Final answer:
The respiratory therapist should change the device when the reservoir on a non-rebreathing mask is collapsing completely with each breath.
Step-by-step explanation:
The respiratory therapist should change the device when the reservoir on a non-rebreathing mask is collapsing completely with each breath. The non-rebreathing mask is designed to provide high concentrations of oxygen to the patient by using one-way valves to prevent the rebreathing of exhaled air. If the reservoir is collapsing, it means that the valves are not working properly and the mask is not delivering the desired oxygen concentration effectively.