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:  Sodasorb_rotate

  1. Sofnolime 408 Mesh NI L Grade
  2. Sofnolime 812 Mesh NI D Grade
  3. HP Sodasorb (4/8 Reg HP)
  4. Dragersorb 400
  5. Limepak
  6. 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.

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 breakdown, 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.

Bag of granules_rotate
Sample bags of sodalime removed from absorbent buckets, awaiting testing.

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 1600’s.

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!

Picture12
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 sometime 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.

 

 

 

 

 

 

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