Types of Occupancy Sensors

Ultrasonic Occupancy Sensors

• Ultrasonic occupancy sensors operate on the Doppler principle; that is, they send ultrasonic high-frequency signals out into a space and then track the movements in that space based upon variations in the signals' frequencies, which are then taken to be movements within the space the device is monitoring. The advantages of this type of sensor include the fact that it does not require a direct line of sight, but can instead "see" around corners. However, some of the drawbacks of this system include an inability "see" through fabric partitions and a very high loss of effectiveness in high-ceilinged rooms. These sensors are best suited to rooms with relatively low traffic and work better at tracking movement to and from the sensor.

Passive Infrared Sensors

• Passive infrared (PIR) sensors work on a completely different principle than ultrasonic sensors, using changes in heat as the basis for tracking as opposed to changes in sound. They track the level of heat in a space and are then alerted to fluctuations in that heat. PIR sensors require a clear line of sight in order to work properly because they cannot "see" around corners or through partitions of any kind. They are also not proficient at detecting minor movements at a distance of greater than 15 feet. Therefore, these sensors are best used in smaller, more enclosed rooms and spaces, outdoor areas and warehouse aisles. However, these sensors cannot be located too close to any sort of heating, ventilating and air conditioning (HVAC) vent.

Dual-Technology Sensors

• Dual-technology sensors, which use both PIR and ultrasonic sensing technology, alert themselves to movement only when both elements say so. The upside of these types of occupancy sensors is that they are almost incapable of false alarms. They are very useful for spaces such as classrooms or conference rooms where there may be a great deal more traffic than other places.