Carefully installed PIRs offer excellent performance in many security applications.

They’re the most common security sensing devices you can buy but too many installers still get it wrong when they’re putting in PIRs. And just going for dual technology sensors instead of installing PIRs properly is poor practice.

FOR many installers it’s far easier to install dual technology sensors in an environment that may be a challenge for a passive infrared detector. However, putting a dual technology sensor into an environment in which the PIR side is often in a state of false alarm is a mistake. Unless you’ve got a gated microwave sensor with smart processing, the microwave component of the sensor is far less discerning than the PIR side.

Microwaves have the nasty habit of passing through walls, windows and wooden barriers making them extremely vulnerable to false alarms if not installed sympathetically. What this means is that a dual technology sensor needs to be installed just as carefully as a passive infrared sensor or you risk putting in a security system that will have its good hand tied behind its back. Use passives intelligently, installing them using techniques based on a sound understanding of the PIR’s inherent nature.

First let’s consider the uses of the PIR bearing in mind these detectors are designed to receive reflected infrared radiation when its wavelengths and energy levels match those generated by human movement. As a rule, PIRs are designed to be installed indoors (unless microprocessor controlled and waterproofed) to cover moderately broad open areas and longish corridors as well as very small spaces. They’re also ideal for use when factors like noise, inanimate movement and vibration won’t permit the use of microwave alternatives.

PIRs can be masked against known IRE sources, are small in size, have the ability to be recessed or flush mounted, and are sold in 360-degree ceiling mount variations. Infrared detection devices can also be used when there are large areas of glass forming a boundary to the target area. Used in such an environment they won’t detect anything on the other side of the glass in the way microwaves and ultrasonics certainly will.

Doing it right

There are a number of installation tips that will keep false alarms to a minimum while allowing PIRs to work to their maximum potential. Successful use of passive infrared detection demands a stable IRE environment so as soon as you get into the installation environment look around and make certain there are no heaters or powerful lights in the room likely to generate excessive infrared energy levels.

PIRS can’t be located facing receive unfiltered sunlight and nor should they be used anywhere there will be sudden temperature changes on surfaces within the target area. Exposure to glass surfaces should be space divisions in office or home environments environments – not perimeter glass – unless a verandah keeps direct sun off. Other potential problems with PIRs include humidity, unsuppressed electrical equipment that could cause interference (this includes things like fluorescent lights), and dirt and dust in the environment. You can effectively seal PIRs and conformal coat their boards if you’re installing them in tough environments like bathrooms, garages or covered ways.

Another mistake is to install PIRs in environments in which cats, dogs, birds and flying insects are present unless the sensors have been specifically designed to resist these false alarm signals through sensitivity adjustments, microprocessor control or lens design. In the past, pet immunity meant keeping the IRE sensitivity above knee or thigh level with lens design, creating what was called a “pet alley”. This was effective up to a point, especially with small dogs, but climbing animals like cats are likely to leap onto lounge chairs or tables causing false alarms.

Microprocessor-controlled PIRs recognise the waveform signal output from a pyro in the presence of IRE making it possible to recognize animals. It’s true that well-designed sensors can monitor this waveform. They recognise the waveform’s amplitude, cycle, frequency and wavelength and can compare what they “see” to what has been programmed into their memory as the signal level they are seeking. A few cats or a big dog will generate a different type of signal – in part because of their insulating fur – and the sensor will ignore them. It’s not easy, however, for the installer or end user to be sure just how smart the processing of many PIRs is and it’s likely some pet detectors lack sensitivity to certain types of movement in the target area.

Interestingly, SEN conducted tests on a number of standard PIRs and pet detection types installed at 2.5m in a 25m x 18m hall some years ago. We found that it was possible to sneak past some sensors of both types, especially towards the outer limits of their detection zones. Some sensors performed better than others and price wasn’t a reliable guide. Security managers would be wise to conduct their own walk and crawl tests, especially on larger sites where sensors will be challenged on warm days.

