09 July 2004

Where there's smoke, there's (not always) fire: An inside look at Smoke Detectors

Mike Dziekan

Part 1. Flame and heat detectors

Most of us know that a smoke detector, detects fires, and most of us would be wrong! To drive my point home, how many of us have inadvertently set off the smoke detector while taking a hot, steamy shower? Probably quite a few. I'll ask again, where was the fire? There wasn't any!

Although smoke detectors do respond to smoke from fires, they will also respond to a wide variety of non-fire situations. This was the reason for the line in the title, "Where there's smoke, there's ( not always ) fire."

Smoke detectors

To be precise, we should state that "smoke" detectors detect the presence of airborne particulate matter that may be produced by fire or by other sources. It is important to understand that smoke detectors are not necessarily fire detectors.

The industry jargon when dealing with smoke sensing devices is to simply call them "initiating devices." The National Fire Protection Association (NFPA) is a 75,000 member organization that has developed considerable information on such devices. A general smoke detector information FAQ is available from NFPA.

In addition to coming up with all sorts of necessary technical jargon, NFPA is the standard for anything related to fire protection and safety. NFPA publishes its "codes and standards" that dictate how to handle every aspect of the installation of a fire detection and warning system installation, as well as ancillary devices. NFPA updates the NFPA 72 code every three years. The current version is 2002.

Those of you enjoy getting migraines and hitting your thumb with a hammer might be interested in reading the national fire alarm code related to placement of smoke detectors. The code relevant to smoke detectors is NPFA 72 (Chapter 5. Initiating Devices).

Be prepared to shell out $46.25 (Non-member price). Unless you are a licensed fire alarm installer, LAHJ (Local Authority having Jurisdiction), or you work with/for a fire department, you probably don't want to get into this many detailed specifications (trust me on this, there are people who write books on how to read and interpret the NFPA 72 Code!).

NFPA 72 was written to provide requirements for the installation, performance, testing, inspection, and maintenance of the fire alarm system. If you want to know if a fire alarm system is required for a given occupancy, then NFPA 101, Life Safety Code and other related codes make that determination. Underwriters Laboratories (UL) also has additional information dealing with smoke detectors and fire alarm systems .

Sensor types

Before we go further, we need to discuss a few basics. First, here are the most common types of sensors for the detection of smoke and/or fire:

•  Optical Flame detectors

•  Ultraviolet (UV) Sensitive

•  Visible Sensitive

•  Infrared (IR) Sensitive

•  Heat Detectors

•  Fixed or Rate compensated

•  Rate of Rise

•  Smoke detectors

•  Photoelectric Type

•  Ionization Type

•  Combination Type or "Combo Detector"

There is also ongoing research on the acoustic detection of fires . To the best of my knowledge, this is still research and has not led to any commercial applications.

To further break down the detector groupings, there are two sub-groups known as "Spot type" and "Line type" initiating devices. The NFPA definitions of Spot and Line type are as follows:

A) NFPA Preferred Definition of a Line type device - A device in which detection is continuous along a path. Typical examples are rate-of-rise pneumatic tubing detectors, projected beam smoke detectors, and heat-sensitive cable .

B) NFPA Preferred Definition of a Spot type device - A device in which the detecting element is concentrated at a particular location. Typical examples are bimetallic detectors, fusible alloy detectors, certain pneumatic rate-of-rise detectors, certain smoke detectors, and thermoelectric detectors.

A Spot type detector will provide coverage for a limited area, or small spot, while the line sensing type can protect or monitor very large areas, such as large atriums. Spot type detectors have a maximum theoretical rated coverage of 900 sq. ft (30 ft x 30 ft) in large open rooms. If placed in a narrow hallway, the maximum allowed rated coverage might be increased.

The Line type sensors are typically of the "Projected Beam" or heat sensitive cable variety. In the average residential home, all detectors will most likely be of the Spot type (unless you are Bill Gates). The maximum theoretical rated coverage area for the System Sensor manufactured projected beam detector can be as large 19,800 sq. ft. Note: This maximum coverage area for Spot type and Line type detectors is only a general statement, and should not be used in every circumstance.

Heat detectors

Heat detectors can be electrical or mechanical. The most common type are thermocouples that senses ambient temperature and provide an alarm signal if the ambient temperature rises above the alarm threshold. Heat detectors are further broken down into two main classifications, "rate-of-rise" detectors, and "fixed" or "rate compensated."

•  Rate-of-rise heat detectors react to the sudden change or rise in ambient temperature from a normal baseline condition. Any sudden temperature increase that matches the predetermined alarm criteria will cause an alarm. This type of heat detector can react to a lower threshold condition than would be possible if the threshold were fixed. A typical alarm may sound when the rate of temperature rise exceeds 12 ° to 15 ° F degrees per minute.

•  Fixed threshold, or rate compensated heat detectors react to a preset threshold and will not activate until the preset threshold is crossed, regardless of the rate of temperature increase. If there is too much thermal lag in the design, the alarm threshold can be exceeded before an alarm condition is indicated.

If you have a gas- or oil-fired furnace or water heater in your house, you may have noticed an odd looking detector just above it. This is a heat detector. If a photoelectric or ionization type smoke detector were used, then there might be frequent nuisance alarms caused by emissions from the flame.

Each type of heat detector has its advantages, and one cannot say that one type of heat detector should always be used instead of another. If you were to place a rate-of-rise (ROR) heat detector above a large, closed oven, then every time the door is opened a false alarm could be generated due to the sudden heat transient. In this circumstance the fixed threshold detector would probably be best. If a room is protected with a fixed heat detector filled with highly combustible materials, then a fast flaming fire could exceed the alarm threshold due to thermal lag. In this case the ROR heat detector may be preferred.

Optical flame detectors

Optical flame detectors react to the light produced by fire. They may detect ultraviolet (UV), infrared (IR) or visible light emitted by a flame. The selection of a UV, IR or visible optical flame detector depends on the type of materials that may combust or burn. Optical flame detectors respond only to a flaming fire and not smoke. If the combustible materials produce large quantities of smoke, then an IR sensor is preferred. This because IR penetrates smoke, and visible and UV light are blocked or scattered.

Selecting a detector

The final decision about which type of optical sensor should be used, must consider many factors, including the size of the room, height of the ceiling, composition of combustible materials, activities that may cause false alarms (such as using an arc welder in a room protected with a UV or visible flame detector), the air handling system (HVAC), and so forth. The many variables involved in the selection of sensors may instill in you a deeper respect for those individuals charged with the task of coming up with the NFPA codes and standards.

It is important to note that neither optical flame and heat detectors cannot detect smoke. They can only react to a flaming fire, not an incipient one. For additional information on heat detectors and optical flame detectors, see http://app.scdf.gov.sg/data/html/pChapterfile/42.pdf . For more on how fires are handled in low gravity environments, check out Fire Safety in the Low-Gravity Spacecraft Environment .

NFPA 72 also dictates where detectors must be placed for proper operation and, more importantly, where a detector must not be placed. If a detector is placed improperly, a fire might not be detected, even though the detector is fully functional and working!

Part 2

Of what use is a smoke detector if it is placed in an area where the smoke can never reach it? When purchasing a smoke detector for residential use, be sure to read the manufacturer's instructions. There is a wide variety of "smoke detector" like sensors, so I am going to limit the discussion to only photoelectric and ionization smoke detectors. In Part 2 we will start our discussion with photoelectric smoke detectors.