CHIEF ENGINEER Unit 18 Fire Q5

An obscuration type smoke detector, also known as a light obscuration or beam detector, is a type of fire detection device that operates by measuring the reduction in light intensity caused by smoke particles in the air.  

1. A Guide to Smoke Detector Types – Who Will You Meet

Working Principle:

1. Light Source and Receiver: The detector has a light source (typically an infrared LED) and a light receiver (photodiode) positioned at a distance from each other, creating a beam of light across a monitored area.   1. Optical beam smoke detector – Wikipedia en.wikipedia.org
2. Smoke Detection: When smoke enters the beam path, the smoke particles scatter and absorb some of the light, reducing the intensity of the light reaching the receiver.   1. Remote Sensing of the Light-Obscuring Smoke Properties in Real-Scale Fires Using a Photometric Measurement Method – SpringerLink link.springer.com
3. Alarm Trigger: The detector’s electronics continuously monitor the received light intensity. If the intensity drops below a predetermined threshold (indicating a certain level of smoke obscuration), the detector triggers an alarm, signaling a potential fire.

Advantages:

• Large Area Coverage: Beam detectors are ideal for monitoring large open spaces with high ceilings, such as warehouses, atriums, or aircraft hangars, as they can cover long distances (up to 100 meters or more).   1. How Conventional Beam Detectors Enhance Fire Safety in Large Spaces – TNA Fire Alarm www.tnafirealarm.com
• Early Detection: They can detect smoke at relatively low concentrations, potentially providing early warning of a fire before it becomes fully developed.
• Cost-Effective: Compared to installing multiple point-type smoke detectors in large spaces, beam detectors can be a more cost-effective solution.

Disadvantages:

• Limited Sensitivity to Smoldering Fires: Beam detectors may be less sensitive to smoldering fires that produce little visible smoke.
• Susceptible to False Alarms: Dust, insects, or other airborne particles can also obscure the light beam, potentially triggering false alarms. Proper alignment and maintenance are crucial to minimize false alarms.   1. Smoke detectors false alarms through dust – Zehnder Clean Air Solutions www.zehnder-cleanairsolutions.com
• Not Suitable for All Environments: They are not ideal for areas with high airflow, steam, or dust, as these factors can interfere with the light beam and affect detection accuracy.

Applications:

• Large open spaces with high ceilings
• Warehouses
• Atriums
• Aircraft hangars
• Industrial facilities

Conclusion:

Obscuration type smoke detectors are a valuable tool for fire detection in large open areas. They offer the advantages of wide coverage, early detection, and cost-effectiveness. However, their limitations in certain environments and potential for false alarms must be considered when selecting the appropriate fire detection system for a specific application.

A light scatter type smoke detector, also known as an optical smoke detector, is a common type of fire detection device that works on the principle of light scattering by smoke particles. It’s designed to detect the presence of smoke in the air, which is an early indicator of a fire.  

1. How Optical Smoke Alarms Work – Safelincs

2. Dual-Wavelength Smoke Detector Measuring Both Light Scattering and Extinction to Reduce False Alarms – MDPI

How it works:

1. Light Source: The detector contains a light source, typically a Light Emitting Diode (LED), which emits a beam of light into a sensing chamber.   1. How Optical Smoke Alarms Work – Safelincs www.safelincs.co.uk
2. Sensing Chamber: This chamber is designed to minimize the entry of light under normal conditions. It has a light-sensitive device, usually a photodiode, positioned at an angle to the light beam.   1. How Optical Smoke Alarms Work – Safelincs www.safelincs.co.uk
3. Smoke Entry: When smoke enters the chamber, the smoke particles scatter the light in various directions.   1. What Is Scattering Of Light? – Rayleigh Scattering Theory | BYJU’S byjus.com
4. Light Detection: Some of this scattered light reaches the photodiode, which generates an electrical signal.   1. How Optical Smoke Alarms Work – Safelincs www.safelincs.co.uk
5. Alarm Trigger: The detector’s electronics analyze the signal from the photodiode. If the signal exceeds a predetermined threshold, indicating a sufficient level of light scatter caused by smoke, the alarm is triggered.   1. How Optical Smoke Alarms Work – Safelincs www.safelincs.co.uk

Advantages of Light Scatter Smoke Detectors:

• Sensitive to Smoldering Fires: These detectors are particularly good at detecting smoldering fires, which produce a lot of smoke but may not have visible flames initially.   1. Ionization vs photoelectric – NFPA www.nfpa.org
• Widely Used and Affordable: They are the most common type of smoke detector due to their effectiveness and relatively low cost.
• Fewer False Alarms: Compared to ionization type smoke detectors, they are less prone to false alarms caused by cooking fumes or steam.

