Diving | John Clarke Online

Does Your Rebreather Scrubber Operate in Its Goldilocks Zone?

gliese581d — Exoplanet Gliese 581d, orbiting the red-dwarf star Gliese 581, only 20 light-years away. (The existence of this planet is currently in dispute.)

In space, there is a so-called Goldilocks zone for exoplanet habitability. Too close to a star, and the planet is too hot for life. Too far from its star, and the planet is too cold for life, at least as we understand biological life, life dependent on water remaining in a liquid state. Earth is clearly in the Goldilocks zone, and so is a purported planet Gleise 581d, from another solar system.

Carbon dioxide absorbing “scrubber” canisters in rebreathers have similar requirements for sustaining their absorption reactions. If it’s too hot, the water necessary for the absorption reaction is driven off. Too cold and the water cannot fully participate in the absorption reactions.

Those with some knowledge of chemistry recognize that cold retards chemical reactions and heat accelerates them. But that does not necessarily apply to reactions where a critical amount of water is required. Water thus becomes the critical link to the reaction process, and so maintaining scrubber temperature within a relatively narrow “Goldilocks” zone is important, just as it is for life on distant planets.

Temperature within a scrubber canister is a balance of competing factors. Heat is produced by the absorption of CO2 and it’s conversion from gas to solid phase, specifically calcium carbonate. A canister is roughly 20°C or more warmer than the surrounding inlet gas temperature due to the heat-generating (exothermic) chemical reactions occurring within it.

Heat is lost from a warm canister through two heat transfer processes; conduction and convection. Conduction is the flow of heat through materials, from hot to cold. Hot sodalime granules have their heat conducted to adjacent cooler granules, and when encountering the warm walls of the canister, heat passes through the canister walls, and on to the surrounding cold water.

You can think of this conduction as water flowing downhill, down a gravity gradient. But in this case, the downhill is a temperature gradient, from hot to cold. If the outside of the canister was hotter than the inside, heat would flow in the opposite direction, into the canister.

Copper is a better conductor of heat than iron (it has a higher thermal conductivity), explaining why copper skillets are popular for cooking on stoves. Air is a poor conductor of heat, explaining why neoprene rubber wet suits, filled with air bubbles, are good insulators. Air-filled dry suits are an even better insulator.

Capturecan1 — Chemical absorption reactions heat an otherwise cold canister (yellow is hot, red is warm, black is cold.) (Copyright John R. Clarke, 2014).

Convection is the transfer of heat to a flowing medium, in this case gas. You experience convective cooling when you’re working hard, generating body heat, and a cool dry breeze passes over your skin. Convective cooling can, under those circumstances, be delightful.

When you walk outside on a cold, windy day, convective cooling can be your worst enemy. Meteorologists call it wind chill.

There is wind chill within a canister, caused by the flow of a diver’s exhaled breath through the canister. In cold water the diver’s exhaled breath leaves the body quite warm, but is chilled to water temperature by the time it reaches the canister. Heat is lost through uninsulated breathing hoses exposed to the surrounding water.

As you might expect, if the canister is hot, that convective wind chill can help cool it. If the canister is cold, then the so-called wind chill will chill it even more.

Capturecan2 — Copyright John R. Clarke, 2014.

The amount of heat transferred from a solid object to gas is determined by three primary variables; the flow rate of the gas, the density of the gas, and the gas’s heat capacity. Heat capacity is a measure of the amount of heat required to raise the temperature of a set mass of gas by 1° Celsius.

Both the heat capacity and density of the gas circulating through a rebreather changes not only with depth (gas density), but with the gas mixture (oxygen plus an inert diluent such as nitrogen or helium). The heat capacity of nitrogen, helium and oxygen differ, and the ratio of oxygen and inert gas varies with depth to prevent oxygen toxicity. Nitrogen and helium concentrations vary as well, as the diver attempts to avoid nitrogen narcosis.

Q is heat transferred by convection, and the terms on the right are, in sequence, diver ventilation rate, gas density, heat capacity of the inspired gas mixture at constant pressure, and the difference in temperature between the absorbent and environmental temperature.

The interaction of all these variables can be complex, but I’ve worked a few examples relevant to rebreather diving. The assumptions are a low work rate: ventilation is 22 liters per minute, water temperature is 50°F (10°C), oxygen partial pressure is 1.3 atmospheres, and dive depths of 100, 200 and 300 feet sea water. The average canister temperature is assumed to be 20°C (68°F) above water temperature, a realistic value found in tests of scrubber canister temperatures by the U.S. Navy.

The heat capacities for mixtures of diving gases come from mixture equations, and for the conditions we’re examining are given in the U.S. Navy Diving Gas Manual. (This seems to be a hard document to obtain.)

At 100 fsw, the heat transfer (Q) for a nitrogen-oxygen (nitrox) gas mixture is 34.2 Watts (W). For a helium-oxygen mixture (heliox), Q is 27.4 W. At 200 fsw, Q for nitrox is 59.9 W, and for heliox Q is 50.3 W. At 300 fsw, Q for nitrox gas mixture is 85.5 W, and for heliox, is 59.9 W.

Interestingly, the heat transferred from the absorbent bed to the circulating gas is the same at 300 fsw with heliox as it is at 200 fsw with nitrox.

Photo courtesy of David L. Conlin, Ph.D., Chief – National Parks Service Submerged Resources Center. Photo by Brett Seymour, NPS.

Dr. Jolie Bookspan briefly mentioned the fact that helium removes less heat from a diver’s airways than does air in her short article on “The 36 Most Common Myths of Diving Physiology” (see myth no. 20). Conveniently, heat exchange equations apply just as well to inanimate objects like scrubber canisters as they do to the human respiratory system.

