Final answer:
A pressure gauge is the device used to measure and display pressure readings for the quantity of breathing air in an SCBA.
Step-by-step explanation:
The device that measures and displays pressure readings, indicating the quantity of breathing air available in an SCBA (Self-Contained Breathing Apparatus), is known as a pressure gauge. This gauge provides users with vital information to ensure there is a sufficient supply of air while using the apparatus. Devices like barometers are used to measure atmospheric pressure, but for applications such as SCBA, where it is crucial to know the gauge pressure (pressure relative to atmospheric pressure), a pressure gauge calibrated to measure the pressure within the system is used.