Abstract: The invention disclosed is an indicator for monitoring the capacity of a CO.sub.2 absorbent to absorb CO.sub.2. The indicator is typically used in association with a closed circuit breathing apparatus, including a container for a CO.sub.2 absorbent. The indicator comprises temperature sensing means immersed in the absorbent, a temperature to voltage converter for converting a sensed temperature reading to an electrical voltage signal, temperature trend processing means for processing said electrical voltage signal into a selected electrical output signal characteristic of a temperature trend; and display means for converting said selected electrical output signal to a predetermined visual display indicative of the capacity of the CO.sub.2 absorbent to absorb CO.sub.2.