Senior Science Disasters Part 4: Warning Devices

describe the energy transformations involved in the operation of a range of commonplace warning and protection devices, including – smoke detectors – fire alarms – sprinkler systems

Smoke Detectors Identifying the presence of a fire in schools, homes and public buildings. They are used to sense the presence of smoke and alert the residents to give them time to escape. It is now compulsory to install at least one wired-in smoke alarm in each new home

Smoke Detectors All smoke detectors have a sensor to detect the smoke and a loud electronic horn to alert people and wake them up. The units are powered by a battery alone or with connection to your household electrical wiring and a backup battery. Smoke detectors are cheap and effective detectors of fire.

Smoke Detectors Most smoke detectors contain a radioactive source and good at detecting small amounts of smoke from flaming fires. These ionisation detectors consist of an ionisation chamber and a radioactive source of ionising radiation to produce ions (electrically charged particles). The ions are reduced in number by smoke particles in the air.

Smoke Detectors The man-made radioactive isotope americium –241 decays over time as the nuclear structure breaks up emitting particles of matter and energy. Alpha particles emitted move through the air ionising large numbers of air molecules.

Smoke Detectors 1.The ionisation chamber has two metal plates with a voltage across them. 2.The radioactive source releases alpha particles in between the two plates. 3.The alpha particles ionise the molecules of oxygen and nitrogen in the air. That is, the alpha particles knock off an electron, and produce one free electron (negative charge) and a molecule missing an electron (positively charged ion).

Smoke Detectors 4.The negative electron is attracted towards the plate with the positive charge and the positive ion will be attracted towards the plate with the negative charge. 5.The electronics sense the small amount of electrical current that this movement of ions causes. Smoke, however, will disrupt this current. 6.Particles of smoke react with ions removing them from the air. 7.The smoke detector senses the drop in current and sets off the horn alarm.

Smoke Detectors

Fire Alarms These detectors usually contain thermistors (thermal resistors) that have electrical resistance sensitive to change in temperature. Fixed temperature detectors: Preset to sound an alarm when the air temperature exceeds a pre-set value. They have two electrical contacts separated by a chemical that melts at a high temperature. Once the electrical contacts join together, the alarm is activated. Rate of rise detectors: Identify an abnormally fast temperature climb over a short time period, eg. over a few minutes.

Fire Alarms Flame detectors: Require a clear, direct, unobstructed view of the infra-red or ultraviolet radiation from the fire. They are expensive and mostly used in fuel depots, mines and aircraft maintenance areas where smoke or dust may lead to nuisance alarms by other, cheaper, detectors. Smoke detectors: Detect the presence of a fire much faster than heat/fire/thermal detectors

Why would a heat/fire/thermal detector not be recommended as the only detector for areas where humans live and work? A heat/fire/thermal detector alone is not recommended for areas where humans live and work because they only operate when the temperature is already very high and flames have started. This could be too late to safely evacuate people and successfully extinguish the flames.

Sprinkler System