The ALERT system can provide comprehensive security protection for SCBA users. The personal monitoring unit, which is connected with the gas cylinder, can display the cylinder pressure by mechanical and digital gauge. The device can monitor the user's motion status, the ambient temperature, altitude and battery status, and it can signal visually and acoustically under danger conditions. Through wireless communication, the BASE station can receive the information, status and alarms from the personal monitoring units and display the data on screen. On-line users can be managed and controlled in groups. If necessary, evacuation alarm can be sent to all the crews, a team or an individual user.