From these types of heat transfer calculations it is easy to see that for a given depth, work rate and oxygen set point, it is better to use a heliox mixture than a nitrox mixture if you’re in cold water. That may sound counterintuitive considering helium’s high thermal conductivity, but the simple fact is, the helium background gas with its low density carries away less heat from the canister, and thereby keeps the canister warmer, than a nitrox mixture does. The result is that canister durations are longer in cold water if less heat is carried away.

In warm water, the opposite would be true. Enhanced canister cooling with nitrox would benefit the canister.

An earlier post on the effect of depth on canister durations raised the question of whether depth impedes canister performance. The notion that increased numbers of inert gas molecules block CO2 from reaching granule absorption sites has little chemical kinetic credence. However, changing thermal effects on canisters with depth or changing gas mixtures does indeed affect canister durations.

I’ve just given you yet another reason why helium is a good gas for rebreather diving, at least in cold water. Unfortunately, these general principles have to be reconciled with the specific cooling properties of all the rebreather canisters in current use. In other words, your canister mileage may vary. But it does look like the current simple notions of depth effects are a bit too simplistic.

How Cold Can Scuba Regulators Become?

The Arctic science diving season is in full swing (late May). Starting in September and October, the Austral spring will reach Antarctica and science diving will resume there as well.

Virtually all polar diving is done by open-circuit diving, usually with the use of scuba.

As has often been reported, regulator free flow and freeze up is an operational hazard for polar divers. However, even locations in the Great Lakes and Canada, reachable by recreational, police and public safety divers, can reach excruciatingly cold temperatures in both salt and fresh water on the bottom.

Decades ago a reputed Canadian study measured temperatures in a scuba regulator, and found that as long as water temperature was 38° F or above, temperatures within the second stage remained above zero.

Recent measurements made on modern high-flow regulators at the U.S. Navy Experimental Diving Unit show that the thermal picture of cold-water diving is far more complex than was understood from the earlier studies.

NEDU instrumented a Sherwood Maximus regulator first and second stage with fast time response thermistors. The regulators were then submerged in 42°, 38°, and 34° F fresh water, and 29° F salt water, and ventilated at a heavy breathing rate (62.5 liters per minute), simulating a hard working diver.

In the following traces, the white traces are temperatures measured within the first stage regulator after depressurization from bottle pressure to intermediate pressure. That site produces the lowest temperatures due to adiabatic expansion. The red tracing was obtained at the inlet to the second stage regulator. The blue tracing was from a thermistor placed at the outlet of the “barrel” valve within the second stage regulator box. Theoretically, that site is exposed to the lowest temperatures within the second stage due to adiabatic expansion from intermediate pressure to ambient or mouth pressure.

Regulators were dived to 198 ft (60.4 meters) and breathed with warm humidified air for 30-minutes at the 62.5 L/min ventilation rate. The regulator was then brought to the surface at a normal ascent rate.

To make the breathing wave forms more distinct, only one minute of the 30-minute bottom time is shown in the following traces, starting at ten minutes.

The first two tracings were at a water temperature of 42° F. In the first tracing, bottle pressure was 2500 psi, and in the second, bottle pressure was 1500 psi. (For all of these photos, click the photo for a larger view.)

Color coding of thermistor locations, all internal to the regulator.

When bottle pressure was reduced from 2500 psi to 1500 psi, all measured temperatures increased. The temperature at the entrance to the second stage oscillated between 0° and 1°C. At 2500 psi that same location had -1 to -2°C temperature readings.

The next two tracings were taken in 29° F salt water. The coldest temperatures of the test series were in 29° F water with 2500 psi bottle pressure.

As a reminder, 32°F is 0°C, -22° C is equal to -7.6° F, and -11°C is 12.2°F. At a bottle pressure of 2500 psi, the temperature inside the second stage (blue tracing) never came close to 0° C. So we’re talking serious cold here. No wonder regulators can freeze.

This material was presented in condensed form at TekDiveUSA 2014, Miami. (#TekDiveUSA)

Nightmarish Thoughts of Being Eaten

There is a downside to situational awareness.

I discovered this fact while 868 miles north of the Arctic circle, 600 miles south of the North Pole. It took place in Ny-Ålesund, Svalbard, a part of the well-known island Spitsbergen.

I was helping the Smithsonian Institution train divers in polar diving. My job was to teach them about scuba regulator performance in frigid water.

A fact of life in Ny-Ålesund, the most northern continuously occupied settlement, a research village, is that Polar Bears are always a threat. In fact, one came through town during our visit to Svalbard. The Greenland sled dogs, tied down outside, were understandably, and quite noisily, upset. The bear walked right past them.

After the excitement of that nighttime polar bear prowl had begun to wane, the incident remained as a not so subtle reminder during seemingly routine activities. For you see, polar bears are emotionless killers; to them, we are prey. Tracking and eating a human gives it no more pause than us picking blackberries alongside the road. For adult polar bears, humans are simply a conveniently-sized food item, not nearly so fast and wily as their typically more available meals, seals.

Unlike the ploy of divers bumping potentially predatory sharks on the nose to dissuade them from biting, bumps on the nose don’t work with polar bears. Without a gun by your side, a walk in Svalbard is a walk on the wild side, and not in a good way.

I was observing and photographing boat-based diving operations from the end of a long pier jutting 375 feet (115 m) into the Kongsfjorden. Normally in March the fjord is ice covered, but the year I was there (2007) there was no ice to be seen except at the nearby glacier.

I had been standing at the pier’s end for a while taking photographs, and soaking up the polar ambiance, when I looked back and realized that from a safety standpoint, I was vulnerable. That is when situational awareness began to kick in.

We were in a deserted, industrial portion of the town. The old coal mining operations were shut down long ago. Other than the divers on and in the water, I was the only one around. And I was stuck out on the end of a very long pier, with no means of escape.

If an intruding and hungry bear made its appearance at the land side of the pier, I would be trapped. Although I was dressed for cold, I was not dressed for cold water. That water was, after all, ice water. Polar bears, on the other hand, are excellent swimmers in polar water. So after I’d jumped into the water, which I would have if faced with no alternative, it would have taken the bear only a few furry strokes before he would have me. While he or she would find my body parts chilled on the outside, my internals would still be pleasantly warm as they slid down its gullet.

