Well designed security sensor from Bosch.

When it comes to selecting internal alarm sensors, what are the features installers should be looking for? Are all internal sensors the same or do some offer superior catch performance and false alarm resistance?

What makes for a quality internal alarm sensor? That depends on the environment the sensor is installed in – the more challenging the environment, the better the design and quality need to be. In many internal applications, passive infrared sensors are the perfect solution, while for more challenging internal environments, you need to look at dual technology detectors.

When selecting a PIR for internal use look for dual or quad multiple pyro-electric sensing. These reduce false alarms through increased stability. Other features you should rate highly include creep zones, a good-sized detection window, not just a slot or a small hole; and RFI, surge and low-voltage protection. You also want zone adjustment that allows range to be changed or particular zones to be ignored, latching (with LED indicator), walk test capability (and LED indicator).

You want a high signal-to-noise ratio, anti-masking, tantallum capacitor circuits for EMI/RFI rejection, trouble log, silent alarm relay, enhanced processing, temperature gain compensation and a tamper switch. Other beneficial features include site adjustable sensitivity, first-to-alarm memory, low voltage signal and auto self test.

The best PIRs will have dual edge or quad sensing zones and an array that offers a significant number of look down zones, as well as more than one sensing range and the greatest possible number of discrete zones. They’ll have surge and low voltage protection, adjustments letting you alter the range of mask zones, as well as LED-supported latching. A plug-in test meter is an advantage.

Once the temperature gets over 35C, any PIR is going to be seriously disadvantaged even if temperature compensation is fattening up that tiny pyro signal through an amplification circuit. If things are going to be warm or there will be direct sun on glass, you should think quality dual technology. Dual technology sensors combine a pair of technologies that are sensitive to 2 different types of disturbance – microwave and PIR.

The thinking behind this is to ensure that each sensor supports the weaknesses of the other to eliminate false alarms. It works thanks to the intrinsic nature of each of these 2 sensing technologies. PIR elements sense the level of IRE changing between zones over a set time at a set speed. There aren’t many environmental disturbances that mirror this sort of activity – but heat sources, especially warm air currents, spell trouble for PIRs.

Microwaves work differently. They cover an area with a signal and then pick up variations between the signal sent and the signal reflected back. It’s called the Doppler shift. Microwave sensor technologies offer strong detection performance – they’ll pretty much detect anything that moves. In terms of dual technology, what’s vital is that PIRs are sensitive to movement across their zones, while microwave devices activate if they pick up a Doppler shift that moves either towards the sensor or away from it.

It’s important that the appropriate sensor technology (PIR or dual technology) is selected for a given application. The one-type-fits-all installation undertaken because the end user may want all sensors to look the same, or because of under quoting to win the installation, can result in poor overall system performance that may require site re-visits.

As important as appropriate selection of sensor technology is the correct sensitivity adjustment and walk testing of a sensor onsite. If sensors are left at their factory settings, they are more likely to false alarm due to over sensitivity that is inappropriate for the size of room they are installed in or because of environmental conditions in the installed environment.

The best security sensors are rigorously challenged in state-of-the-art test rooms built to exceed the toughest industry standards. Establishing whether this takes place is important – it’s worth installers use the sensors they install at home and at their own offices, so they can work out their strengths and weaknesses.

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