How does a person know the air supply in an SCBA is low?

A person knows the air supply in an SCBA is low primarily through the low air alarm or gauge readings. SCBAs are equipped with pressure gauges that provide an accurate reading of the air supply remaining in the cylinder. Additionally, SCBAs typically include audible alarms that activate when the air pressure falls to a critical level, effectively alerting the wearer to the low air supply. These features are specifically designed to ensure the safety of the user by providing timely warnings about the need for air supply replenishment.

While other factors such as changes in breathing resistance or facepiece indicators may provide supplemental information about the breathing apparatus's performance, they are not as direct or reliable as the low air alarm and gauge readings for assessing air supply levels. Monitoring the gauge ensures that the user can plan accordingly and respond proactively before the air supply is depleted.