Being a sensible person, I called the boat drivers over and put them on alert; should a polar bear appear at the far, land-side end of the pier, they should pick me up post haste. Otherwise, there would be no way I could safely escape from my vulnerable position. No photograph is worth dying for.

Being nice fellows, they agreed they would keep an ear out for my shouts. They then returned to their duty of waiting for and recovering the divers.

As the boat eventually sped off with its load of thoroughly chilled divers, I realized that I had been deluding myself all along. At their distance and with the noisy interference of the boat motor, my shouts would have been inaudible. And from their low position on the water, they would have been unable to see what I was so agitated about; until it was too late.

My return back to the safety of the diving center was a cautious one; with the full realization that I was exposed and vulnerable for the entire route. Fortunately, safety was only a third of a mile away, but that was a long 500 meters, which gave my alert mind plenty of time to focus on walking quietly, and avoiding being eaten.

Nothing focuses the mind like knowing that close by, hidden by piles of snow, could be lurking a camouflaged predator looking for lunch.

This Youtube video shows a Polar Bear searching for food in Ny-Ålesund during the brief Arctic summer.

Root Causes: Some Accidents Are No Accident

Interesting flights and interesting dives provide an opportunity for post-event introspection; debriefing if you will.

Professionally, I am called upon to analyze fatalities and near-misses for the Navy and, occasionally, the Air Force. Personally, I spend even more time analyzing “what ifs” for my own activities.

For example, recently I was preparing a video of one of my more beautiful nighttime flights with a passenger, departing the coal-mining regions of Pennsylvania, heading south over the valleys and mountains of Appalachia as the early morning sun began to brighten our part of the world. Editing that video gave me a chance to reflect on the pre-flight and in-flight decisions I made that day. There were many decisions to be made, and those decisions resulted in not only a safe flight, but a spectacular flight.

But like most things, there was also a risk, calculated, and weighted, and recalculated as conditions in flight and on the ground changed in the face of aggressive weather.

In very real ways, single pilot IFR (instrument flight rules) flight is akin to cave diving. They are both technically challenging, rewarding solo activities. However, you better be on your game, or else not play.

I was cave diving before cave diving was cool; before it was considered a technical diving specialty, before safety rules and high quality equipment was available. Trimix, scooters, and staged decompression were all decades in the future, and frankly the safety record at that time was atrocious. I am alive because I had the good sense to limit my penetration; “just a little” was enough of a sobering experience, about which I have previously written.

But this posting is not about moderation; it is a warning to those who would, for whatever reason, deliberately make bad decisions, one after the other. If after a chain of such deliberate misadventures, a fatality results, then I would say that fatality is no accident. It is a procedure; a flawed process of decision making with a more or less guaranteed fatal outcome.

Dr. Tom Iliffe, Texas A&M University at Galveston

Lest you lose interest in reading this post because you believe all cave divers are loonies, rest assured that could not be further from the truth. Where I work we have four very active cave divers, highly intelligent, experienced, diving deep breathing trimix (helium/nitrogen/oxygen) when necessary using scuba and rebreathers. They are safe divers who are on the cutting edge of diving research when they’re not diving for pleasure. In fact, two of them are the U.S Navy’s diving accident investigators, so they know all too well about underwater misadventures.

Friends met early in my career have been the cave explorers of the 70’s and 80’s; names you may know like Bill Gavin and John Zumrick. Another long-time friend from the Navy’s Scientist in the Sea Program, and of whom I am quite envious, is Dr. Tom Iliffe, a biologist constantly on the front edge of underwater cave biology. (My draft novel, Children of the Middle Waters, includes a story about his beloved Remipedes.)

All these cave divers have survived due to their sane and balanced approach to risk management; moderation in all things. But sadly, not all divers I’ve come to know, one way or the other, have been so sensible and measured.

One was a wonderfully gracious man, a Navy diver who had a hobby: free diving. He’d tell me how he enjoyed surprising divers in the main cave at Morrison Springs, Florida when he would swim up to them and wave, while wearing no breathing equipment at all except that with which he was born.

I’m sure they were shocked; I know I would be.

After a while, as he gained experience with this solo recreation, he began to confide in me, and ask me questions about events he’d experienced. He told me how pleasant it was sometimes when he would surface. I warned him about shallow water blackout, loss of consciousness on ascent, and explained the physical laws that made breath-hold diving so dangerous; at least in the manner in which he practiced it.

FlMorrisonSpring — Morrison Springs, Florida. Photo licensed under Wikimedia Commons.

The last day I saw him alive, he once again came in for consultation, and told me about the euphoria he had experienced a few days before. I was of course extremely concerned and told him that what he described sounded like a near death experience. The next time he might not be lucky enough to survive, I told him. Later I heard more of that story; the previous weekend he had been found floating unconscious on the surface, but was revived.

Soon after that, this diver was again found, but this time his dive had proven fatal. His personal agenda for thrills exceeded all bounds of either training or common sense. And those thrills killed him.

The only solace I could find was that he wanted to share his experience and bravado, but he clearly was not interested in really hearing the truth, no matter how hard I worked to educate and dissuade him. While some might call this young man’s mental status as a perpetual death wish, I would argue that he never consciously thought he would die; at least not that way. Life was good, in his perspective, and I suspect he thought he was smart enough to make sure it continued that way.

Unfortunately when we were talking, we did not know just how close the end was.

5791863806_6fcd3452fc_o — Jackson Blue Spring, Marianna, Florida. Photo by Paul Clark, released under Creative Commons license.

The same was true I suspect for another well-liked diver who was the subject of a fatality report I helped write several years later. It was a rebreather fatality at Jackson Blue Spring in Marianna, Florida. The decedent was reportedly an experienced diver. I won’t belabor the story because the NEDU report is available on the internet (released by his family and available on the Rebreather Forum).

