Abstract: A method of determining the concentration of a vapor in a gaseous fluid comprises the steps of passing a sample of the fluid through a channel having a crystal oscillator therein coated with a substance which absorbs the vapor, monitoring the deviation of the frequency of the oscillator from a base-line frequency representative of a fluid sample having a zero concentration of the vapor and converting the frequencydeviation, if any, to a numerical value indicative of the concentration of the vapor in the fluid.

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.