentration of CO2 leaving the CO2 absorbent bed within a scrubber canister is plotted as a function of time for five “canister runs” for the same model rebreather. A fresh canister should absorb all the CO2  a diver exhales, leaving CO2-free gas to be inhaled by the diver on the next breath. As the absorbent becomes depleted, the scrubbing process loses efficiency and CO2 begins bypassing the canister. The amount of CO2 being inhaled by the diver begins rising exponentially, as shown in Figure 1.

For this example, canister duration tests were conducted at 70° F, at a fixed depth, with Sofnolime 812™ as the chemical absorbent, and at both a fixed minute volume of gas (representing the simulated diver’s breathing rate) passing through the canister bed, and a fixed rate of CO2 injection representing a fixed work rate and oxygen consumption. Therefore, you would expect results to be very similar from run to run, but Figure 1 shows variation in the amount of CO2 leaving the canister with time.

Figure 2. Fit of the summary data of Figure 1 to a single exponential curve. Click for a larger image.

The average data for the canister curves fit a simple exponential equation fairly well (Figure 2). We were thus justified in using an exponential equation to explore how canister duration might vary from dive to dive. Basically, the equation considered how the amount of CO2 absorbent in the canister, and the rate of CO2 production by the diver, would work together to determine the canister duration, with all else being fixed. The amount of CO2 produced depended on the rate of oxygen consumption, and from the respiratory exchange ratio which determines how much CO2 is produced for a given amount of consumed oxygen.

Fortunately we have data for those variables, in some cases coming from divers using the same rebreather as shown in Figure 1. We have estimates of oxygen consumed during prolonged swims. Most importantly, we have measures of the variability associated with all that data. For instance, Figure 3 shows the bell shaped curve for oxygen consumption data measured by an NEDU researcher during distance swims by Navy divers. We deduced the curve for this exercise from the reported statistics (mean or average, and standard deviation). Similar curves were obtained for the other factors that influence canister duration, except for water temperature. That was assumed constant.
Figure 3. Oxygen consumption bell curve.

We then treated all the known factors and their known variability to a mathematical process called Propagation of Error (H.H. Ku, Notes on the Use of Propagation of Error Formulas, Journal of Res. of the Nat. Bur. Stds., 1966.)

The result was Figure 4 which requires careful study to appreciate what it’s telling us.

If everything about a diver and his diving equipment were “average” then their UBA canister might be expected to follow the white canister breakthrough curve on the far right, identified as P = 0.500. Since that curve represents an average, fifty percent of canisters would be expected to last longer than that curve (fall to the right of the curve) and fifty percent would be expected to fall to the left of it; i.e., to last the same or shorter amount of time. Approximately 16% of the canister breakthrough curves would be expected to fall to the left of the black line identified as P = 0.159, and 2.3% would fall on or to the left of the yellow line (P = 0.023).

Figure 4. Results from the application of propagation of error formulas.Click to enlarge.

Now comes food for thought. What if, as Garrison Keillor says, you’re a child from Lake Wobegon, and are above average in your oxygen consumption? If your dive lasted to the point where the average canister broke through at 0.5% CO2 (about 255 min, white curve intersection with the horizontal blue-green line), then you might be seeing a dangerously high inspired CO2 of 3-4% (vertical blue-green line), depending on how far from average you are.

If you chose to dive for the average time for a canister to reach 2% CO2 (magenta lines), then your actual inspired CO2 could be 7 to 12%, an extremely dangerous CO2 exposure as described in a preceding post.

Keep in mind that in this particular example water temperature was constant. If you dive in a variety of water temperatures your canister duration will vary even more. If your work rate changes widely over the course of a dive, then the canister duration will be essentially unpredictable.

So regarding how long your canister will last on any given dive: Are you feeling lucky?

 

 

 

 

 

 

 

This material was presented by JR Clarke and DE Warkander in a 2001 meeting of the Undersea and Hyperbaric Medical Society. Undersea and Hyperbaric Medicine, 28:81, suppl., 2001.