Nevertheless, the sequence of events leading to his demise involved a surprisingly long list of decision points which should have prevented the fatal dive from occurring. As each opportunity to change the course of events was reached, poor choices were made. In combination those choices led inexorably to his demise.

By now we know that even the U.S. Navy is not immune to poor decision trees. In fact, I would argue that wishful thinking is a common factor among people with intelligence and technical ability, and those with a “get it done” attitude. People who fix problems for a living are seemingly resistant to admitting that sometimes the bridge really is too far, and some problems are better fixed in the shop than in the field.

Gareth Lock of Cranfield University, Bedfordshire, U.K. is currently collecting data on diving incidents through a questionnaire on “The Role of Human Factors in SCUBA Diving Incidents and Accidents”. Like me, he has both an aviation and diving background. Gareth is serious about trying to understand and reduce diving accidents. Links to a description of his work, and his questionnaire can be found here and here. If you are a diver, please consider contributing much needed information.

I Too Landed at the Wrong Airport

As a professional in underwater diving, and an amateur airman, I’ve been thinking a lot lately about the causes of accidents and “near-misses”. If you’re reading this in early 2014, you are no doubt aware of several recent incidents of commercial and military jets landing at the wrong airport. In the latest case there was a potential for massive casualties, but disaster was averted at the last possible moment.

As they say, to err is human. From my own experience, I know the truth of that adage in science, medicine, diving, and the subject of this posting, aviation. Pilot errors catch everyone’s attention because we, the public, know that such errors could personally inconvenience us, or worse. But lesser known are the sometimes subtle factors that cause human error.

I can honestly tell you exactly what I was doing and thinking that caused errors at the very end of long flights. Those errors, none of which were particularly dangerous or newsworthy, were nonetheless caused by the same elements that have been discovered in numerous fatal accidents. Namely, what I was seeing, was not at all what I thought I was seeing.

N1144Y_300dpicrop — The small but capable Cessna 150B.

Long before the advent of GPS navigation, cell phones and electronic charts, I was flying myself and an Army friend (we had both been in Army ROTC at Georgia Tech) from Aberdeen Proving Ground, MD to Georgia. I was dropping him off in Atlanta at Peachtree-Dekalb Airport, and then I would fly down to Thomasville in Southwest Georgia where my young wife awaited me.

Since it was February most of the planned six hour flight was at night. We couldn’t take-off until we both got off duty on a Friday.

I had planned the flight meticulously, but I had not counted on the fuel pumps being shut down at our first planned refueling spot. After chatting with some local aviators about the closest source of fuel, we took off on a detour to an airport some thirty miles distant. That unplanned detour was stressful, as I was not entirely sure we’d find fuel when we arrived. Fortunately, we were able to tank up, and continue on our slow journey. We were flying in my 2-seat Cessna 150, and traveling no faster than about 120 mph, so the trip to Atlanta was a fatiguing and dark flight.

As we eventually neared Atlanta, I was reading the blue, yellow and green paper sectional charts under the glow of red light from the overhead cabin lamp. Lights of the Peachtree-Dekalb airport were seemingly close at hand, surrounded by a growing multitude of other city lights. Happy that I was finally reaching Atlanta, I called the tower and got no answer. No matter, it was late, and many towers shut down operations fairly early, about 10 PM or so. So I announced my position and intentions, and landed.

The runway was in the orientation I had expected, and my approach to landing was just as I had planned. However, as I taxied off the runway, I realized the runway environment was not as complex as it should have been. We taxied back and forth for awhile trying to sort things out, before I realized I’d landed 18 nautical miles short of my planned destination.

Capture — My unplanned refueling stop in South Carolina placed me far enough off course to take me directly over an airport that looked at night like my destination, Peachtree-Dekalb, Atlanta. (Solid line: original course, dashed line: altered course.)

I had so much wanted that airport to be PDK, but in my weariness I had missed the signs that it was not. I had landed at Gwinnett County Airport, not Peachtree-Dekalb.

No harm was done, but my flight to Thomasville was seriously delayed by the two extra airport stops. It was after 1 AM before I was safe at the Thomasville, GA airport, calling my worried wife to pick me up.

She was not a happy young wife.

A few years later, I added an instrument ticket to my aviation credentials, and thought that the folly of my youth was far behind me. Now, advance quite a few decades, to a well-equipped, modern cross-country traveling machine, a Piper Arrow with redundant GPS navigation and on-board weather. I often fly in weather, and confidently descend through clouds to a waiting runway. So what could go wrong?

DSC_4168-Edit — Piper Arrow 200B at home in Panama City, Fl.

Wrong no. 2 happened when approaching Baltimore-Washington International airport after flying with passengers from the Florida Panhandle. Air Traffic Control was keeping me pretty far from the field as we circled Baltimore to approach from the west. I had my instrumentation set-up for an approach to the assigned runway, but after I saw a runway, big and bold in the distance, I was cleared to land, and no longer relied on the GPS as I turned final.

As luck would have it, just a minute before that final turn we saw President George W. Bush and his decoy helicopters flying in loose formation off our port side. I might have been a little distracted.

In the city haze it had been hard to see the smaller runway pointing in the same direction as the main runway. So I was lining up with the easy-to-see large runway, almost a mile away from where I should have been. It was the same airport of course, but the wrong parallel runway.

I was no doubt tired, and somewhat hurried by the high traffic flow coming into a major hub for Baltimore and Washington. Having seen what I wanted to see, a large runway pointed in the correct direction, I assumed it was the right one, and stopped referring to the GPS and ILS (Instrument Landing System) navigation which would have revealed my error.

The tower controller had apparently seen that error many times before and gently nudged me verbally back on course. The flight path was easily corrected and no harm done. But I had proven to myself once again that at the end of a long trip, you tend to see what you want to see.

