Rebreather divers and purchasing agents new to carbon dioxide absorbent are rightfully mystified by some of the terminology. As for the terms aDsorbent vs aBsorbent, that is a debate for another day, call it what ever you like... most experienced rebreather divers just call it "sorb". There are a lot of parameters that go in to describing a specific absorbent, but the most confusing are granule size, indication, and grade.

Granule Sizes -- Most absorbent is a mix of different sized granules. Modern specifications describe the particle size using millimeters and distribution using percentages. For example, by far the most popular granular mixture for rebreathers is about 80% of the particles ranging from 2.0 mm to 1.4 mm in size with nearly all of the particles being between 2.5 mm and 1.0 mm in size. There is a sieve standard for woven wire mesh where #8 aperture size is defined as 2.0 mm and #12 is 1.4 mm. This range of particle sizes is often referred to as 8-12 or sometimes 812. Another absorbent widely used in commercial diving is mostly larger particles from 4.0 mm to 2.0 mm with nearly all the particles being between 5.0 mm and 2.0 mm. This is mesh aperture size #4 to #8, referred to as 4-8 but often written as 408. There are many other sizes because a specific size or shape of granule is not always better than another, there are important trade offs that depend on the application.

Indication -- Some absorbents can optionally be formulated to contain dyes that temporarily change color to visually indicate chemical reactions that occur as carbon dioxide is being absorbed. The indicator colors are White-to-Violet abbreviated as WV or Pink-to-White abbreviated as PW; the absence of indicator is abbreviated as NI for Non-Indicating. Absorbent manufacturers claim the dyes used in their indicating versions do not have any adverse effects and do not affect performance. However, the purchasing requirements for some organizations now specifically prohibit the presence of any indicator dye in CO₂ absorbent. Because the color change will reverse once the chemical reaction ceases, rebreather divers should not consider indicator a reliable method for evaluating scrubber stack usage.

Grades -- In the world of absorbent, the grade just identifies a specific formulation (ingredients, binders, moisture content, particle size and shape) that is highly optimized for a specific application. In some cases the same formulation may have multiple grades that differ only in their documentation as meeting a particular standard or purchasing requirement. There are many different formulations intended for different applications such as anesthesia, mine safety, submarine air purification, commercial diving, and rebreather diving to name a few. Using the wrong absorbent, such as substituting an anesthesia grade for a diving grade, can result in adverse consequences.

Sofnolime is the Molecular Products brand name for its soda lime formulations of carbon dioxide absorbent. A widely available Molecular Products absorbent grade used in commercial diving hyperbaric chamber systems is "Grade CD" which is generically referred to as 408. Some early rebreather designs still specify 408 absorbent, but today this is rare. The Sofnolime absorbent grade used in modern rebreather designs is "Grade 797", which is generically referred to as 812 by divers. The Molecular Products description of the CO₂ absorbent soda lime formulation in 1.0 mm to 2.5 mm granule size (1025) sold by Dive Gear Express is Sofnolime^™ 8-12 Mesh NI (Grade 797), whose primary usage application is semi-closed circuit and fully-closed circuit rebreather diving.

Be very careful, we have seen the various terms mixed up in to nonsense descriptions such as "Sofnolime 797, Grade 408WV" or "812 CO2 Adsorbent, Grade CD". In these cases the absorbent is probably the larger 4-8 mesh indicating formulation that is primarily used in commercial diving applications; typically lower cost but much less absorption capacity. The mesh size alone is also not adequate identification, there are alternative formulations of similar particle sizes intended for applications other than diving. Watch out too for indicating vs non-indicating, if it doesn't clearly state non-indicating (NI) in the description then you possibly could receive indicating.

For more detailed information about CO₂ absorbents, especially for diving, read our article titled Rebreather Absorbent Selection and Handling.