Tyco MR601TEx Product Information

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MR601TEx INTRINSICALLY SAFE ENHANCED OPTICAL SMOKE DETECTOR

PRODUCT APPLICATION AND DESIGN INFORMATION

1. INTRODUCTION

The MR601TEx Intrinsically Safe High Performance Optical
Smoke Detector forms part of the M600Ex series of plug in
detectors for ceiling mounting. The detector plugs into the
5BEx 5” Universal IS Base and is intended for two-wire
operation on the majority of the control equipment currently
manufactured by the company. The Intrinsically Safe High
Performance Optical detector is available in one sensitivity
setting only.

2. INTRINSIC SAFETY

The detectors are for use in potentially explosive gas and dust
atmospheres (zone 0 gas, zone 20 dust).

The detectors are designed to comply with EN/IEC 60079­0:2006, EN/IEC 60079-11:2007 and EN/IEC61241-11:2006 for
Intrinsically Safe apparatus. They are certified:

ATEX code:

II 1 GD

Certificate: BAS01ATEX1134X

Gas/Dust code: Ex ia IIC T5

Ex iaD 20 T100°C

IECEx Certificate: IECEx BAS 07.0056X

These detectors are designed and manufactured to protect
against other hazards as defined in paragraph 1.2.7 of Annex II
of the ATEX Directive 94/9/EC.

2.1 DETECTOR USE

It is recommended that the detector is used in conjunction with
a suitable isolator or shunt diode safety barrier in a certified
Intrinsically Safe system, ie, System 620.

2.2 SPECIAL CONDITIONS OF SAFE USE

The apparatus has a plastic enclosure which constitutes a
potential electrostatic hazard. The enclosure must be cleaned
only with a damp cloth.

3. OPERATING PRINCIPLE

The MR601TEx operates by sensing the optical scatter from
smoke particles generated in a fire. While the optical scatter
detector can give good detection performance for the majority
of fires, some fast burning fires produce little visible smoke and
some produce very black smoke, neither of which are easily
detected by the optical scatter detector. (Such fires are
represented in EN54-7 by Polyurethane and Heptane type fires
respectively). These fires do, however, produce high heat
outputs with an associated rise in air temperature.

The detector has been designed to offer improved detection of
such fires, by detecting the rapid rate-of-rise of air temperature
and under these conditions, increasing the smoke detection
sensitivity. This gives an earlier detection of such fires and a
broader detection capability than a standard detector.

The MR601TEx detector has two sensing systems as follows:

• An optical chamber with associated electronics

to measure the presence of smoke by light
scatter.

• A thermistor with its associated electronics to

detect the presence of hot air draughts or high
temperatures.

Fig. 1 Optical System Schematic

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SMOKE

PATH

SAMPLING

VOLUME

FINS

Fig. 2 Measuring Chamber Showing Smoke Flow Path

3.1 OPTICAL SYSTEM

The MR601TEx detects visible particles produced in fires
by using the light scattering properties of the particles. The
detector uses the optical arrangement shown
diagrammatically in Fig. 1.

The optical system consists of an infra-red emitter and
receiver, 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 receiver by the baffles. When
smoke is present in the sampling volume a proportion of the
light is scattered, some of which reaches the receiver. For a
given type of smoke, the light reaching the photodetector is
proportional to the smoke density. The amplified output
from the sensor can be used to activate an alarm circuit at a
predetermined threshold.

FINS

REFERENCE

SENSING

Fig. 3 Thermal Measuring System

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Registered Company: Thorn Security Ltd. Registered Office: Dunhams Lane Letchworth Garden City Hertfordshire SG6 1BE

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STATUS
LED

IR
EMITTER

PHOTO
DIODE
SENSOR

AMPLIFIER

GAIN

TIMER/
OSCILLATOR

SYNCHRONOUS
DETECTOR

COMPARATOR

REF

SENSING
THERMISTOR

REFERENCE
THERMISTOR

1

2

COMPARATOR

THRESHOLD

LED

OUTPUT
SWITCH

COUNTER

LINE A IN

LINE A OUT

LINE B IN/OUT

BRIDGE

BASE

R

L1
L2

L

REMOTE LED

Fig. 4 Block Schematic of Detector

3.2 FEATURES OF MEASURING CHAMBER

The MR601TEx uses vertical chevrons to exclude ambient
light.

Smoke incident on the detector is channelled into the
detector by the outer cover fins (Fig. 2) and passes through
the vertical chevrons. The smoke is deflected into the
optical chamber and through the sampling volume before
passing out the other side of the detector.

The emitter (Fig. 1) is a GaAlAs solid state type operating
in the near infra-red (880nm peak), while the detector is a
matched silicon photodiode. These devices, together with
their associated lenses, are held in place by the chamber
mouldings. The design of the optical system is such that the
presence of small insects such as thrips, should not cause
false alarms.

3.3 THERMAL MEASURING SYSTEM

Refer to Fig. 3.

This is designed to detect the presence of horizontally
moving hot air draughts moving across the ceiling which
occur in a fast burning fire.

The measuring system consists of two fast responding
negative temperature thermistors. A sensing thermistor is
located above the labyrinth under the cover in the airstream
and will detect any sudden changes in the air temperature or
draughts of hot air moving across the ceiling. The second
thermistor is located out of the airflow within the smoke
labyrinth and has a longer time constant and is used as a
temperature reference to compare the sensing thermistor
against. At a given temperature differential between the
two thermistors, the comparator will switch and increase the
gain of the amplifier, thereby increasing the sensitivity of
the sensor. Fins located on the top of the labyrinth are
designed to increase air turbulence and the efficiency of the
sensing thermistor.

3.4 CIRCUIT OPERATIONS

A simplified block schematic of the detector is given in
Fig. 4.

The emitter is subjected to a pulse stream only every 10s in
order to reduce the quiescent current. The pulse signal
received by the photodiode is fed to a high-gain
amplifier. If smoke is present, the pulse signal received
varies in proportion to the smoke density.

The amplifier output is fed via an integrator, the output of
which is compared to a preset threshold
level. Sophisticated synchronous detection techniques are
used to reduce the effects of noise and spurious transients.

The gain of the front end amplifier is controlled by the
thermistor bridge circuit. When the temperature differential
between the two thermistors exceeds a certain value, the
amplifier gain increases. Under these conditions the High
Performance Optical detector is more sensitive to the
presence of smoke and is said to be in ‘Enhanced Mode’.

When the detector is in the ‘Enhanced Mode’, the detector
will only alarm if a smoke signal is present. The presence
of rising temperature alone cannot cause an alarm.

If the signal amplitude exceeds a threshold level, then the
emitter samples the smoke every two seconds. The sample
period remains at two seconds if the signal is above the
threshold. When the counter has counted three consecutive
pulses above the threshold, the output stage is latched into
the alarm condition. If however, the amplitude of the
second or third pulse is below the threshold, then the pulse
period reverts to 10 seconds and the counter resets. The
switching of the output stage lights the alarm LED and
provides drive for the remote LED indicator.

The critical front end of the circuit is run off a 12V regulator
to make it independent of supply voltage.

The detector is polarity conscious.