Several years later I had been slogging through lots of cloud en-route to Dayton, Ohio. I had meetings to attend at Wright Patterson Air Force base. It was again a long flight, but I was relaxed and enjoying the scenery as I navigated with confidence via redundant GPS (three systems operating at the same time).

As I was approaching Dayton, Dayton Approach was vectoring me toward the field. They did a great job I thought as they set me up perfectly for the left downwind at the landing airport. But then I became a bit perturbed that they had vectored me almost on top of the airport and then apparently forgotten about me. So I let them know that I had the airport very much in sight. They switched me to tower, and I was given clearance to land.

As I began descending for a more normal pattern altitude, the Dayton Tower called and said I seemed to be maneuvering for the wrong airport. In fact, I was on top of Wright Patterson Airbase, not Dayton International.

Rats! Not again.

Dayton airports — Wish my electronic Foreflight chart on my iPad had these sorts of markings.

Well, the field was certainly large enough, but once again I had locked eyes on what seemed to be the landing destination, and in fact was being directed there by the authority of the airways, Air Traffic Control (ATC). And so I was convinced during a busy phase of flight that I was doing what I should have been doing, flying visually with great care and attention. However, I was so busy that my mind had tunnel vision. I had once again not double checked the GPS navigator to see that I was being vectored to a large landmark which happened to lie on the circuitous path to the landing airport. (I wish they’d told me that, but detailed explanations are rarely given over busy airwaves.)

Oddly enough, if I had been in the clouds making an instrument approach, these mind-bending errors could not have happened. But when flight conditions are visual, the mind can easily pick a target that meets many of the correct criteria like direction and proximity, and then fill in the blanks with what it expects to see. In other words, it is easy in the visual environment to focus with laser beam precision on the wrong target. With all the situational awareness tools at my disposal, they were of no use once my brain made the transition outside the cockpit.

To be fair, distracting your gaze from the outside world to check internal navigation once you’re in a critical visual phase of approach and landing can be dangerous. That’s why it’s good to have more than one pilot in the cockpit. But my cockpit crew that day was me, myself and I; in that respect I was handicapped.

Apparently, even multiple crew members in military and commercial airliners are occasionally lulled into the same trap. At least that’s what the newspaper headlines say.

My failings are in some ways eerily similar to reports from military and commercial incidents. Contributing factors in the above incidents are darkness, fatigue, and distraction. When all three of these factors are combined, the last factor that can cause the entire house of cards, and airplane, to come tumbling down, is the brain’s ability to morph reality into an image which the mind expects to see. Our ability to discern truth from fiction is not all that clear when encountering new and unexpected events and environments.

The saving grace that aviation has going for it is generally reliable communication. ATC saved me from major embarrassment on two of these three occasions.

I only wish that diving had as reliable a means for detecting and avoiding errors.

How Does Your Rebreather Scrubber Handle the Deep?

If you’ve planned a deep dive, to say 100 meters or deeper, you may have wondered just how your rebreather scrubber will handle that depth. Since pressure is equalized across a carbon dioxide (CO₂) absorbent canister within a rebreather, it won’t implode. But what about the chemical absorption reactions occurring within the scrubber?

The rebreather scrubber is a vital part of your underwater life support system, so that question is a pretty important one. And the answer is very hard to find.

I recently traveled to Ireland to act as an external examiner for a Ph.D. student’s Doctoral Dissertation defense in the Department of Mechanical, Biomedical, and Manufacturing Engineering of the Cork Institute of Technology. The very talented graduate student was Shona Cunningham, and her dissertation was titled, “Carbon Dioxide Absorption and Channeling in Closed Circuit Rebreather Scrubbers”. She’s an athlete, musician, and perhaps most importantly for you readers, an avid diver.

Her work is the first computational fluid dynamic representation of scrubber canister thermokinetics. A portion of her dissertation work has already been published. Apparently, it was partially inspired by some of my computer simulation descriptions posted on this blog, which can be found here, here, and here.

Dr. Cunningham’s analytical approach (using Ansys CFX) showed that ambient pressure (depth) could reduce the effectiveness of scrubber canisters. In support of that finding were the words from the Dive Gear Express website regarding the Diverite O2ptima using the ExtendAir scrubber cartridge.

“As pressure increases the total number of molecules, the relative concentration of CO₂ molecules in the loop is reduced, slowing the chemical absorption process. Thus as depth increases, scrubber efficiency will decrease.”

The U.S. Navy has no experience with the Diverite O2ptima, but they have information on other rebreathers using granular absorbent. That experience shows that there is no reliable depth effect across all rebreathers and all absorbents.

U.S. Navy MK 16. US Navy photo by Bernie Campoli.

For example, in one rebreather there was indeed a 17% decrease in endurance using large grain absorbent (Sofnolime 408) at 50°F in descending from 190 fsw to 300 fsw (58 to 92 msw) breathing air. However, there was no decrease in duration when using fine grain absorbent (Sofnolime 812) under the same conditions. (On an actual dive, air would never be used at 300 fsw, but air was used in this study for scientific reasons.)

In another rebreather using Sofnolime 812, for a change in depth from 99 fsw to 300 fsw (30.3 to 92 msw) there was a 29% increase in duration at 75°F, a 10% increase at 55°F, and a 15% decrease at 40°F. Although air diluent was used at 99 fsw, 88/12 heliox diluent was used at 300 fsw.

From another manufacturer, I obtained information on two of their rebreathers. At 4°C, 1.6 L/min CO₂ injection rate (corresponding to a fairly heavy work rate), 40 L/min ventilation rate using air diluent, there was a 27% decrease in one rebreather in going from 15 to 40 msw (50 fsw to 132 fsw), and a 11% decrease in another of their rebreathers in dropping from 40 msw to 100 msw.

In another rebreather tested under the same conditions except for depth, the canister duration dropped 39% between 15 and 40 msw.

