Axial, radial, hydrostatic, pre-packed and cross-flow. Why can’t I just put a larger can of sorb on my rebreather? Manufacturers test their scrubber designs by using oxygen consumption estimates, depth and temperature variables. Because CO2 is a metabolic byproduct, manufacturers can predict how much carbon dioxide will be created after metabolizing a given amount of oxygen.
However you might have noticed dramatically differing specifications for various rebreathers and their scrubbers. Clearly five pounds of scrubber does not equal five pounds of scrubber when it comes to duration at various depths.
Rebreather manufacturers invest a lot of time and money on the gas dynamics within the scrubber. The gas needs to flow through easily to meet the needs of work of breathing, but it also needs to dwell within the material long enough for the chemical reaction to take place. This has to work at various depths and the gas densities that go with it. If less-dense gas molecules speed through the scrubber without adequate dwell time, then blow by may occur. This is why some rebreathers also have a critical depth limitation. Their scrubbers may reach their limit of effectiveness at a particular depth. You may be able to survive a mellow, relaxed dive at a low exertion rate, but if trouble ensues and your metabolic rate increases, then you may exceed the design parameters of a unit.
So I want to leave you with two thoughts. First, you can’t simply add a larger sorb bucket to a rebreather and expect the WOB and scrubber mechanics to work as described by the manufacturer. You may be taking your life into untested waters. Second, when you hear about somebody doing a heroic deep dive on a particular unit, it does not mean that you could survive the same dive. They may have been incredibly cool but only one small bit of additional workload from exceeding the limits of their scrubber. The manufacturer’s tests and ratings are done to keep you safe. Stay within their limits. -- Jill Heinerth