Principles Of Operation - Simplex 4098 Applications Manual

Principles of Operation

Introduction
This section describes how the different types of detectors and sensors work. For additional
information on TrueAlarm sensor operation, refer to the TrueAlarm Concepts publication
(PER-91-024).
Heat Detector
The heat detector senses the heat or the Rate-of-Rise (ROR) in the air temperature of the
Operation
environment in which it is located. The heat detector is comprised of electronic circuitry and a
mechanical package that is designed to sense the ROR of the air temperature in an expedient and
reliable fashion. Upon detection of an abnormal increase in air temperature, or ROR in air
temperature, the electronics indicate an alarm by increasing the amount of current draw from the
monitor zone it is connected to. The monitor zone is a supervised detection circuit that is tied
back to a main control panel that takes appropriate action to indicate an alarm has been reported,
if the zone current is substantially increased.
Being of an electronic design, the temperature of the air is sensed by using two negative
temperature coefficient thermistors. The resistance of the thermistors goes down with an increase
in temperature. One thermistor is placed in a position such as to sense the open air temperature
very rapidly (RT1). The second thermistor is positioned in a small cavity that protrudes out from
the main body of the detector (RT2). The location of RT2 allows for fast detection of a quick
change in the air temperature, but yet for a slow or medium rate of temperature change, the
detector does not trip due to the ROR feature. For a slower change in temperature, the detector
trips into alarm due to a set fixed temperature that is sensed by RT1. For a fast temperature rise,
when a difference in temperature sensed by RT1 and RT2 has reached a predetermined amount,
the detector trips into alarm.
Photoelectric Smoke
These devices operate on a light scattering principle. The smoke sensing chamber contains an
Detectors/Sensors
infrared LED source with a peak spectral emission of 880 nanometers. This source is placed at an
angle from a spectrally matched photodiode receiver. During a NO SMOKE condition, only light
reflected from the chamber walls enters the receiver and shows up as a small photocurrent. As
smoke particles enter the sensing chamber and cross the light beam of the LED, more light reaches
the receiver due to scattering. The receiver circuitry converts this photocurrent into a signal
voltage. In a detector, when this voltage reaches a preset level, an alarm is produced. In a sensor,
this signal voltage goes into an 8-bit, A to D (analog to digital) converter. A digital representation
of this signal voltage is then transmitted to the fire alarm panel for further processing.
Ionization Smoke
These devices use a small radiation source, Americium-241, which emits alpha particles that
Detectors/Sensors
ionize air molecules between two electrically charged electrodes. With the application of a
DC voltage .to these electrodes, a small ionization current flows within the chamber. As smoke
enters the chamber, a decrease in ionization current results. This current is converted into a signal
voltage by a transimpedance circuit. In a detector, when this signal voltage drops below a preset
level, an alarm is produced. In a sensor, this signal voltage goes into an 8-bit A to D (analog to
digital) converter. A digital representation of this signal voltage is then transmitted to the fire
alarm panel for further processing.
Carbon Monoxide
The CO Sensor and Sounder Bases with a CO Replaceable Sensor (CRS) module work in
Gas
conjunction with existing detector heads. The CO Sensor Base enhances fire detection and
Detectors/Sensors
provides the ability to combine fire and toxic gas leakage detection in a single device. The CO
sensor measures the concentration of carbon monoxide gas in the air in part per million (PPM).
When toxic gas operation is selected, the FACP panel may generate either a supervisory, utility or
priority 2 alarms. The alarm level is calculated within the panel based on the time integrated CO
levels that have been measured at the detector.
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