So, there is some support for a drop in duration with depth, but in other cases, there is either no effect or an increase in duration with deeper depths. Clearly, if the high number of inert gas molecules coming with a pressure increase makes it more difficult for CO₂ to reach absorption sites, then that would be a simple and unavoidable fact of physics. But that cannot be the whole story. What is likely to be going on, a hypothesis, is being developed for a later posting.

Should the effect of depth on your particular rebreather matter to you? Logically it shouldn’t. Even on a deep dive, the majority of the dive time is spent shallow, decompressing.

However, consider the case where you conduct a deep dive with an anticipated short bottom time, but something bad happens on the bottom. You or your dive team becomes fouled, ensnared in lines. Or there is a partial cave collapse trapping you. The benefit of a rebreather over scuba is that it gives you time to sort out your problem. Gas consumption is not nearly as great a concern as with open-circuit breathing apparatus.

However, as the minutes tick by as you work deep to get yourself or a team member free, you might wonder, “How is my scrubber handling this depth?” In the middle of a crisis is no time to be making assumptions about the status of a major part of your life support system.

Ask your manufacturer how your canister performs at depth. You have a right to know, and that information just might prove useful someday.

Cold Water Regulator Blues

It’s a black art, the making of scuba regulators for use in polar extremes; or so it seems. Many have tried, and many have failed.

Once you find a good cold water regulator, you may find they are finicky, as the U.S. Navy recently discovered. In 2013 the Navy invested almost two hundred hours testing scuba regulators in frigid salt and fresh water. What has been learned is in some ways surprising.

OLYMPUS DIGITAL CAMERA — Looking at a pony bottle that saved a diver when both his independent regulator systems free-flowed at over 100 feet under the thick Antarctic ice.

The Navy has been issuing reports on cold water regulator trials since 1987. In 1995 the Navy toughened its testing procedures to meet more stringent diving requirements. Reports from that era are found at the following links (Sherwood, Poseidon). (Here is a link to one of their most recent publicly accessible reports.)

The Smithsonian Institution and the Navy sent this scientist to the Arctic to help teach cold water diving, and to the Antarctic to monitor National Science Foundation and Smithsonian Institution funded trials of regulators for use in the under-ice environment. What those studies have revealed have been disturbing: many regulator models that claim cold water tolerance fail in the extreme environment of polar diving.

The Navy Experimental Diving Unit (NEDU) has developed testing procedures that are more rigorous than the EN 250 tests currently used by European nations. (A comparison between US Navy and EN 250 testing is found on this blog). All cold water regulators approved for U.S. military use must meet these stringent NEDU requirements.

Nevertheless, we learned this year, quite tragically, that the Navy does not know all there is to know about diving scuba in cold water.

For example, what is the definition of cold water? For years the U.S. and Canadian Navies have declared that scuba regulators are not likely to freeze in water temperatures of 38° F and above (about 3° C). (The 1987 Morson report identified cold water as 37° F [2.8° C] and below). In salt water that seems in fact to be true; in 38° F scuba regulators are very unlikely to fail. However, in fresh water 38° F may pose a risk of ice accumulation in the regulator second stage, with resultant free-flow. (Free-flow is a condition where the gas issuing from the regulator does not stop during the diver’s exhalation. Unbridled free flow can quickly deplete a diver’s gas supply.)

DSCN3524 — The regulator on the left free-flowed, the one on the right did not.

While a freshly manufactured or freshly maintained regulator may be insensitive to 38° F fresh water, a regulator that is worn or improperly maintained may be susceptible to internal ice formation and free-flow at that same water temperature. There is, in other words, some uncertainty about whether a dive under those conditions will be successful.

IsolatorValve1 crop — An isolator valve that can be shut to prevent loss of gas from a free flowing regulator.

That uncertainty can be expressed by a regulator working well for nine under-ice dives, and then failing on the tenth. (That has happened more than once in Antarctica.)

That uncertainly also explains the U.S. Antarctic Program’s policy of requiring fully redundant first and second stage regulators, and a sliding isolator valve that a diver can use to secure his gas flow should one of the regulators free flow. There is always a chance that a regulator can free flow in cold water.

A key finding of the Navy’s recent testing is the importance of recent and proper factory-certified maintenance. Arguably, not all maintenance is created equal, and those regulators receiving suspect maintenance should be suspected of providing unknown performance when challenged with cold water.

This finding points out a weakness of current regulator testing regimes in the U.S. and elsewhere. Typically, only new regulators are tested for tolerance to cold water. I know of no laboratory that routinely tests heavily used regulators.

Phoque_de_Weddell_-_Weddell_Seal — Weddell seal on the Antarctic sea ice. Photo copyright Samuel Blanc. (From Wikimedia Commons).

Considering the inherent risk of diving in an overhead environment, where access to the surface could be potentially blocked by a 1400 lb (635 kg), 11 foot (3.4 m) long mammal that can hold its breath far longer than divers can, perhaps it is time to consider a change to that policy.

About to descend through a tunnel in 9-feet of ice on the Ross Ice Shelf.

A huge Weddell Seal blocks the diver’s entry hole. He looks small here, but like an iceberg, most of his mass is underwater.

Keep Your Powder Dry, Rebreather Divers

Compared to decompression computers, digital oxygen control, and fuel cell oxygen sensors, carbon dioxide absorbent is low-tech and not at all sexy. Perhaps because it is low in diver interest, it is poorly understood. In rebreather diving, a lack of knowledge is dangerous.

The U.S. Navy Experimental Diving Unit (NEDU) is intimately familiar with sodalime, the crystalline carbon dioxide absorbent used in a wide variety of self-contained breathing apparatus for both diving and land use. NEDU routinely tests sodalime during accident investigations, during CO2 scrubber canister duration determinations, or during various research and development tasks. They have developed computer models of scrubber canister kinetics and patented and licensed technology for use in determining how long a scrubber will last in diving and land applications.

