Detecting fire

We need to be able to detect a fire so that we can stop it or get out in time. Often we are the detectors. We can see smoke and flames, we can smell smoke and we can feel heat.

Sometimes we need some help. When a fire breaks out, we might not be at home, we might be asleep or we might not be paying much attention to what is going on around us.

Fire detectors have been developed using science knowledge and technology.

Heat detectors

Heat detectors involve the science principles of heat transfer, especially conduction and convection.

One of the first heat detectors was a sprinkler system used in the 1860s. This detector used the melting of materials to detect heat. A metal solder with a low melting point was used to keep caps in place in a network of water-filled pipes. In the event of a fire, the solder melted, allowing the caps to drop away and the water to flow. Melting is still used today, but is not so common.

Most heat detectors (sprinklers) today use the principle of expansion. A fragile glass bulb is incorporated in the sprinkler. It contains fluid that expands with heat. The bulb breaks, releasing water from a sprinkler head. Temperatures that can break this bulb are set to range from 57°C to over 200°C.

Heat detectors can also be used as an alarm. Expansion of metals or gas, caused by a rise in temperature, exerts pressure that can be used to close the contacts of an electrical circuit. This sets off an alarm.

The electrical properties of materials such as ceramics and polymers change when heated. This can be used in sensors to detect temperature change.

Good and bad things about heat detectors

Heat detectors are quite reliable and cheap to buy. They don’t activate easily so they don’t keep going off when they don’t need to.

The problem is that they have a slow response time because hot fire gases have to travel to the sensor.

Scientists and technologists are still working to improve heat detectors.

Smoke detectors

We can see smoke – smoke detectors are designed to ‘see’ smoke as well. A number of different science ideas have been used in different sorts of smoke detectors to sense the presence of smoke.

Photoelectric smoke detector: One idea is to create a light beam that can be obscured by the presence of smoke particles. An electrical signal changes because of the interruption of the light beam and sets off an alarm. A more sophisticated version uses laser beams to detect the smoke.

Ionisation smoke detector: This sensor ionises the air (adds an electrical charge to the air molecules) in a detection chamber. Two charged plates in the chamber and the ionised air generate a current across the chamber. If smoke particles are present, they capture some of the ionised air molecules and reduce the current. This change in current activates the alarm.

Good and bad things about smoke detectors

Smoke detection is sensitive and will give an alarm soon after a fire starts. Ionisation detectors are cheap and reliable.

The design of smoke detectors is complex. Many aspects of the smoke need to be considered, for example:

• particle size
• colour
• distance from fire
• temperature.

The sensitivity of smoke alarms can be a problem. The alarms cannot easily tell the difference between smoke and other particles such as steam or dust. They can be a nuisance and go off when there is no fire.

Other detectors

Other detectors are available too. One detector can see or detect flames and is often used in aircraft hangars or timber mills where a single spark can be detected immediately.

Another detector is one that can respond to smell in much the same way that our noses do. These detectors sense the gases present in fires. They are very sensitive, but are expensive and may require regular maintenance. They are usually used in chemical industries, although a sensor that senses carbon monoxide can be bought for homes and industrial premises.