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How does a smoke detector work?

Inside the smoke detector there is a smoke chamber which contains an infra-red diode which emits a permanent light, which usually does not hit the light-sensitive photocell. If smoke particles enter the measurement chamber, the light from the infra-red diode is reflected is such a way so that it hits the highly-sensitive photo element. The electronics behind the optical measurement process interprets this as a risk of a fire. An alarm is then triggered. Who would go for two years without cleaning their car windscreen? Martin Dohmeier from Pyrexx GmbH. Everyone knows the situation: a film of dirt which impedes vision builds up on the inside of the windscreen after a few weeks. In the same way, ambient gases such as nicotine and particles of grease build up on the light-sensitive measurement components in the smoke detector, which lead to a gradual deterioration of the device's performance. In the worst case, this can mean that the measurement electronics are already heavily impeded after around 18 months and the smoke detector reacts to a fire only with a long delay. The consequence is that people may be injured or even killed! A high quality smoke detector always sends out an alarm at the same level of smoke density, recognises its level of contamination and gives out an acoustic signal which indicates its state at an early stage before its function as a warning device is endangered through a build up of dirt. Despite such technology, checks still have to be carried out as part of a careful maintenance regime to determine whether the device is undamaged, that the electronics are intact and above all that the air inlets are free of foreign bodies.

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How does a smoke detector react to cigarette smoke?

As long as large quantity of cigarette smoke is not deliberately blown into the smoke detector from the immediate vicinity, then the device will not send out an alarm. We have intensively tested the sensitivity to cigarette smoke of the smoke detectors we use and have established that even when there are numerous smokers in a small room, the device seldom triggers an alarm.

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What environmental influences can trigger an alarm in the smoke detector?

As smoke detectors measure the transparency of the surrounding air by means of a photocell, gases which are very dense can trigger an alarm from the device. With dense water vapour and vapour from cooking, roasting, baking and grilling an appropriately high concentration of such gases in the surrounding environment of the smoke detector can set off an alarm as a precaution. Building dust, spray mist from spray painting, insect and room scent sprays which get into the smoke detector can also trigger an alarm and also contaminate the device. In a few cases, it has been known for an alarm to be triggered due to a strong through-draft or great variance in temperature.

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Where should a smoke detector be installed? And where not?

As the sense of smell is not active in people an animals when they are asleep, and because two to four breaths of toxic smoke can lead to severed brain damage or death, it is important that a smoke detector is installed in every bedroom on the ceiling and as far as is possible in the centre of the room. Furthermore, detectors should monitor escape routes, such as corridors and stair wells, and be placed every four to six meters. Additional protection can be offered by also installing smoke detectors in rooms which are not used as bedrooms or escape routes, such as living rooms and dining rooms. Smoke detectors do not belong in the kitchen or bathroom, as environmental influences such as large amounts of steam can trigger an unwanted false alarm. The hallway outside of the kitchen is a good place to install a smoke detector, because the proximity to the cooker and at the same time the distance from it means that an alarm is only triggered in real situations. Smoke detectors should always be installed for every 25m² of floor space and the distance between the devices should, if possible, be less than 6m. “In a room filled with smoke the visibility is down to zero. You cannot see your hand in front of your face, let alone the exit. It is therefore important to be aware of a fire early enough, so that you can escape”. Wolfgang Weiden, Chief of Operations of the Bergisch Gladbach Fire Service.

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Who is liable for a fire service call-out if the smoke detector makes a false alarm?

Based on our experience, there are a occasional cases that we know if where, for example, an insect or room spray has been used. As the the smoke detectors permanently measures the transparency of the surrounding air with a photoelectric cell, such spray mist can be interpreted as smoke by the electronics and thereby trigger an alarm. In certain cases other residents have heard the smoke detector alarm and have hastily alerted the fire service without checking whether there is a real fire. The person who does not check if there is actually a real danger and calls out the fire service is always the person liable for the costs of the call-out.

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