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Criminals regularly manage to enter houses without setting off the alarm systems, a serious problem when one has to deal with insurance providers. Fingers are invariably pointed at the security companies, who are accused of having malfunctioning equipment in the houses of their clients. But, now and then, a foreign beach umbrella is found on the victim’s premises or a mattress will have been dragged from a bedroom into the lounge area. The burglars know what they are doing. They know how to render an alarm system ineffective and these umbrellas and mattresses provide the clues as to how they can achieve this.

Over the past 30 years, the standard, most cost-effective method of intruder detection worldwide has been the Passive Infrared Detector, or PIR sensor, which is essentially a heat detector. Any object that has a temperature above zero radiates infrared in the form of heat. Thus, a human body, with its temperature of 37 degrees, is an excellent infrared emitter. The PIR, together with its associated electronics and lens, detects the infrared emitted by the human body and when that heat source moves against a background with a lower temperature, an alarm circuit can be triggered.

The role of the umbrella or mattress suddenly becomes clear. They are used to block the perpetrator’s body heat from the detector’s field of view. He can thus move undetected around a room.

Over the years, the PIR sensors have become more efficient and it is the systems that were installed many years ago that cause the most problems. There have been improvements in the electronic systems that interpret what the sensor is detecting. The same applies to the lenses that determine the detection pattern. A lot of other factors influence the detection probability – the height and angle of the detector/sensor, the ambient temperature of a room, the amount of shielding [the size of the umbrella or mattress] that the intruder is utilising and so on. However, a PIR sensor remains dependent on body heat. Therefore, successful burglaries will on occasion happen if PIR sensors are used.

Microwave Detectors; A More Effective Alternative

The future is less bleak; thanks to a totally different detection method that uses microwave radiation. Microwave detectors emit high frequency radio waves that detect movement instead of body heat. This is a new technology that has been used for a number of years, mainly in conjunction with PIR detectors, to minimise false alarms. Purely microwave alarm systems have until recently been prohibitively expensive. But now, more flexible and cost effective microwave detectors have come on to the market. The local security companies recommend their use, particularly in the living-rooms of houses, as these are generally the target spaces. They are also recommended for outside use.

For peace of mind, it may be sensible to upgrade one’s security system to a combination of PIR and microwave sensors.

[Ed: It would be useful to hear from local security companies what these microwave detectors cost to install and to run — eletricity use. Do they demand more of a battery? This is vital given the state of the folly that is Eskom.

If you have answers to this — please post a comment or contact me.]