There are monitors, since everyone likes to use intubated patients as examples, the two main types of anesthetic scrubbers both have a blue dye that appears when the scrubber is spent.
The Sodasorb dye is not necessarily a good indicator that the absorbent material is spent. We used indicating Sodasorb in our hyperbaric patient recirculators for a number of years, and even though it would turn purple after one dive in some patients, the CO2 in the patient head tents remained well within our specified limits. We used the canisters for a total of three 2-hour dry hyperbaric dives. We've since switched to the high-performance Sodasorb.
Also, the Navy stopped using the indicating Sodasorb in rebreathers in the late 1990's due to problems with odors - I remember switching to Sofnolime for the Mk-16s at that time. http://archive.rubicon-foundation.org/dspace/handle/123456789/4972
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