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.
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.
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.
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).
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.