Heat Detector

The heat detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions, the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance of the thermistors and causing the detector to change to the alarm state.

                 Rate of Rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detector earlier.
Fixed heat detectors only change to the alarm state at a preset temperature.

UV/IR (Ultraviolet/infrared) Flame Detector.

UV/IR Detector, which makes use of an ultraviolet radiation sensitive phototube in addition to an infrared detector. This combination provides a flame detector which is highly immune to false alarms. The UV portion of the detector, as described in the previous section, is combined with an infrared detector, which responds to changes in the intensity of infrared radiation. By sensing very specific wavelengths in both the UV and IR spectra and then processing these Signals with a microcomputer, a very high degree of discrimination is achieved. Incorporated in the IR circuitry is a flicker discrimination circuit. This permits the detector to ignore steady IR sources such as hot objects. The inherent flickering of a flame provides the necessary modulation to activate the IR circuit. Since a flame is a copious source of both ultraviolet and infrared radiation, discrimination is provided when both UV and IR emissions are detected. If only UV is detected, as in the case of arc welding, no alarm is given. If only IR is detected, such as a large modulating hot object, no alarm is given. However, if both conditions are met in the correct combination and intensity, as determined by an algorithm in the microcomputer, a fire is identified and the alarm outputs are activated. 

Current Level          Detector status

        0mA                           Fault

        2mA                           Optical Integrity (io) Fault.

        4mA                            Normal Operation

        8mA                            IR Alarm Only

        12mA                          UV Alarm Only

        16mA                          Pre-Fire Alarm

         20mA                         Fire Alarm

UV ( Ultraviolet) Flame Detector.

Flame Detector which contains an ultraviolet photo tube that responds to radiation in the 185 to 260 nano meter region when radiation from a flame strikes the cathode plate within a UV detector tube, electrons are ejected from the cathode plate. These electrons are accelerated towards the positively charged anode of the tube. They collide with molecules of an ionization gas, with which the tube is filled. This emits more electrons and produces an avalanche condition. More   electrons are released which creates a momentary electron flow from the cathode to the anode. This momentary current (pulse) recurs at a rate proportional to the intensity of the UV radiation. This output converted to standard mA  output.

IR GAS DETECTOR

The infrared (IR) method of gas detection relies on the IR absorption characteristics of gases to determine their presence and concentration. IR gas detectors consist of an IR light source (transmitter) and light detector (receiver) to measure the intensity both at the absorption wavelength and a non-absorbed wavelength. If gas is present in the optical path, it will affect the intensity of light transmitted between the light source and the detector. This change in intensity provides the data for determining that a specific gas or type of gas is present. This method works only for gases that can absorb infrared radiation. Most hydrocarbon based gases absorb IR radiation at around 3.4 micrometers, which is transparent to both water and carbon dioxide vapors.Combustible IR gas detection can take one of two forms: either the point detector or the open path detector. The primary difference between point detectors and open path is the size of the IR path and its relationship to the gas/vapor sample source. The self-contained point detector has a smaller IR path than the open path detector, and is used to monitor fixed areas of space. The open path detector usually consists of a separate transmitter and receiver, which monitor much larger areas of space. Consequently, in some instances a single open path detector serves the function of multiple point detectors.

Working Principle of Combustible or Flammable Gas Sensor

Many gases and vapors are combustible. The catalytic bead converts the combustible materials to heat. A change in heat is then converted to a change in resistance, which can be measured.

Taking a matched pair of catalytic beads and coating one so that it does not respond to the presence of combustible gases can compare the change in resistance between the two beads. The bead that is coated is called the reference bead and the other is called the active bead. Because the beads are a matched pair, they will respond equally to changes in ambient temperature, humidity, and pressure. This makes the sensor virtually immune to changing environmental conditions.

  By connecting one end of each catalytic bead together, a series circuit is formed. This circuit is supplied with a constant current. The voltage drop across each of the beads will be identical in the absence of combustible gases. As combustible material is converted to heat, the resistance of the active bead increases and causing a voltage drop across each bead to be different. This difference is proportional to the amount of combustible gas that is present.

 The voltage from the sensor is amplified and fed to an Analog to Digital (A/D) converter and then made available to the microprocessor. The baseline and the gain for the amplifier are set using digital potentiometers. They are adjusted by the microprocessor during calibration.

OPTICAL SMOKE DETECTOR

In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm.

 In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell (light receiving cell), positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.