Theory of operation, Signal processing – Hochiki IFD-E User Manual
Page 8
8
Hochiki Europe (UK) Ltd
2-3-0-808/ISS3/OCT11
Selectable Functions
DIL Switch Settings
Relay RL2 Function:
1
2
RL2 off (No fault relay) – For lowest detector current consumption.
0
0
RL2 off, or UV pre-alarm, flame or electrical sparks detected.
1
0
RL2 energised on IR pre-alarm
0
1
RL2 detector fault relay (Energised if detector powered and no faults)
1
1
Detector Supply Current (Detector Status): [-/ = see Output Mode below]
3 4
Low current mode, 3mA / 9mA (RL1 Only), 8mA / 14mA (RL1 & RL2)
0
0
Two-wire current signalling only. No relays operating. 4-20mA, 4/20mA
1
0
Two-wire current signalling 8-20mA, 8/20mA and both relays operating.
0
1
Two-wire current signalling 8/28mA and both relays operating.
1
1
Output Mode:
5
(-) Proportional analogue supply current. Non-latching fire alarm signalling. (-)
0
(/) Step change, supply current. Latching fire alarm signalling. (/)
1
Response Time:
6
7
Slowest ≈ 8s
0
0
Medium ≈ 4s
1
0
Fast ≈ 2s
0
1
Faster response times reduce the optical interference immunity.
Very Fast ≈ 1s
1
1
Sensitivity:
See EN 54-10
8
Low Class 3
0
High Class 1
1
denotes Factory settings
Table 3 DIL Switch Settings
Theory of Operation
The detector responds to low-frequency (1 to 15 Hz.) flickering IR radiation emitted from flames during
combustion.
IR flame flicker techniques enable the sensor to operate through a layer of oil, dust, water vapour, or ice.
Most IR flame sensors respond to 4.3µm light emitted by hydrocarbon flames. By responding to 1.0 to
2.7µm light emitted by every fire all flickering flames can be detected. Gas fires not visible to the naked
eye e.g. hydrogen may also be detected.
The triple (IR³) IR photoelectric detector, responding to neighbouring IR wavelengths, enables it to
discriminate between flames and spurious sources of IR radiation.
The combination of filters and signal processing allows the sensor to be used with little risk of false
alarms in difficult situations characterised by factors such as flickering sunlight.
Signal Processing
The detector views the flame at particular optical wavelengths. The more differing optical wavelength
signals available the better the detector is at discriminating between flames and false optical sources.
The detector processes the optical signal information to determine if a flame is in view. This is achieved
by comparing the signals with known flame characteristics stored within the detector.