Det-Tronics U7698A, B, C, E Unitized Frequency IR Detector/Controller - Manual (Page 4)

These characteristics are important because fire is a
chemical process, and chemical processes can vary
significantly, depending on the elements and conditions.
A simple fire detector can be designed to detect any
one of these elements, but unless this detector is able to
discriminate, it will be subject to false alar ms.
Discrimination can be achieved by using more than one
element as criteria to detect a fire.
The U7698B detects two elements of fire: IR radiation
and flame flicker. IR radiation is found throughout
nature, but few things emit IR at the flicker frequency of a
fire.
To ensure a high degree of reliability, precision optical fil-
ters limit IR response to a narrow band from 4.2 to 4.7
microns. This bandwidth is significant for two reasons:
1. Hydrocarbon fires emit intense IR radiation in this
band.
2. Atmospheric transmission of IR from the sun in the
4.2 to 4.7 micron range is nearly zero. See Figure 1.
Refer to Figure 2 for a block diagram of the U7698B.
Optical filters limit the radiation reaching the sensor to
the range of 4.2 to 4.7 microns. The pyroelectric sensor
converts the infrared energy to an electrical signal.
A flicker filter checks for flame flicker. Any signal that
does not meet the flicker frequency requirements, such
as steady state IR emissions, is rejected. The flicker rate
is set at the factory for levels characteristic of a hydro-
carbon fire (2 to 10 Hz). This flame criteria for the IR
detector results in reliable response to the presence of
fire combined with a high level of immunity from false
alarms.
If the signal meets the requirements for a fire output (IR
radiation of adequate amplitide, with proper wavelength
and flicker frequency, and lasting for approximately 3
seconds), the signal processing circuitry triggers an out-
put circuit, which energizes the alarm relay coil.
OUTPUTS
The U7698B is furnished with two output relays. The
relays have Form C (normally open/normally closed)
contacts and are rated for 2 amperes at 30 vdc. The
normally de-energized fire relay is energized upon
detection of a fire that meets the previously described
flame recognition criteria. It is also field programmable
for either latching or non-latching operation. When
latching operation is selected, the relay is reset by
removing input power for a minimum of 2.0 seconds.
The normally energized fault relay is de-energized upon
loss of input power to the detector or if the sensor mod-
ule is removed.
OPTICAL
FILTERS
IR
SENSOR
FLICKER
FILTER
FIRE
RELAY
NO
COM
NC
A1290
SIGNAL
PROCESSING
CIRCUITRY
POWER
SUPPLY
FAULT
RELAY
INPUT
SUPPLY
VOLTAGE
NO
COM
NC
oi
TEST
oi
Figure 2—U7698B Block Diagram
5.0 4.0 3.0 2.0 1.5 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
ATMOSPHERIC
TRANSMISSION
WAVELENGTH (MICRONS)
SOLAR RADIATION REACHING THE EARTH
VISIBLE INFRARED ULTRAVIOLET
100
75
50
25
0
A1295
INFRARED SENSOR RESPONSE
Figure 1—Atmospheric Transmission of the Sun's Radiation
2