Australian standards specify that Class II cabinets be fitted with audible and visible alarms that warn of loss of safe airflows. Most Class II cabinets in Australia use pressure switches to operate alarms. These devices are relatively insensitive and are often set up by service technicians to minimise the probability of an alarm condition that is the result of the alarm set-point being close to the normal exhaust airflow. When set up in this way, cabinets could be operated in an unsafe condition without an alarm being triggered.
Gelaire BH Series cabinets do not use pressure switches or have the inherent limitation of unreliable alarm sensitivity. Sensitive rotating vane anemometers detect the smallest variation in airflow and a microprocessor-based control system automatically maintains safe airflows.