Quantcast NAVFAC Energy Criteria Team Application Guide for Occupant Sensors

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1 July 1996
1.00 Introduction - Occupant (or occupancy) sensors have been used to turn lights on
and off for a number of years, yet many problems still exist over their misapplication and
improper installation. Remember that occupant sensors (O.S.) are viable energy savers
only where human occupancy is intermittent. Do not employ O.S. in an area where people
are expected to be moving about continuously throughout the day. Classrooms, offices,
restrooms, warehouses, hangars, and equipment and storage rooms are examples of areas
where O.S. will save energy by turning off lights when no human presence is detected.
This application guide will address the available technologies and selection factors for
occupant sensors in an abbreviated form. More detailed occupant sensor application
guidance is included in the references listed at the end of this guide.
1.01 Sensor Technologies - The two primary technologies used to detect occupancy are
ultrasonic and passive infrared (PIR) employed singly or with each other in a dual-
technology sensor.
a) Ultrasonic Technology - Emits ultrasonic radiation (high-frequency sound) to
sense occupants in an area. Frequency range of operation is between 20 and 28 kHz.
They are most sensitive to motion toward and away from sensor. An ultrasonic sensor is
volumetric, meaning it floods an area within its coverage pattern. This allows it to detect
persons behind partitions and other obstructions. Any moving object within its coverage
area will disturb the sound wave pattern, creating a Doppler shift and altering the signal
returning to the sensor. False activation may also occur from vibration, air movement,
high sound levels, and audible sounds having ultrasonic components. Ultrasonic sensors
have sensitivity adjustments to minimize environmental effects. Reducing sensitivity will
also reduce the coverage area. If ultrasonic sensors are mounted close to each other, they
must operate at different frequencies to avoid interfering with each other.
b) Infrared Technology - An infrared O.S. is sensitive to body heat and works best
when the person is moving across the sensor pattern. PIR sensors have different lenses
available to generate specific patterns of coverage: fan shaped pattern for smaller areas
(private offices), 360 degree pattern for large areas (open plan offices), and long narrow
pattern for areas such as hallways. Lenses may also be field modified to block out part of
its sensor's view area to avoid a heat generating device or an area that must remain lighted
continuously. Because it is sensitive to body heat, the infrared O.S. is also sensitive to any
rapid fluctuations in temperature within its field of view. Be sure no rapidly changing heat
sources are in the area where the infrared O.S. is to be used. Solar heating of glass and
metal surfaces can cause sensor activation. The sensor can be blinded by the sun shining
directly on it. To be effective, PIR sensors should be used in areas with no obstructions to
occupant movement, since it is a line-of-sight device. Small animals won't usually activate
PIR sensors, although a bird flying close to one could turn it on.


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