MIRROR: G K Bhat
In our previous articles we discussed a lot about various types of detectors. These detectors are small in size and can cover a limited area. Let us think a scenario of wide range area required to be covered. For example a big warehouse, where lot of materials are stored and there is no partition wall.To cover such area, the beam detectors are used and these detectors are highly effective in such buildings.
These detectors work on the principle of light obscuration, where the presence of smoke blocks some of the light from the beam, typically through either absorbance or light scattering. Once a certain percentage of the transmitted light has been blocked by the smoke, a fire is signalled.
There are two types of detectors
- Trans receiver type (Called double ended) or end to end type detectors.
- Trans reflector type (Called single ended ) or Reflective type
Trans receiver type: These detectors generally consisting of a transmitter, which transmits infra-red beam and a receiver to receive the transmitted infra-red ray from the transmitter. An end-to-end optical beam smoke detector is a system that has a separate light transmitter and receiver. They are used in applications where there is little available room to install a wide area detector – as the receiver is on a separate element each individual unit is quite small. The small size of the detector is also an advantage for aesthetic installations, where fire protection is required without introducing unsightly or overtly modern devices. Aesthetic considerations are especially important for cultural and heritage sites. End-to-end detectors include open-area smoke imaging detection, in which two wavelengths of light are used to detect smoke. UV and IR wavelengths of light react to smoke differently, and the comparative difference helps to verify real smoke by comparing the reflections and seeing a difference in the profile. UV and IR respond identically to things like blockage (ladder in front to detector path), bugs (blocking beam), fog, steam, and other things that commonly cause false alarms, so the two wavelengths of light are used together to detect smoke accurately.
In above image, there is a transmitter and a receiver. The rays which are transmitted are received by the receiver. The receiver is connected to a circuit board, which monitors the receiver continuously. When there is any variation in the receiver, the circuit breaks and the path connected to the alarm unit and control penal are get activated , resulting in generation of the alarm .
Reflective type: A reflective optical beam smoke detector incorporates a light transmitter and the detector on the same unit. The light path is created by reflecting light emitted from the transmitter off a retroreflector that is placed opposite the detector
The Transmitter (T) and the Receiver (R) are contained within one unit: the Transceiver (TR). The transmitted infrared light beam is reflected back by a reflector (prism) mounted directly opposite this unit, up to 100 metres away. The Receiver is connected to a Control Unit that can be either integrated into the Transceiver or installed at ground level.
Reflected Optical Beam Smoke Detectors utilises less wiring for reduced installation costs (power and wiring are only required at the Transceiver end) and only requires the Transceiver to be aligned. However the Reflected Optical Beam Smoke Detector can be vulnerable to reflective items close to the IR beam path. As the Reflector has a wide acceptance angle, it does not need to be aligned as accurately as both ends of a Projected Optical Beam Smoke Detector.
We will discuss in detail above detectors in our next topic .
G K Bhat