 

I See Dead People – Sort Of

The exit to the Morrison Springs cave. (photo credit: ZoCrowes255)

The young man in a swimming suit was lying lifeless at the bottom of a fissure on the floor of Morrison Springs, a popular underwater cave in Walton County, Florida. If his eyes had been open, he would have been staring straight up at me. But mercifully, his eyes were shut, as in sleep.

My diving buddies from the Georgia Tech Aquajackets dive club and I were breathing air from scuba tanks at about 110 feet sea water. We were in a portion of the cave that received no indirect light from the cave opening. Without the cave lights in many of the diver’s hands there would have been total darkness.

Who knew that on my second so-called “open water” dive I would find myself deeper than 100 feet in a cave, using the dispersed light from my buddies’ dive lights to examine a very fresh looking corpse? He looked to be about our age, late teens, high school or college age. A rock outcropping hid his body from about mid-hip level down. But the top portion of a bathing suit, his lean stomach, chest, and boyish-looking face and head was plainly visible.

There must have been some current at the bottom of the crevice because his brown hair was waving gently, being the only sign of motion from the deathly pale white boy with closed eyes, waiting patiently to be recovered to the surface.

I and the other divers stretched our arms and shoulders as far into the crevice as we dared, reaching towards the young man, hoping we could grab onto some part of his body. But it was futile – he was at least a foot out of our reach. Finally, checking our dive watches, we saw it was time to swim toward the cave entrance and start our ascent.

Since there was no scuba gear on him he must have been a free-diver, a breath-hold diver who entered the cave then passed out and sank to the deepest, most inaccessible portion of the cave. As I and the other divers rose along the limestone borders of the cave I watched the darkness surround the young man’s cold body once again. I felt lonely, almost as if I could feel his spirit’s loneliness.

As I reached the surface I turned to the closest diver, removed my regulator from my mouth, and panted, “How are we going to recover that body?”

His response stunned me.

“What body? That was no body – that was a Navy 6-cell flashlight!

How could it be? I would have signed a sworn affidavit to the police describing everything I had seen, in detail, just as I’ve reported it to you many years later. The visual details, the textures, the emotions will not leave me.

But they were not real.

As for why that happened, the only thing I can assume is that for a nineteen-year old novice diver, descending in the dark to 110 feet, in a cave, might be just a bit more than the diver’s mind is prepared for. The nitrogen in air is narcotic if found in high enough concentration, so I was undoubtedly suffering from nitrogen narcosis. Plus, at the time the entrance to the Spring was macabre, with a large photo of a diver with his back filleted open by a boat propeller, and signs prominently displaying warnings of the large number of fatalities in the cave from poorly trained and equipped divers exceeding their limits.

My mind was prepared to witness tragedy, and the normally mild nitrogen narcosis of 110 feet may have  been just the trigger needed for a vivid hallucination.

I have had no hallucinations since then, from diving or anything else, except for one medical procedure reported on in this blog. But what remains remarkable to me was my absolute conviction that what I had seen in that cave was real. Consequently, I now know very well  that what people testify as being real, whether they are diving or not, may in fact be only imagined.

How Much is Too Much? (Carbon Dioxide – The Diver’s Nemesis)

The amount of carbon dioxide (CO2) that can be safely inhaled by rebreather divers is a continuing point of conjecture, and vigorous argument. Unfortunately, the U.S. Navy  Experimental Diving Unit has confused that issue, until recently.

A non-diver might wonder why a diver should inhale any CO2. After all, the air we breathe contains only a small fraction of CO2 (0.039%). A rebreather is best known for emitting no bubbles, or at most very few bubbles depending on the type of rebreather. It does that by recirculating the diver’s breath, adding oxygen to make up for oxygen consumed by the diver, and absorbing the carbon dioxide produced by the diver. The CO2 scrubber canister is vital to keeping the diver alive. As pointed out in the first post in this series, carbon dioxide is toxic; it can kill.

