Issue: 3/04

It's obvious to most people that the smoke detectors that work in our homes are not suitable to operate in large factories or other industrial environments. What's not so obvious, however, is deciding which devices would be most appropriate under diverse conditions. Understanding what different smoke detectors are designed to do and how well their design and functions match up with the requirements and conditions of different facility environments goes a long way toward knowing where and when to use a certain type of detector.

Zone-specific smoke detectors, for example, are designed for use in particular settings, and they address the need to match the characteristics of fire and smoke detection systems to particular applications in unique environments. Making this distinction between detectors is critical when building managers, installers, engineers, salespeople and other professionals figure out how smoke detector requirements differ between facility types, such as office buildings, warehouses and museums, as well as between rooms or spaces within each facility, including clean rooms, vacant offices, auditoriums and cafeterias.

An important time to determine which type of smoke detector is right in a specific situation is when a building is undergoing renovations. A detector previously installed in an office space, for example, will likely provide inadequate protection if the space is converted to a hotel. By neglecting to implement zone-specific fire detection technology in new construction projects or to upgrade systems during renovations, the overall integrity of today's integrated facilities can be jeopardized. This can render data, electrical and other building systems vulnerable due to protection gaps, possibly resulting in tremendous cost and aggravation should a fire occur.

To counter this, those responsible for recommending or purchasing smoke detectors should identify potential sources of fire in each zone and the expected usage of the space. After these issues have been identified, the process of finding the right smoke detector that contains the right sensor type which, of course, also complies with local codes can begin.

Figure 1. For photoelectric smoke detectors operating on the light-scattering principle, a light-emitting diode is beamed into an area not normally "seen" by a photosensitive element. (Illustration courtesy of System Sensor.)


Understanding Sensor Types

The two types of sensors commonly found in smoke detectors are ionization and photoelectric models, both of which use different methods to identify particles of combustion. A third type, a thermal sensor, monitors heat registration to identify fire scenarios. When detectors use a combination of any of these sensors, it is deemed a multi-criteria detector.

A typical ionization sensor chamber consists of two electrically charged plates and a radioactive source--typically Americium 241--for ionizing air between positive and negative plates. As these charged air molecules collide, some gain electrons, while others lose electrons, resulting in an even amount of positive and negative ions. Because opposites attract, the ions migrate to the charged plates, creating a measurable ionization current.

When smoke enters the chamber, the ions effectively "stick" to the smoke particles, reducing the number of ionized particles, which results in a decrease in electrical current. Once the current decreases by a predetermined amount, an alarm initiates.

Ionization sensors almost immediately recognize fast-flaming fires that are characterized by combustion particles in the .01- to .3-micron size range and can reliably detect smoke from most common combustible products. But due to their operating principles, ionization sensors offer limited capabilities when installed in high altitude locations, areas with high air velocity or near kitchens.

Photoelectric sensors use a continuous light beam to determine the presence of smoke using one of two methods. The simplest photoelectric technique is light obscuration, which shoots a direct beam of light to a receiving photodiode. As smoke particles enter the sensor chamber and block the continuous light source, the strength of the received signal weakens, initiating an alarm.

Light-scattering photoelectric models beam light away from the receiving diode. When smoke enters the laser beam, light reflects off particles into a receiving photodiode, causing an alarm to sound if enough light is registered to surpass the threshold of the sensor.

Photoelectric sensors very quickly recognize smoldering fires that are characterized by combustion particles in the .3- to 10.0-micron size range, but cannot "see" the full range of smoke at the same intensity as an ionization sensor. Photoelectric sensors instantly identify visible white smoke, while dark smoke produced by fires containing plastics and rubber is not recognized as quickly. Overcoming this problem requires amplifying the photoelectric sensor's sensitivity to detect dark smoke, which, unfortunately, increases the risk of false alarms.

The third type, called a thermal sensor, identifies heat energy as opposed to particles of combustion. Thermal sensors initiate an alarm after temperatures reach a pre-determined level or surpass allowable rate-of-rise temperature increases. A rate-of-rise thermal sensor consists of an air chamber, a flexible metal diaphragm and a moisture-proof, calibrated vent. Because air expands when it is heated, any fumes from a developing fire that enter the air chamber will expand faster than the sensor can be vented, creating pressure that initiates an alarm at pre-determined levels.

Thermal detectors, which are rarely used as single-sensor devices, have carved a niche in harsh environments. They should, however, never be installed in sleeping quarters, attics or spaces that have large temperature swings.

Traditionally, ionization sensors were specified for fast-flaming atmospheres, such as chemical storage areas, while photoelectric detectors were designed for offices and other areas where smoldering fires are more common. The reality is that both types of these single-sensor detectors must pass the same industry tests, and the response time of any detector to specific stimuli will be relatively uniform across the industry.

The main difference between the two is the stimuli that initiate false alarms. Dust or dirt can accumulate on the radioactive element in ionization detectors, causing these devices to be too sensitive, and intensely lit areas or steam can falsely set off photoelectric devices.


The Role of Multi-Criteria Detectors

Ionization detectors had dominated the market for about 50 years because of their perceived ability to better detect fires and distinguish between real and false stimuli. In simulated fire scenarios, however, photoelectric models have performed just as well as ionization detectors. As a result, during the past decade, the ionization market share has been cut in half, down to 25%.

