A fire alarm detector (sometimes called a “fire sensor”) is a crucial element of any fire alarm system—alerting building occupants and responders at the earliest stage of ignition. Detectors vary in detection principle, from temperature and smoke to open flame. This guide outlines the most common detector types—smoke, heat, linear, flame, and manual—how they operate, and the situations for which they’re best suited.
1. Smoke Detectors
Smoke detectors are the most widespread fire sensors, working by analyzing particles produced during combustion. Because smoke flows with air currents, these devices are sensitive to ventilation conditions, making proper placement critical.
Types of Smoke Detectors
Optoelectronic (Photoelectric) Detectors
- Use an infrared (IR) beam and photoreceiver in a smoke chamber.
- Smoke particles scatter the IR light toward the photodiode, triggering an alarm when the intensity crosses a threshold.
- Suited for smoldering fires but can false-trigger in dusty or steamy areas.
Ionization Detectors
- Employ a small radioactive source (e.g., Americium-241) creating ion pairs in two chambers: one sealed (reference) and one open (measuring).
- Smoke reduces ion flow, causing an alarm once levels deviate from the reference.
- Highly sensitive to fast-flaming fires; dust or humidity can cause nuisance alarms.
Spot vs. Linear
- Spot (Point) Detectors: Typically the familiar round devices on ceilings.
- Linear Smoke Detectors: Use IR beams across a room or corridor to detect reduced light from smoke. Ideal for large, high-ceiling areas (e.g., warehouses).
2. Heat Detectors
Heat detectors respond to changes in temperature—especially useful in dusty or humid environments where smoke detectors may be unreliable.
Types of Heat Detectors
Fixed (Maximum)
- Triggers once temperature exceeds a preset threshold (e.g., 135–160°F).
- Basic versions use a fusible link that melts; more advanced models have an adjustable setpoint.
Rate-of-Rise (Differential)
- Activates if temperature climbs rapidly, indicating a fast-developing fire.
- If ambient heat increases gradually (e.g., normal environment changes), no alarm is raised.
Combination (Max-Differential)
- Uses both fixed and rate-of-rise elements, enhancing early detection for varying fire scenarios.
Recommended Usage
- Areas where smoke might dissipate or get diluted (drafty spaces, kitchens, or dusty areas).
- Storage areas containing materials that produce more heat than smoke.
3. Linear Fire Detectors
Linear fire detectors scan along a line or cable, detecting temperature or smoke across a distance. They come in several designs:
A. Linear Heat Detectors
Thermal (Electronic) Cable
- Changes electrical resistance with temperature changes.
- An associated control module interprets these variations and triggers an alarm upon reaching a threshold.
Electromechanical Cable
- Two conductors enclosed in heat-sensitive insulation.
- When the insulation melts at high temperature, wires contact, signaling an alarm.
Pneumatic Tube
- A sealed tube filled with gas, connected to a pressure sensor.
- Heating expands the gas, raising pressure beyond a limit—alarm triggers.
B. Linear (Beam) Smoke Detectors
- Active IR devices shooting a beam across the protected space.
- Two-Position (Bi-Static): Transmitter and receiver face each other.
- One-Position (Reflex): Transmitter/receiver in one housing, with a reflector on the opposite side.
- Common for large open areas (e.g., atriums), typically up to 300 ft (100 m) range.
4. Flame Detectors
Flame detectors target open fire by sensing UV or IR light emitted during combustion. They are highly specialized and relatively expensive, often deployed in:
- Industrial facilities with flammable liquids or gases.
- High-hazard areas with immediate flame presence risk.
Types of Flame Sensors
Infrared (IR)
- Detect IR emissions in a particular band (commonly 4.4 μm).
- Some models can identify a 10 cm² flame at up to 65 ft (20 m) within 3 seconds.
Ultraviolet (UV)
- Monitor wavelengths around 200–280 nm.
- Effective at up to 650 ft² (60 m²) coverage or more, depending on design.
5. Manual Call Points (Pull Stations)
Manual fire alarm “pull stations” (or manual call points) allow human-initiated alarms. They are typically:
- Brightly colored (red), labeled “FIRE ALARM.”
- Positioned along evacuation routes at ~5 ft (1.5 m) from floor level, typically on the right-hand side of exits.
- Placed every 50 ft (15 m) indoors and up to 150 ft (45 m) outdoors (depending on local code).
Activation & Design
- Usually includes break-glass or push-button style activation, often behind a transparent cover.
- Once triggered, it stays latched in alarm until reset by an authorized person.
Best Practices & Considerations
Detector Output & Addressability
- Threshold (non-addressable) sensors have a simple “alarm” or “normal” signal.
- Addressable detectors send more data (fault, maintenance alert, or exact sensor location).
Repeatability & Reusability
- Most modern sensors are multi-use, resetting automatically or via a panel command.
- Some single-use devices (like certain thermal links) must be replaced after activation.
Installation Environment
- Smoke Detectors: Avoid dusty or steamy areas.
- Heat Detectors: Great for high-dust or kitchen-like settings.
- Flame Detectors: Ideal for open-air or specialized industrial hazards.
- Manual Call Points: Must be visible and accessible, never hidden behind panels or décor.
Maintenance & Codes
- Follow NFPA 72 (in the US) or local equivalents for placement, testing intervals, and documentation.
- Inspect and clean detectors regularly, especially in dusty or high-traffic areas.
Conclusion
Effective fire alarm systems usually deploy a combination of smoke, heat, linear, flame, and manual detectors to cover all potential fire scenarios—whether slow, smoldering, or explosive combustion. Each detector type has unique advantages and constraints:
- Smoke (spot or beam) for a wide range of flaming/smoldering fires.
- Heat (fixed, rate-of-rise, or combination) for dusty or heat-intensive areas.
- Linear (cables or beams) for large spaces and tough-to-reach zones.
- Flame detectors for immediate recognition of open fires or explosive ignitions.
- Manual Call Points enable occupants to trigger alarms at the first sign of trouble.
By selecting detectors aligned with the environment’s fire risks and operational needs, building owners can ensure a faster response to fires and better protection for people and property. For personalized advice on designing or upgrading a fire alarm system, visit safsale.com—where our experts can tailor solutions that conform to local codes and best practices.