A CO2 scrubber  keeps the recirculating CO2 levels low by chemically absorbing exhaled CO2. However, the scrubber has a finite lifetime – it can only absorb so much CO2. Once its capacity has been exceeded, CO passing through the canister accumulates exponentially as the diver continues to produce CO2 from his respiration.

The question rebreather divers want answered is, “How much of that bypassed CO2 can I tolerate?” As we’ve discussed in previous posts, 30% CO2 can incapacitate you within a few breaths. I can personally verify that if you’re exercising you may not notice the effect 7% CO2 has on you, until you try to do something requiring coordination. I’d equate it to the effect of drinking too many beers. There is little controversy about CO2 levels of 5-7% being bad for a diver.

For levels below 5-7% CO2, the U.S. Navy has not been real clear. For instance, 2% CO2 is the maximum CO2 allowed in diving helmets. If CO2 were to climb higher the diver would most likely feel a need to ventilate the helmet by briefly turning up the fresh gas supply to clear CO2.

Since at least 1981, NEDU has defined the scrubber canister breakthrough point in rebreathers as 0.5% CO2. That means that at some point, which varies with CO2 injection rate, ventilation rate, water temperature, and grain size of CO2 absorbent, CO2 begins leaking past the canister, not being fully absorbed during its passage through the canister. Once that leakage starts, the amount of CO2 entering the diver’s inspired breath rises at an ever increasing rate unless work rate or other variables change. By the time the CO2 leaving the canister has reached 0.5%, the canister has unequivocally “broken through”.

I pointed out in my last post that even 0% inspired CO2 may be too much for some divers when they are facing resistance to breathing. And all rebreathers are more difficult to breathe than other types of underwater breathing apparatus because the diver has to force his breath through the rig’s scrubber canister and associated hoses. The deeper the dive the denser the breathing gas and the worse breathing resistance becomes.

In free-flow diving helmets like the old MK 5, and the short-lived MK 12, the diver did not breathe through hoses and scrubber canisters. But those helmets had a high dead space and to keep helmet CO2 at tolerable levels a fresh gas flow of 6 actual cubic feet per minute (acfm; 170 liters per minute) was required. The U.S. Navy allowed up to 2% CO2 in the helmet because 1) the helmets did not have a high work of breathing and 2) due to simple physics the helmet CO2 couldn’t be kept very low.

For rebreathers, none of the above apply. A high breathing resistance is inevitable, at least compared to free-flow helmets, and once CO2 starts rising there is nothing you can do to decrease it again, short of stopping work.

In 2000, NEDU’s M. Knafelc published a literature review espousing that the same limit for inspired CO2 which applies in helmets could be used in rebreathers. Nevertheless, in 2010 NEDU’s D. Warkander and B. Shykoff clearly demonstrated that in the face of rising inspired CO2 concentrations work performance is reduced, and blood levels of CO2 rise, in some cases to dangerous levels. More recent work by the Warkander and Shykoff duo have extended those studies into submersion, however those reports are not yet publicly available.

As a result of both physiological theory and confirmatory data in young, physically-fit experimental divers, NEDU has not relaxed the existing definitions of scrubber canister breakthrough, 0.5% PCO2. Furthermore NEDU will adhere to the current practice of using statistical prediction methods to define published canister durations, methods which are designed to keep the odds of a diver’s rebreather canister “breaking through” to no more than 2.5%, comparable to the odds of decompression sickness following Navy multi-level dive tables. Details of this procedure will be explained in later postings.

 

Knock Yourself Out (Carbon Dioxide – The Diver’s Nemesis)

Most rebreather divers start off their diving career with open-circuit diving; that is, with scuba. And some of them pick up bad habits. I happen to be one of those divers.

