Air flow monitor provides reliable source to monitor positive or negative air flow of compact fans or filter fans. Proper installation and connection with an optical LED or low audible signal device, a bi-direction air flow monitor will activate – deactivate an electrical contact if the air flow of the fan falls below 0.8 m/s , thus either turning the signalling device on or off Recommended Use: Normally used to turn an alarm or signalling device On to indicate loss of air flow.