Tag: canister duration

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.

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.