With scuba you start the dive with a very limited amount of air in your scuba bottle. New divers are typically anxious, breathe harder than they have to, and blow through their air supply fairly quickly. More experienced divers are relaxed and enjoy the dive without anxiety, and thus their air bottles last longer than they do with novice divers.

So early in a diver’s experience he comes to associate air conservation with a sign of diver experience and maturity. When you are relaxed and physically fit, and your swimming is efficient, your breathing may become extraordinarily slow. Some call it skip breathing — holding your breath between inhalations.

I was once swimming among the ruins of Herod’s Port in Caesarea, and my dive buddy was a Navy SEAL. I started the dive under-weighted, so I picked up a 2000 year old piece of rubble and carried it around with me as ballast. In spite of the very inefficient style of swimming which resulted, my air supply still lasted longer than that of my SEAL buddy.

At first I was annoyed that I had to end the dive prematurely, but then I began to feel somewhat smug. I had used less air than a frogman.

As a physiologist I knew that I may well have been unconsciously skip breathing, which would have raised my arterial carbon dioxide level, potentially to a dangerous level. But all ended well, and I could not help being glad that I was not the one to call the dive.

It is important for rebreather divers to understand that they don’t have to be breathing elevated levels of carbon dioxide to run into physiological problems with carbon dioxide. It’s the carbon dioxide in your arterial blood that matters. It can render you unconscious even when you’re breathing gas with no carbon dioxide at all.

MK 16 rebreather diver

Normally the body automatically ensures that as you work harder, and produce more carbon dioxide in your blood stream, that you breathe more, forcing that CO2 out of your blood, into the lungs, and out through your mouth. It works like an air conditioner thermostat; the hotter it gets in the house, the more heat is pumped outside. In other words, arterial and alveolar CO2 levels are controlled by automatic changes in ventilation (breathing.) In fact you can predict alveolar levels of CO2 by taking the rate at which CO2 is being produced by the body and dividing it by the ventilation rate. This relationship is called the Alveolar Ventilation Equation, or in clinical circles, the PCO2 Equation.

Normally, CO2 production and ventilation is tightly controlled so that normal alveolar and arterial CO2 is about 40 mmHg, mmHg being a unit of so-called partial pressure. 40 mmHg of arterial CO2 is safe. [One standard atmosphere of pressure is 760 mmHg, so ignoring the partial pressure of water vapor and other gases, a partial pressure of 40 mmHg of CO2 is equivalent to exhaling about 5% carbon dioxide.]  

When a diver is working hard while breathing through a breathing resistance like a rebreather, as ventilation increases respiratory discomfort goes up as well. For most people, when the respiratory discomfort gets too high, they quit working and take a”breather”. But there are some divers who hate respiratory discomfort, and don’t mind high levels of arterial CO2. We call these people CO2 retainers.
Navy experimental deep sea divers; photo credit: Frank Stout

As an example, I once had as an experimental subject a physically fit Navy diver at the Naval Medical Research Institute during a study of respiratory loading. The test was conducted in a dry hyperbaric chamber under the same pressure as that at 300 feet of sea water. The experimental setup in the chamber looked somewhat like that in the figure to the right although the diver I’m talking about is not in this photo.

The diver was exercising on the bicycle ergometer while breathing through a controlled respiratory resistance at 300 feet in a helium atmosphere. The diver quickly learned that by double breathing, starting an inspiration, stopping it, then restarting, he could confuse the circuitry controlling the test equipment, thus eliminating  the high respiratory loading.

As he played these breathing pattern games my technician was monitoring a mass spectrometer which was telling us how high his expired CO2 concentration was going. The exhaled CO2 started creeping up, and I warned him that he needed to cut out the tricky breathing or I’d have to abort the run.

The clever but manipulative diver would obey my command for a minute or so, and then go back to his erratic breathing. He joked about how he was tricking the experiment and how he felt fine in spite of the high CO2 readings.

That was a mistake.

