The effect of over- or underfilling the soda lime canister on CO2 absorption in two closed-circuit oxygen rebreathers.

O2 diving incidents investigated by our laboratory were related to improper filling of the soda lime canister in closed-circuit oxygen rebreathers. We studied the effect of overfilling or underfilling the canister on CO2 absorption using a continuous flow of 5% CO2. With a full canister in the Oxyger 57, CO2 began to rise at 130-160 min, reaching 1% at 240 min and 1.5% at 270 min. Similar results were obtained after a reduction of 100 g in the quantity of soda lime packed into the canister. After reductions of 200, 300 and 400 g, the rise in CO2 concentration occurred earlier as a function of the amount of the reduction. The level of CO2 in the OxyNG 2 began to rise after 250 min with a full canister, reaching 1% at 340 min and 1.5% at 370 min. After a reduction of 100 g there was a delay in the rise of CO2, which reached 1.5% at 390 min. However, when the reduction was 200, 300 and 400 g, the rise in CO2 concentration tended to occur earlier as a function of the amount of the reduction. For both rebreathers, when the quantity of soda lime was reduced by 200 g or more, there was a considerable difference in timing between the two test measurements for each weight reduction, due to variations in channeling. For an excess of soda lime, moderate pressure was applied manually to achieve a full canister plus 300 g in the OxyNG 2. The initial rise in CO2 concentration started early, at 60 min with a full canister plus 300 g compared to 150 min with a full canister; 1% CO2 was reached at 120 min, compared to 210 min with a full canister. As the use of rebreathers becomes increasingly widespread in diving, close attention should be paid to proper filling of the soda lime canister.