There are several reasons for this decline, including environmental concerns. Because ionization sensors require AM 241 to charge air molecules, problems arise with the disposal of dated models. Radioactive AM 241 has a half-life of 400 years and requires special disposal practices.

Another reason for the market shift is the development of multi-criteria smoke detectors. Multi-criteria smoke detectors process inputs from two sensors using software algorithms that translate signals into pre-determined responses. Responses from a multi-criteria smoke detector are programmed to react to pre-defined scenarios that correspond with environmental conditions. Responses range from activating an alarm to giving input to a "feedback loop" that refines future detection parameters.

The most redeeming advantages of multi-criteria smoke detectors are quick signal-processing periods and the rejection of false or nuisance alarms. By rejecting false alarms, multi-criteria detectors alleviate many headaches due to unnecessary fire department involvement and the chaos associated with building evacuation. Also, local municipalities may enforce strict false alarm regulations that include substantial fines.

Figure 2. When smoke particles enter the smoke path, light strikes the particles and is reflected onto the photosensitive device, causing the detector to respond. (Illustration courtesy of System Sensor.)


Mix and Match Sensor Strengths

Finding the right multi-criteria detector entails more than choosing a brand; it is also important to consider the fact that the combination of sensor inputs dictates how the unit performs in a variety of fire scenarios. While both ionization and photoelectric sensors can detect the full range of fire scenarios, each is better for specific applications. When packaged with a secondary sensor such as a thermal unit, however, performance levels can be easily discerned.

Both sensors in ionization/thermal detectors identify fast-flaming fires quickly, but neither is adept at detecting smoldering fires. This leaves a wide gap in fire protection. And because a multi-criteria detector cross-references inputs to determine its actions, similar data actually slows the reaction time to fast-flaming fires. Remember, multi-criteria detectors reject nuisance alarms by design, and when one sensor relays information, the other must adjust its parameters to confirm beyond a doubt that a fire is present. Having two similar inputs in a multi-criteria detector increases alarm thresholds for each sensor to confirm the presence of a fire.

Therefore, a photoelectric/thermal model, which has one input that recognizes slow-burning fires and another that identifies fast-flaming characteristics, is the best combination for a multi-criteria detector. By having dissimilar inputs, one sensor can check with the other to confirm or deny the existence of a fire. As soon as the thermal sensor detects a rapid rise in temperature, the sensitivity of the photoelectric sensor increases, prompting it to scan for smaller particles of combustion. The combination of photoelectric/thermal sensors also helps safeguard against false alarms, because the lack of a strong thermal signal will reject a photoelectric alarm resulting from false stimuli such as cigarette smoke or airborne dust.


What Should Be Installed and Where?

Single-sensor photoelectric detectors provide adequate protection for areas such as offices and lobbies, whereas ionization detectors are better suited for areas that could produce fast-flaming fires, such as laboratories. For years, both device types have offered reliable protection, but advances in technology have now limited these models to specific applications.

Photoelectric/thermal multi-criteria detectors have gained industry acceptance, claiming more of the market each year during the past decade by offering complete protection from both fast-flaming and smoldering fires. This type of detector provides comprehensive protection in most atmospheres, including offices, storage areas, atriums, warehouses, museums and churches.

A multi-criteria detector, however, is not the best choice for all applications. Some spaces have many uses, meaning multiple detection parameters are needed to offer complete protection.

For example, a light manufacturing area will be vacant during the weekend, but bustling during the workweek, resulting in higher airborne particulate counts, increased ventilation activity and additional heat produced from machine operations and personnel.

Such transitions change a zone's atmosphere and decrease a sensor's ability to detect a fire due to changes in temperature, air velocity, stratification and air particulate count. Essentially, the local environment for which the sensor was specified changes into an unfamiliar environment because engineers specify systems for the "uptime" in each zone.

Recent technological advancements have lead to the development of self-adjusting, microprocessor-based detectors that rely on a complex set of onboard algorithms to adapt to operating parameters, such as sensitivity, pre-alarm modes, day/night adjustment and signal verification.

By continuously sampling the local environment, self-adjusting multi-criteria detectors automatically "learn" the environment to optimize performance using advanced software to continuously monitor and adjust its detection parameters and alarm thresholds without human intervention. Self-adjusting multi-criteria detectors eliminate human error by tweaking parameters to exact levels, as opposed to relying on maintenance staff making manual adjustments based on educated guesses.

In addition, other algorithms process the alarm level responses. These algorithms are used to minimize the effects of unwanted transient nuisance alarms while enhancing the response time to fires. This is accomplished by monitoring the current value of the sensor as well as trends in the signal, such as increasing heat or a decreasing photoelectric signal.


Buying Peace of Mind

The responsibility to safeguard building tenants and their possessions falls on all those involved in either designing, recommending and/or purchasing smoke detector technology. To live up to this responsibility, these professionals need to consider detectors for use in unique circumstances and building and room types.

Today's integrated, "smart" buildings often house next-generation electrical, mechanical and plumbing systems. Yet very often, older smoke detectors that do not reflect the technological changes that have been made in recent years are thought to be good enough. It is the responsibility of all those involved in making decisions about smoke detection to achieve a functional balance when designing and purchasing systems for use in particular settings under different circumstances and that adhere to local codes.

Just one unfortunate incident can prove that a failure in a single portion of a facility's overall technological makeup can have devastating results. Professionals responsible for making decisions about smoke detectors must insist on systems that offer the greatest return on investment--peace of mind.