Disadvantages:

• May Not Detect Fast-Flaming Fires Quickly: Light scatter detectors might be slower to respond to fast-flaming fires that produce less smoke initially.
• Susceptible to Dust and Dirt: The sensing chamber can become contaminated with dust or dirt, potentially leading to false alarms or reduced sensitivity. Regular cleaning is necessary.

Applications:

• They are widely used in residential and commercial buildings for general fire protection.
• Particularly suitable for bedrooms, living rooms, and hallways.

In summary, a light scatter type smoke detector is a reliable and cost-effective fire detection device that works by sensing the scattering of light caused by smoke particles. Its sensitivity to smoldering fires and relatively low rate of false alarms make it a popular choice for various applications.

Both obscuration type and light scatter type smoke detectors are optical smoke detectors, meaning they use light to detect the presence of smoke. However, they differ fundamentally in how they utilize light to identify smoke particles.  

1. Smoke Detectors – AFA Protective Systems

2. A Guide To Smoke Detectors And Fire Alarms – Electronics For You

Obscuration Type Smoke Detector (Beam Detector)

• Principle: Works on the principle of light blockage or obscuration. It has a light emitter and a receiver aligned in a straight path, creating a beam of light. When smoke enters this path, it obscures or blocks a portion of the light beam, reducing the intensity of light reaching the receiver. This reduction triggers the alarm.   1. Smoke detector – Wikipedia en.wikipedia.org2. Optical beam smoke detector – Wikipedia en.wikipedia.org3. Initiating Devices: Smoke Detectors, Duct Smoke Detectors, Beam Detectors – DGA Blog blog.dga.com
• Best for: Large, open areas like warehouses, atriums, or high-ceilinged spaces where covering a long distance with a single beam is advantageous.   1. What Are The Differences Between Each Type Of Detector? – Rhino Fire Control rhinofirecontrol.co.uk
• Sensitivity: Can be less sensitive to smoldering fires with low smoke density.
• Potential Challenges: Susceptible to false alarms due to dust, insects, or misalignment of the beam.

Light Scatter Type Smoke Detector

• Principle: Operates on the principle of light scattering (Tyndall effect). A light source is positioned in a chamber with a photodetector placed at an angle, normally shielded from the light source. When smoke enters the chamber, the smoke particles scatter the light, and some of this scattered light reaches the photodetector. This increased light intensity triggers the alarm.   1. How Optical Smoke Alarms Work – Safelincs www.safelincs.co.uk2. Detector Heads – Fire Safety Bridgend saintdavidsfire.co.uk3. Smoke detectors explained www.ifsecglobal.com4. An optical fire detector with enhanced response sensitivities for black smoke based on the polarized light scattering – ResearchGate www.researchgate.net
• Best for: Smaller, enclosed spaces like homes, offices, or ship cabins.
• Sensitivity: More sensitive to smoldering fires that produce a lot of smoke, even at lower densities.
• Potential Challenges: Can be susceptible to false alarms from dust or dirt buildup inside the chamber.

Key Differences Summarized:

• Principle of Detection: Obscuration detects a decrease in light intensity, while light scatter detects an increase in light intensity.
• Smoke Detection: Obscuration detectors measure the blockage of light by smoke, while light scatter detectors measure the light scattered by smoke particles.   1. A Guide To Smoke Detectors And Fire Alarms – Electronics For You www.electronicsforu.com
• Ideal Applications: Obscuration detectors are better for large open spaces, while light scatter detectors are suitable for smaller, enclosed areas.
• Sensitivity to Fire Types: Obscuration detectors may be less sensitive to smoldering fires, while light scatter detectors excel at detecting them.
• Potential False Alarms: Obscuration detectors are susceptible to false alarms from misalignment or obstructions in the beam path. Light scatter detectors can have false alarms due to dust or dirt in the chamber.   1. Dual-Wavelength Smoke Detector Measuring Both Light Scattering and Extinction to Reduce False Alarms – MDPI www.mdpi.com

Both types of detectors play crucial roles in fire safety, and the choice between them often depends on the specific application and the type of fire most likely to occur in that environment.