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Sofnolime® 797 is the commercial name for soda lime produced by Molecular Products with triangular cross-section particle sizes in the range 1.0-2.5mm | 0.04-0.1in and 16-20% moisture content to optimise reaction speed. Used for most rebreathers including JJ-CCR, Revo, AP Inspiration, Hollis, Poseidon, SF2-ECCR.

Soda lime is an alkali mixture of sodium hydroxide (caustic soda) and calcium hydroxide (slaked lime) which removes carbon dioxide (and other acidic contaminants) from gas streams via an exothermic, water facilitated, base catalysed chemical reaction. 

Relevant factors to consider in CO2 absorbent for diving applications are particle size and moisture content.  Particle size affects breathing resistance and the shape of the reaction front though a vertical scrubber.

For very large particles (eg 3.6mm | 0.14in) there is less impediment to straight line gas flow so while there will be little breathing resistance, the reaction front will exhibit a vertical profile. In a vertical profile gas flow bypasses and fails to make use of large areas of the scrubber and quickly achieves breakthrough. 

The chemical reaction between soda lime and CO2 takes approximately 0.5 seconds and varies depending on moisture and heat.  The speed of gas flow through large particles may not provide sufficient time for this reaction to take place. 

For very small particles (eg 1.0mm | 0.04in) there is a lot of impediment to gas flow causing excessive breathing resistance. However, the reaction front will exhibit a slanted profile making best use of all absorbent across the width of the canister until breakthrough is made in the center.

Is it better or OK to use medical grade Sofnolime for diving?

Molecular products have advised that using cheaper medical grade Sofnolime® 2550 USP in diving applications is both a false economy and unsafe.  Sofnolime® 2550 USP is optimised for low flow anaesthesia applications at standard atmospheric pressure, and is not suitable for variable or high flow rates and higher operating pressures as are found in diving applications. Sofnolime® 2550 USP will not maintain post scrubber CO2 levels below the safe limit under demand conditions and exhibit a significantly reduced time to breakthrough. 

Molecular Products “Use of medical grade Sofnolime® in Diving Applications” (press release) January 2010.

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