When you’re talking, you’re not breathing. Since his breathing was already marginal, his end-tidal CO2, an estimate of alveolar CO2, shot up in a matter of seconds from 60 to 70 and then 90 mmHg, over twice what it should have been. When my technician told me the diver’s exhaled CO2 was at 90 mmHg, I yelled “Abort the run”. But the diver never heard that command. He was already unconscious and falling off the bike on his way to the hard metal decking inside the hyperbaric chamber.

The diver thought he was tricking the experiment, but in fact he was tricking himself. Although he felt comfortable skip breathing, he was rapidly pedaling towards a hard lesson in the toxicity of carbon dioxide.

Keep in mind, this diver was breathing virtually no carbon dioxide. His body was producing it because of his high work level, and he was simply not breathing enough to remove it from his body.

In upcoming posts we’ll look at what happens when inspired CO2 starts to rise, for instance due to the failure of a carbon dioxide scrubber canister in a rebreather. I already gave you one example in the CO2 rebreathing study of my first post in this series. There’s lots more to come.

 

 

 

 

 

 

 

 

 

 

 

 

 

Carbon Dioxide – The Diver’s Nemesis Pt. 1 (Meduna’s Mixture)

Of all the gases humans excrete, the most bountiful, and arguably the most deadly, is exhaled carbon dioxide.

There is a forgotten bit of American medical history that reveals the bizarre features of the toxicity of carbon dioxide. In 1926, before the advent of modern psychiatric medications, some American psychiatrists began experimenting with the use of inhaled carbon dioxide for the treatment of schizophrenia and psychoses. At the time, there were no effective treatments other than electroshock.

Dr Ladislas J. Meduna

One of the most successful of these researchers was Dr Ladislas J. Meduna, a Professor of Psychiatry at the University of Illinois College of Medicine in Chicago.

High levels of carbon dioxide (CO2) did in fact have some success in treating schizophrenia, but it also produced Out of Body (OBE) and seemingly spiritual experiences. The following text is quoted from a book called Carbon Dioxide Therapy. A Neurophysiological Treatment of Nervous Disorders, published in 1950 and authored by Meduna.Meduna administered by mask between 20 and 30 breaths of a gas mixture of 30% CO2, 70% O2. From pg. 22 of his book we find,

“Any attempt to define the sensory phenomena during CO2 anesthesia, in terms of dream, hallucination, illusions, etc., would be futile. The actual material would support any hypothesis. Some of the sensory phenomena would direct us to define them as hallucinations. Some of these phenomena are felt by the patients as “real dreams”; others obviously are dreamy repetitions of real events in the past or of past dreams. I believe therefore that any classification of these phenomena in terms of dream or hallucination would be not only meaningless, but directly misleading; the patient is not “sleeping” in the physiological sense, nor is he in the state of consciousness which we usually assume to be present in true hypnagogic hallucinations.”

click to enlarge

“One subject, after 20 respirations of the gas, reported seeing a “bright light, like the sun.”

“It was a wonderful feeling. It was marvelous. I felt very light and didn’t know where I was. For a moment I thought: ‘Now isn’t that funny. I am right here and I don’t know whether I am dreaming or not.’ And then I thought that something was happening to me. This wasn’t at night. I was not dreaming. And then it felt as if there were a space of time when I knew something had happened to me and I wasn’t sure what it was. And then I felt a wonderful feeling as if I was out in space.”

“After the second breath” — reported a 29 year-old healthy female nurse who had taken a treatment – “came an onrush of color… then the colors left and I felt myself being separated; my soul drawing apart from the physical being, was drawn upward seemingly to leave the earth and to go upward where it reached a greater Spirit with Whom there was a communion, producing a  remarkable, new relaxation and deep security. Through this communion I seemed to receive assurance that the petite problems or whatever was bothering the human being that was me huddled down on the earth, would work out all right and that I had no need to worry.”

“In this spirituelle I felt the Greater Spirit even smiling indulgently upon me in my vain little efforts to carry on by myself and I pressed close the warmth and tender strength and felt assurance of enough power to overcome whatever lay ahead for me as a human being.”

