Thorn security MR300 Product Application Manual

DOCUMENT CONTROL NUMBER
MR300 OPTICAL SMOKE DETECTOR RANGE
PRODUCT APPLICATION & DESIGN INFORMATION
1.

INTRODUCTION

The MR300 range of optical smoke detectors forms part of the
M300 series of plug-in detectors for ceiling mounting. The
range is intended for two-wire operation on the majority of the
control equipment currently manufactured by the company. De-
tectors having different smoke sensitivities are offered as is an
intrinsically-safe type for use in hazardous atmospheres.
2.

OPERATING PRINCIPLE

2.1

OPTICAL SYSTEM

Detectors in the MR300 range detect visible particles produced
in fires by using the light scattering properties of the particles.
All detectors in the range use the same optical system which is
shown diagrammatically in Fig. 1.
The optical system consists of an emitter and sensor, with a lens
in front of each, so arranged that their optical axes cross in the
sampling volume. The emitter, with its lens, produces a narrow
beam of light which is prevented from reaching the sensor by
the baffles. When smoke is present in the sampling volume a
proportion of the light is scattered, some of which reaches the
sensor. For a given type of smoke,
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the light reaching the sensor is proportional to the smoke den-
sity. The output from the sensor can be used to activate an
alarm circuit at a pre-determined threshold.
2.2
In order to make a practical smoke detector which uses the type
of optical system described above, great care is needed in the
design of the housing. Both the optical components and the
sampling volume must be protected from the environment but
still allow smoke to enter freely into the sampling volume. The
housing must also be designed in such a way that dust settling
on its surfaces will not scatter so much light into the sensor that
false alarms are generated.
The emitter is a Gallium Arsenide [GaAs] solid state type op-
erating at a wavelength of 0.94
todiode. These devices with their associated lenses are held
within the optical array which also provides the baffles of
Fig.1. The design of this assembly is such that the presence of
very small insects [e.g. thrips] will not cause false alarms.
Fig. 1 M300 Detector Range, Optical System
M300 SERIES
01A-02-D4
02
/

FEATURES OF MEASURING CHAMBER

µ
m; the sensor is a silicon pho-
2/97
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