The types of sodalime in NEDU’s experimental inventory are:

Sofnolime 408 Mesh NI L Grade
Sofnolime 812 Mesh NI D Grade
HP Sodasorb (4/8 Reg HP)
Dragersorb 400
Limepak
Micropore

Absorbent undergoes a battery of quality tests at NEDU, most of them in accordance with NATO standardized testing procedures (STANAG 1411). One test is of the distribution of sodalime granule sizes, and another tests the softness or friability of the granules. One test checks the moisture content of the sample, and another tests the CO2 absorption ability of a small sample of absorbent.

The lead photo is a sample bag of sodalime removed from absorbent buckets, awaiting testing.

From time to time, absorbent lot samples fail one or more of these tests. One failure of granule size distribution was caused by changes in production procedures. “Worms” of absorbent rather than granules of absorbent started showing up in sodalime pails. In another case, absorbent was found to have substandard absorption activity, and in yet another, the material was too soft. Too soft or friable material can allow granules to break down, turning into dust.

This would not be a major problem, except that a diver or miner has to breathe through his granular absorbent bed, and dust clogs that bed, making breathing difficult. In the extreme, labored breathing from unusually high dust loading can result in unconsciousness.

What does the above have to do with this post’s title?

Supposedly, the maxim “Trust in God, but keep your powder dry” was uttered by Oliver Cromwell, but first appeared in 1834 in the poem “Oliver’s Advice” by William Blacker with the words “Put your trust in God, my boys, and keep your powder dry!” If indeed Cromwell did say it, then it dates from the 1600s.

A much more modern interpretation, appropriate for rebreather divers, is as follows: buckets of sodalime with a larger than usual layer of dust at the bottom (due to the mechanical breakdown of absorbent granules during shipment), should be kept dry. In other words, don’t dive it!

Micropore rolled carbon dioxide absorbent on the right, granular absorbent on the left.

Presumably, this is not an issue with Micropore ExtendAir CO2 absorbent since it’s basically sodalime powder suspended on a plastic medium. The diver breathes through fixed channels in the ExtendAir cartridge, not through the powder.

Considering the relatively high cost of granular sodalime, a diver might be very reluctant to discard an entire bucket of absorbent with a non-quantifiable amount of dusting. They certainly will not be performing sieve tests for granule size distributions like NEDU, however, one simple solution to a suspected dusting problem might be to sieve the material before diving it. The only requirement would be that only the dust should be discarded, not whole granules. In other words, your sieve must have a fine mesh.

In NEDU’s experience, quality control issues are not necessarily a problem with manufacturing. Where and how sodalime is stored can apparently have an appreciable effect on sodalime hardness. The same lot of sodalime stored in two different but close proximity locations has been found to differ markedly in its friability. Exactly why that should be, is presently unknown.

Regardless of whether the subject is sexy or not, a wise rebreather diver will seek all the knowledge available for his “sorb”, as it’s sometimes called. After all, the coolest decompression computer in the world will do you no good at all if you’re unconscious on the bottom because you tried to outlast your CO2 absorbent.

My Respiratory System is So Embarrassed

stk85284cor — Royalty free image from Punchstock.com

“Respiratory embarrassment” is an uncommon phrase most likely spoken by physicians and physiologists.

This week I found myself telling an engineer that “respiratory embarrassment can lead to an untoward event”. It quickly became apparent from the puzzled stare I received that I was not communicating.

Scientists and some medical personnel tend to do that; fail to communicate. In fact, they do it a lot.

What I was really saying is that in the right circumstances a person could have difficulty breathing, and that difficulty could cause something bad to happen; an “untoward” event. That bad thing would not necessarily be an aircraft crash, or in the case of a diver, a drowning, but it would mean that the pilot’s or diver’s performance would be impaired.

Why didn’t I just say so?

Laziness I suppose. I was using the language clinicians and physiologists are taught in graduate or medical school, and it flows out of our mouths naturally, without effort. Translating those same words into laymen’s terms takes time and effort.

I next started talking about respiratory impedance, a term understood by some but not all engineers, and rarely if ever by laymen. So once again I was not communicating well with all of my audience which was composed mostly of engineers, but not entirely.

That was the case until I used pictures to explain the otherwise difficult concepts of respiratory impedance and physiological embarrassment. The images below seemed to work, so I thought it worthwhile to share those images with you.

For you engineers, respiratory impedance is proportional to the sum of respiratory flow resistance and pulmonary and chest wall elastance.

pb-110104-buried-shulman_photoblog900 — From Shulman photoblog.

So what is that?

Well, for elastance, at least chest wall elastance, think of being buried to your neck in sand. Breathing difficulty comes from the difficulty of moving your chest wall in and out with the weight of sand pressing in on all sides. The pressure of sand impedes your breathing, hence elasticity (the inverse of compliance) is a major component of respiratory impedance.

Based on the photo of the young man pictured on the right, being partly buried for supposedly therapeutic reasons is not a pleasant experience.

Some might disagree. The man on the left is an actor in the 2008 French short film Le Tonneau des Danaïdes by David Guiraud, who seems quite at ease impeding his breathing for the sake of art. I’m guessing he’s either very dedicated, or very well paid.

In diving, respiratory elastance can be elevated by tight fitting wet suits; in aviators by tight fitting chest pressure garments, and in patients, by pulmonary fibrosis brought about by, for example, asbestos exposure.

Another key component of respiratory impedance, that thing that causes respiratory embarrassment, is flow resistance. Sticking your head in the sand would certainly be one way of generating

head-in-sand — *This image is found randomly throughout the web without attribution. The original source is unknown.*

severe respiratory resistance, with its attendant embarrassment.

out-of-breath-286x300 — From news.menshealth.com

Clinically, there are far more common sources of respiratory resistance, for example the narrowing of air passages in the lung caused by asthma. (Sticking your head in sand is probably a reasonable analogy to the sensations experienced during an asthma attack.) Chronic obstructive pulmonary disease (COPD) can also lead to a significant increase in respiratory resistance.