Meduna summarized that preceding case by stating, “In this beautiful experience we can discern almost all the constants of the CO2 experience: (1) color; (2) geometric patterns; (3) movement; (4) doubleness of personality; and (5) divination or feelings of esoteric importance.”

Meduna went on to admit that “Not all of the sensory phenomena experienced by the patients are of celestial beauty and serenity. Some of them are horrifying beyond description.”

In 1971, Chris Lambertsen, M.D., Ph.D., from the University of Pennsylvania School of Medicine, and considered to be the father of special warfare diving by Navy SEALS, published a careful examination of the physiological consequences of the Meduna mixture. He found that inhalation of 30% CO2 in oxygen would cause unconsciousness and convulsions within 1-3 min. The precipitating event for loss of consciousness seemed to be a catastrophic increase in the acidity of the blood due to the large amount of carbonic acid produced by the CO2 inhalation. This raises the possibility that the experiences noted by Meduna were caused by pre-convulsive events within the brain.

Since then the medical community has deemed carbon dioxide “treatments” as not only dangerous but ineffective compared to modern psychiatric medication. Meduna’s mixture is no longer used.

While at the Naval Medical Research Institute, I was my own research subject in a study of the effects of rebreathing  CO2 concentrations up to 8%. That was a carbon dioxide concentration that some Navy SEALS had claimed could be tolerated without impairment.

The simplest scrubber canister in the simplest rebreather, Ocenco M20.2

I was not under water, but riding a stationary bicycle ergometer in the laboratory, simulating breathing on a closed-circuit underwater breathing apparatus (in diving vernacular, a rebreather.) Although oxygen was being added as I consumed it, there was no carbon dioxide scrubber (a container of carbon dioxide absorbing material), so the test was examining what happens when a scrubber canister is no longer functioning properly. At 7% inspired  CO2 I stopped the exercise, feeling a little abnormal. However, I was surprised at how unimpaired I seemed to be; that was, until I attempted to dismount the ergometer. I almost fell and needed help removing myself from the bicycle to a chair.

The single-minded and simple-minded task of exercising had hidden a growing central nervous system impairment. Like someone intoxicated with alcohol, I could not judge my level of impairment until a task requiring some coordination was required.

So we see that high levels of carbon dioxide intoxication can lead to profound disturbances of the central nervous system. In upcoming posts we’ll see how elevated carbon dioxide levels and the control of respiratory ventilation can interact to put rebreather divers at risk.

Much of the above is from a nonfiction book project currently under review. The working title for the book is “Collected Tales of the Spiritual and Paranormal.”

 

Six-Degrees of Freedom

Photo credit Paul Burger, Houston

I’ve had an epiphany of sorts.

I was flying with friends as night was falling. We were over a mile up, the air was clear and still, not a bump to be found. City lights and major roads could be seen from over 45 miles away. We seemed to be suspended in space, with only the movement of lights sliding below our wings betraying the fact that we were traveling at 145 knots over the ground.

The fellow sitting in the seat to my right seemed interested in taking the controls, something he had never done before. I first let him handle the yoke. With the autopilot holding track so we wouldn’t get too far off course I let him see how the elevator worked to raise and lower the nose, controlling pitch. Then as I turned the autopilot off completely, I had him experiment with the rudder pedals to see how that affected the aircraft. They made the plane yaw to the left and right. Next I showed him how the ailerons on the wings work with the rudder on the tail to smooth out turns by applying roll simultaneously with yaw. That created a coordinated turn which is the most efficient and comfortable way to change direction in the air.

He was getting a mini-lesson in flying, and doing quite well for a novice.

Then I told him to point the nose of our bird towards a light on the horizon that would keep us headed in the right direction, towards our home base some 90 nm away.

