How much of a conservative are you?

When it comes to storage and use of the ‘kitty litter’ used in rebreathers to scrub carbon dioxide from the breathing gas, I had until very recently thought of myself as ultra conservative. Turns out this was not necessarily the case.

I was careful with the storage part and careful when packing or loading the scrubber canister of any unit I dived with, but it turns out I misunderstood the actual working life of the absorbent once it was partially used.

Now I should make it clear that the only absorbent I have much experience with is Sofnolime® 797. This is a product made by Molecular Products in the UK and – in my circles at least – is the gold standard for use in closed-circuit rebreather diving. For the record, I use what’s called the non-indicating variety, which means it does not change color when suffused with Carbon Dioxide.

Sofnalime® itself looks a little like a white version of the material used in a cat box (hence its street name), and is actually a triangular cross-sectioned extruded pellet made in part from calcium hydroxide with a little sodium hydroxide mixed in, and is between 1.0 mm and 2.5 mm in size. It is alkaline (a pH between 12-14), slightly water soluble, and non-corrosive – but the dust will irritate the eyes and perhaps the skin, and inhaling it is a definite no-no.

In simplest terms possible, the chemical reaction that takes place inside a rebreather’s scrubber removes carbon dioxide and produces heat and water, and turns the soda lime into chalk (calcium carbonate). Also, for the record, in addition to proper storage and handling of unused scrubber material, used soda lime should be disposed of responsibly. Whenever possible, I take it home and then spread spent scrubber material on the garden where horticultural lime might be indicated, and put the rest in our horseshoe pit.
Ok, now with that clear, let’s focus on my misunderstanding.

Rebreather manufacturers tend to rate the working life of the scrubber material in their units based on the size of the scrubber canister. Literally on the amount of kitty litter their machine holds. In a perfect world, we might ask for a slightly more scientific method to gauge this, but referring to an X-hour scrubber is the norm. Certainly, this is what I was taught… but it is not what I teach; and here’s why.

After speaking with one of the chemists at Molecular, I learned that the method commonly used to indicate the effective life of scrubber material (i.e. Sofnalime®) is incorrect. While a freshly charged scrubber may have X or Y or N hours of potential effectiveness ahead of it, that number of hours is an estimate based on continuous use.

Let’s say for example that a rebreather manufacturer designates its scrubber duration as four hours. This means up to four hours on one dive and NOT two two-hour dives back-to-back on the same scrubber. This, according to Molecular’s chemist, would be “pushing it.” There are several other considerations that should be taken into account when estimating how much ‘life’ is left in one’s scrubber but on straight, no frills, moderate depth dives (such as shallow cave dives in North Florida which would normally be to depths less than 30 metres/100 feet) after one two-hour dive on a ‘four-hour scrubber’ perhaps only an hour and a half is left, and NOT two more hours. After a couple of one-hour dives, a third dive to 45 minutes or so, will all but exhaust the remaining Sofalime® so that in actual use, the effective life – and safest interpretation – of a four-hour scrubber would be less than three hours.

Now, it should be said that estimates of scrubber duration from manufacturers tend to be conservative and are usually based on the worst type of conditions; however, I found it interesting that the guy who oversees the manufacture and testing of the active component in the little chemistry set I lug around on my back to go diving, is more conservative yet.

And I for one will follow his example from here on in.