When you focus on the human respiratory system, the body parts shown in pink below, keep in mind that breathing can be impaired by things occurring inside the body (like asthma, COPD, fibrosis) or outside the body. Any life support system used for aviation, diving, mining, or firefighting imposes an impedance on breathing. That impedance in turn can lead to breathing difficulty, which can result in a failure to complete assigned duties.

Perhaps that’s where the “embarrassment” part comes in.

Created on www.biodigitalhuman.com ©2012.

The Siren’s Call of Rebreather Oxygen Sensors

In Greek mythology irresistibly seductive female creatures were believed to use enchanted singing to beckon sailors to a watery grave.

Why this myth endured through the centuries is difficult to say. However, my theory is that it helped explain to grieving widows and mothers why ships sometimes inexplicably disappeared, taking their crew with them, never to be seen again. By the reasoning of the time, there must have been some sort of feminine magic involved.

The oxygen sensors in closed-circuit, electronically or computer-controlled rebreathers are a magic device of sorts. They enable a diver to stay underwater for hours, consuming the bare minimum of oxygen required. The only thing better than a rebreather using oxygen sensors would be gills. And in case you wondered, gills for humans are quite impractical, at least for the foreseeable future.

I have written, or helped write three diving accident reports where the final causal event in a rebreather accident chain proved to be faulty oxygen sensors. So for me, the Siren call of this almost magical sensor can, and has, lured divers to their seemingly blissful and quite unexpected death.

Those who use oxygen sensors know that if the sensor fails leading to a hypoxic (low oxygen) state, loss of consciousness comes without warning. If sensor failure results in a hyperoxic state (too high oxygen), seizures can occur, again leading to loss of consciousness, usually without warning. Unless a diver is using a full facemask, loss of consciousness for either reason quickly leads to drowning.

EX19 — EX 19 rebreather (U.S. Navy photo)

Due to the life-critical nature of oxygen control with sensors, three sensors are typically used, and various “voting” algorithms are used to determine if all the sensors are reliable, or not. Unfortunately, this voting approach is not fail-proof, and the presence of three sensors does not guarantee “triple” redundancy.

In one rebreather accident occurring during the dawn of computer-controlled rebreathers, a Navy developed rebreather cut off the oxygen supply to a diver at the Navy Experimental Diving Unit, and all rebreather alarms failed. The diver went into full cardiopulmonary arrest caused by hypoxia. Fortunately, the NEDU medical staff saved the diver’s life, aided in part by the fact that he was in only 15 feet of water, in a pool.

In two more recent accidents the rebreathers kept feeding oxygen to the diver without his knowledge. One case was fatal, and the other should have been but was not. Why it did not prove fatal can only be explained by the Grace of God.

The two cases were quite different. In one the diver broke a number of safety rules and began a dive with known defective equipment. He chose to assume that his oxygen sensors were in better shape than the rest of his rebreather. If he had been honest with himself, he would have realized they weren’t. If he had been honest with himself, he would still be alive.

The other dive was being run by an organization with a reputation for being extremely safety conscious. Nevertheless, errors of omission were made regarding oxygen sensors which almost cost the experienced diver his life.

In the well-documented Navy case, water from condensation formed over the oxygen sensors, causing them to malfunction. The water barrier shielded the sensors from oxygen in the breathing loop, and as the trapped oxygen on the sensor face was consumed electrochemically the sensor would indicate a declining oxygen level in the rig, regardless of what was actually happening. Depending on how the sensor voting logic operated, and the number of sensors failing, various bad things could happen.

During its accident investigation, when NEDU used a computer simulation to analyze the alarm and sensor logic, it found that if two of the three sensors were to be blocked (locked) by condensed water, the rig could lose oxygen control in either a hypoxic or hyperoxic condition. Based on a random (Monte Carlo) sensor failure simulation, low diver work loads were more often associated with hypoxia than higher work rates, even with one sensor working normally.

We deduce from this result that “triple redundancy” really isn’t.

The white circles at the top left of this scrubber canister housing are the three oxygen sensors used in an experimental U.S. Navy rebreather.

When the accident rig was tested in the prone (swimming) position at shallow depth, after 2 to 3 hours sensors started locking out, and the rig began adding oxygen continuously. The computer simulation showed that the odds of an alarm being signaled to the diver was only 50%. The diver therefore could not count on being alerted to a sensor problem.

Unfortunately in this near fatal case the rig stopped adding oxygen, the diver became hypoxic and the diver received no alarms at all.

After NEDU’s investigation, the alarm logic was rewritten with a vast improvement in reliability. The orientation of the sensors was also changed to minimize problems with condensation.

Today what is being seen are divers who extend the use of their sensors beyond the recommended replacement date. Like batteries, oxygen sensors have a shelf-life, but they also have a life dependent on use. Heavily used sensors may well be expended long before their shelf-life has expired.

the-siren — The Siren, by John Williams Waterhouse.

Presumably, the birthing pains of the relatively new underwater technology based on oxygen sensors have now passed. Nevertheless, those who use rebreathers should be intimately familiar with the many ways sensors, and their electronic circuitry, can lead divers ever so gently to their grave.

Like sailors of old, there are ways for divers to resist being lulled to their death by oxygen sensors. First among them is suspicion. When you expect to have a great day of diving, you should be suspicious that your rebreather may have different plans for you. Your responsibility to yourself, your dive buddies and your family is to make sure that the rebreather, like a Siren, does not succeed in ruining your day.

The best way to ward off sensor trouble is through education. To that end, Internet sites like the following are useful. Check with your rebreather manufacturer or instructor for additional reading material.

http://rebreathers.es/celulas%20o2/celulas%20o2.htm

http://www.rf30.org/

http://www.deeplife.co.uk/or_files/DV_O2_cell_study_E4_160415.pdf