CO2 Absorbent. The weight of absorbent required is determined by
the weight of carbon dioxide to be absorbed and the efficiency of absorbent
use. Carbon dioxide weight is determined by the number of divers served, the
carbon dioxide production rate per diver, and the operating period.
Efficiency of absorbent use is determined by canister design and operating
conditions; high efficiency requires good distribution of gas flow through
the bed, humidity above 70 percent, temperature above 60 deg. F, and freedom
from moisture condensation in the bed. Figure 6-17 summarizes the
characteristics of Baralyme, lithium hydroxide, and Sodasorb, three
commercial carbon dioxide absorbents. Baralyme can be used if there is a
potential moisture problem because it is not caustic if wetted. Sodasorb can
be used for hyperbaric chambers in which moisture is not a problem.
b. Quantity of CO2. The weight of carbon dioxide generated depends
upon the rate of oxygen consumption, which varies with the rate of effort as
shown in Figure 6-8, and with the respiratory quotient, which is the ratio of
carbon dioxide produced to oxygen consumed. At moderate depths, the maximum
rate of oxygen consumption is about 3 slm; at very great depths this
decreases to 2 slm because of increased breathing resistance. The
respiratory quotient varies from 0.7 to 1.1, averaging 0.9. Thus, the
maximum rate of carbon dioxide production for 3 slm oxygen consumption and
0.9 respiratory quotient, is 2.7 slm, equivalent to 0.71 pounds/hour.
Effect of Temperature. The rate at which carbon dioxide is
absorbed is considerably lower at 40 deg. F than at 70 deg. F. In some
scrubbers sized for adequate performance at 70 deg. F, absorbing capacity at
40 deg. F may be as little as 1/3 that at 70 deg. F.
d. Absorbent Bed Configuration. A variety of absorbent-bed
configurations are in use, and none seem to have advantages that make them
adequate amount of absorbent, uniform distribution of gas flow through the
absorbent bed, and sufficient time for the absorption reactions to occur.