Tag: cold water diving

Diving a Rebreather in Frigid Water: Canister Concerns

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As evidenced by Under the Pole diving expeditions, rebreathers are being used in some of the most isolated and frigid places in the world. Some of those dive missions are surprisingly deep (111 meters, 330 feet) and long, about 2 hours.

That gives me cause for pause.

I suspect most divers are aware of the 1/3 rule for gas consumption on an open circuit (scuba) cave dive. You should use no more than 1/3 of your air supply on the way in, leaving you with 1/3 for the trip out, and 1/3 of your gas supply available in reserve. Sadly, even that amount of reserve has not saved all cave divers.

Now that cave divers are using rebreathers, the rules, at least for some, have changed. Some savvy rebreather cave divers use the rule of doubles: Always have twice as much oxygen, twice as much diluent, and twice as much canister as you think you’ll need. That plus an open-circuit or semi-closed circuit bailout should keep you safe — in theory.

Gas supply is easy to measure throughout a dive; there is a pressure gauge for all gases. But what about canister duration? Most divers assume they will have more canister duration available than gas supply; which means they don’t need to worry about canister duration. That would be a good thing, if it were true. After all, how many manufacturers provide expected canister durations for various work rates and water temperatures? Maybe, none? Or certainly very few.

I would be very surprised if manufacturers could say with certainty that during a two hour dive in -2°C (28°F) water, at depths to 111 meters that the scrubber can provide double the duration needed. That would be four hours in -2°C water, at all potential diver work rates.

Some of you may say, “Under-the-ice-diving is not like cave diving, so the doubles rule is too conservative.” I invite you to think again. Under polar ice, is there ready access to the surface? Not unless you’re diving directly under the through-ice bore hole the entire time.

In the U.S. Navy experience, obtaining useful data on canister durations from manufacturers is difficult. Duration data as a function of temperature is practically nonexistent. Therefore I will share the following information gleamed from scrubber canister testing in extreme environments by the Navy. While this blogger cannot reveal canister durations for military rebreathers, the information on the coefficient of varation (COV) is not protected. (There is no way to figure out what a canister duration is based solely on the COV.)

The following 4-minute video gives a good introduction to the coefficient of variation.

https://youtu.be/XXngxFm_d5c://

All rebreather divers should know that canister performance declines in an accelerating manner as water temperature drops between 50°F and 28°F. But what your rebreather manufacturer may not know is that the innate variability of canister durations also increases as water temperature drops. The Navy has found that trend in all types of rebreathers.

So, while canister durations drop considerably in cold water, you’re also less certain about what your canister’s endurance is going to be, because of the increase in duration variability. When canister duration drops and variability increases, a diver’s margin of safety becomes a gamble. Personally, I don’t like to gamble under water.

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Coefficient of variation (COV, mean duration divided by the standard deviation) of a typical rebreather. Each data point is the mean of five canisters (n=5).

In the U.S. Navy, published canister durations take into account mean canister performance, and variability. That is accomplished through the use of 95% prediction intervals. The greater the variability in canister duration, the lower the published duration.

This method of determining safe canister durations has been in use by the U.S. Navy since 1999. However, I do not know if manufacturers use similar statistically-based methods for publishing canister durations. If they or you do not take duration variability into account as you dive cold, you may be in for a shock. Due to the nature of statistics, you may have 9 deep, cold dives with no CO2 problems, but find yourself in bad shape on the 10th dive.

If you did have a CO2 problem, it wouldn’t necessarily be anyone’s fault: it could just be a result of canister variability in action.

So, diver beware. Give yourself plenty of leeway in planning rebreather dives in frigid waters. After all, you do not want to become a statistic, caused ironically by statistics.

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If you have an interest in understanding the derivation of the prediction interval equation and its application, two videos of lectures by Dr. Simcha Pollack from St. John’s University may be helpful. Part I is found here, and Part 2 is found here.

Thanks to Gene Hobbs and the Rubicon Foundation, NEDU’s original report on the use of prediction limits to establish published canister durations is found here.

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.