He had the plane swaying slightly from side to side, but I did not interfere or correct him. Now that I think about it, he may have been doing it deliberately as he learned how the ailerons and rudders work in unison. And then he said something interesting: “It’s six-degrees of freedom.”

Granted, my friend is a mechanical engineer, and in his student days he had done a project with wind tunnels and model airplanes. That was where he gained both academic and practical experience about the six-degrees of freedom in aviation.

Image credit: Horia Ionescu, Wikipedia Commons

The six degrees involve three degrees of translation, and three of rotation. In the following illustrations, aside from the three rotational axes commonly applied to aircraft, roll, pitch and yaw, the other three axes are also shown. In a ship, motion in those translational axes are called heave, sway, and surge. In an aircraft they have less colorful terms; motion fore and aft, left and right (port and starboard), and up and down. The figure to the right shows all six degrees of freedom irrespective of the craft or method of motion.

Illustration by S. W. Halpern

 

For me, the  epiphany was the realization that my favorite things on earth (or slightly above it) involve six-degrees of freedom. Physically, there can be no greater freedom, and that freedom is found in flying and diving. No wonder I love them.

Birds live in that six-degree of freedom world, and perhaps that’s why we envy them. While we may not envy fish, per se, perhaps it is the six-degrees of freedom that lures so many of us to diving underwater. I well remember the first time I glided over a vertical precipice in crystal-clear water and realized with supreme pleasure that the laws of physics no longer compelled me to tumble over that precipice. Even now, quite a few years later, I still enjoy diving in the Florida Panhandle Springs, and finning directly over a rock face that drops vertically towards a sand bottom some 25 or so feet below. I’ll float over it, looking down, then bend at the waist and glide effortlessly to the bottom.

This is the stuff of flying dreams, of which I am also enamored.

A soul floating in space prior to incarnation, an embryo floating in utero prior to implantation; these are ways we might have once had the same freedom of motion. But soon after becoming a fetus we lose that freedom. There is no where else that freedom of motion can be experienced in a sustained manner than by  flying and diving.

Photo credit: Mass Communication Specialist 1st Class Jayme Pastoric

 

The following video is the best example I’ve found to demonstrate the true meaning of six degrees of freedom. Go to full screen, high def, volume up, and enjoy! (Disclaimer: I have no connection to the featured company or equipment used in the making of this video.)

Children of the Middle Waters

Children of the Middle Waters (working title) is a science fiction/thriller that has been completed and is being submitted today for consideration by Tom Doherty Associates, New York. My friend and mentor, the writer Max McCoy, has provided literary criticism and encouragement for the manuscript. Max, who works primarily in the Western genre, wrote a diving-related thriller called The Moon Pool, which happens to involve in its closing chapter the Navy Experimental Diving Unit, and someone a lot like me.

Below is a blurb briefly describing Children of the Middle Waters.

In the deep-sea canyons and trenches of the Earth lie thousands of alien spacecraft and millions of their inhabitants who have to leave soon or risk being stranded forever, or being destroyed. Due to their physiology they have been unable to directly contact humans, but they are adroit at mental contact and remote viewing, when it suits them.

They need the help of two humans to assure their safe escape, an experienced Navy scientist and a beguiling graduate student.  But introductions through mental means are slow and suspect, as you might imagine.

The U.S. government is well aware of this deep sea civilization, and is desirous of the weapons the visitors possess, which puts the two unsuspecting scientists in the middle of a conflict between powerful
military forces and powerful intergalactic forces. Things could get messy.

Even worse, jealous friends turn on the unlikely duo and put their lives at risk.

Children combines two separate Native American beliefs and legends with current events. It is a complex thriller with science fact and science fiction mixed in with military action and government intrigue. Also revealed are romantic possibilities that far exceed the capabilities of the mundane, everyday world.

Early American Indian beliefs create an ending for this story that no one could anticipate. It is an ending that causes the protagonist to realize everything he has held dear is wrong, in one way or another. At the same time he discovers a reality that is the greatest blessing that man can receive.