See smoke detection clearly
Sandy Damm takes a closer look at smoke detection and the best way to select and maintain your equipment.
Fire safety is an extremely important and complex topic. In a commercial property fire safety and smoke detection comes under the Building Regulations and the BS 5839-1 standard. Once installed and commissioned it can be the on-going maintenance that can prove to be a headache for businesses. If the right smoke detectors are not installed in the right place or used in the right application, the disruption unwarranted alarms can cause has a large impact and will undoubtedly increase maintenance costs.
Traditionally, there are two main types of smoke detectors – optical and ionisation. The two main sensor technologies operate in different ways with the result that they react differently to the kind of fire. An ionisation sensor will react quicker to a flaming fire and is commonly used to sense the invisible early onset of a fire such as overheating equipment, whereas an optical sensor will respond to a smouldering fire and works better in a position where dense smoke could be produced.
So, let’s take a look at these sensors in more details.
Ionisation is the more sensitive sensor technology and will deliver an earlier alarm to an imminent fire. It works by placing a small amount of radioactive material between two electrically charged plates. This ionizes the air and causes a current to flow between the plates. If the current is disrupted by smoke the alarm is set off. As they do include a radioactive source, there is a concern about the environmental impact of manufacturing and disposing of these sensors.
Optical sensors work by using InfraRed technology and operate on the obstruction or light scatter principles. When particles from smoke enter the chamber, the InfraRed light is reduced or scattered thus triggering the alarm. Optical detectors are more likely to produce false alarms from tobacco smoke and steam, while ionisation detectors are more likely to give false alarms if installed in areas in which fumes from cooking processes, (e.g. burning toast) may occur, such as spaces close to kitchens. Ionisation detectors may also give a false alarm if installed in high air flows.
Some optical detectors that operate on the principle of light scatter are more sensitive to light coloured smoke. Very dark smoke, by definition, absorbs light rather than scatters it, but will be readily detected by a smoke detector that operates on the principle of obscuration (e.g. an optical beam type detector), such as the Sontay SD-622 Duct Smoke Detector.
There is an overlap within the range of particle sizes to which optical and ionisation detectors are sensitive. This means that neither type is suitable for all applications.
It all comes down to positioning. Take, for example, duct work in a commercial building. Dust and particles can be an issue for smoke detection sensors if you have the wrong ones installed within the duct. The particles can trigger the alarm and affect businesses negatively, with unnecessary down time and the extra cost of unscheduled maintenance. It’s important to know the differences between the sensors you are installing. So, as a rule of thumb, optical sensors are great for clean environments whilst ionisation sensors work in dirty spaces.
In a nutshell, there are pros and cons to all smoke detection sensors. For instance, optical sensors are more sensitive to slow fires and will work with any coloured smoke, but they are more expensive than ionisation versions, are not as sensitive to fast burning fires, can false trigger in cooking environments and the optics may need routine cleaning, whereas ionisation sensors are more sensitive to fast fires and more cost-effective. However, they are an older type of technology and are becoming less popular especially due to the environmental credentials.
However, there is also third method that is becoming increasingly popular: Aspirating smoke detection (ASD) products have a central detection unit which is continuously drawing samples of air to monitor for smoke particles. ASD’s have pipes with multiple holes and a fan unit that draws the air from the environment. They work by filtering the air to remove any contaminants or dust, in order to avoid false alarms, and then send it to be processed by the detection unit.
This type of smoke detection is highly sensitive and is ideal for environments where quick and sensitive smoke detection is required, for example in areas such as data centres and clean rooms or heritage buildings like museums and theatres. ASD’s can also be useful for harsher environments such as industrial plants, warehouses or parking garages.
Smoke detection is a complex matter and an important one. Think carefully about where you are installing smoke detection sensors and what you are installing – and how it can be maintained in the long run. The advice you can provide your customer will be invaluable.
Sandy Damm is managing director of Sontay