Check Your Detection Zones

Before the sensor goes in, you need to know the layout of the sensor’s zones – these are defined by lens design. Always check the specification sheet before installation and ascertain the sensor’s strengths and weaknesses. The PIR should be installed so it positions the maximum number of zones at right angles to what is the most likely track of an intruder. Passive infrared sensors are far more sensitive to changes between live and dead zones and live and live zones, than they are to changes within these zones. A typical PIRs is designed to activate if a certain number of zones is breached in a given direction at a given speed, so install the sensor in a way that will allow it to pick up the most likely line of an intruder’s passage at right angles.

If the PIR you’re installing has no anti-masking feature, make sure it’s positioned so an intruder will have a hard time reaching it to mask it. Most modern sensors have anti-masking but the cheaper ones will be devoid of this feature and vulnerable to masking by anything from sticky tape to spray paint.

Smart sensors will alarm if they think they’re being deceived in this way. Look for creep zones, too. These lookdown zones are achieved by clever lens design and they make it much harder to approach a PIR from beneath. We tested creep zones during out fieldwork in 2000 and found that they definitely enhanced the sensor’s ability to pick up close movement, though some were slow to respond.

Considerations of Sensor Optics

Lens design is another important issue with PIRs. Using the wrong lens will lessen a PIR’s effectiveness. Some sensors will be supplied with a choice of lens and others will endeavour through their design to offer of both broad coverage and longer range patterns. There are six primary lens patterns – and these can be applied generally to both fresnel lens types and mirror optics:

* Long range, dual zone, narrow coverage lenses are used to monitor narrow corridors with crossing points to protect long rows or windows

* Long range, multizone, narrow coverage lenses are designed for use with long corridors without crossings in which intruders will move towards or away from the sensor

* Multizone curtain lenses are mounted on ceilings above entrance doors or in glazed areas. They’re a detection fence from floor to ceiling and any intrusion through them will generate an alarm

* Broad multizone, wide angle or volumetric: These lens types are the most common found. They are more sensitive to movement across a protected space at right angles to their radial zones. They’re ideal for smaller spaces in domestic homes and small offices

* Ceiling mount PIRs have two-zone, 360-degree coverage, with conical zone arrangement arrayed in 2 circumference lengths.

These sensors are ideal for use in offices that have large numbers of partitions between waist and shoulder height. Multiple installations generally employ overlapping pools of detection. We like mirror optic lenses in tough environments. Having spent time fooling around with some mirror optic PIRs, Help Desk came away impressed with their effectiveness at long range. But in most cases mirrors mean greater expense so there’s a decision to be made with lens choice.

Advantages and Disadvantages

Some of the advantages of PIRS include low false alarm rates when compared to microwave and ultrasonic sensors, miniscule power drains, no energy emissions, no moving parts and simple electronics. PIRs have a longer detection range than ultrasonics, are cheaper than most other technologies, will not interfere with each other when installed in groups and can be adjusted in terms of lens pattern and sensitivity while in the field. There are a number of disadvantages with the technology, too. For a start increases in ambient temperature will effect a PIR’s range and sensitivity. And it’s possible for intruders to wear a suit or carry a shield that reflects body heat to beat PIR sensing. Rokonet has a technology called Anti-Cloaking that works to beat this, though we have not tested it.

There’s not the density or uniformity of coverage with PIRs that you get with other technologies, either. There are dead zones and it’s possible for furniture or pallets of stock to block the sensor’s view. Nor will the PIR alarm if the sensing element fails – something that beam sensors will do. Generally, the PIR sensor’s detection range is limited to 15m x 15m x 50m in wide angle and 50m in narrow beam, or a circular 15m pattern. Other disadvantages include the fact random IRE signals will cause false alarms and there’s a maintenance requirement. Lenses and mirrors need to be kept clean and insects and other pests kept at bay. In dry, dusty environments, particles will be electrostatically drawn to the device where they’ll stick and cloud its vision.

Making a Choice

There are a number of features installation companies should look for and end users should demand when installing PIRs. To begin with, look for sensors with 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.

You want surge and low voltage protection, as well as RFI suppression, pyro electric sensing elements, adjustments letting you alter the range or mask zones, as well as LED-supported latching. Walk-test is a valuable feature and a plug-in test meter is an advantage. Other valuable features include fast-change sensor head, anti-masking (a low power active infrared transceiver), tantallum capacitor-based RFI and EMI protection, a high signal-to-noise ratio, trouble log capability, auto self-test capability and temperature range of -10C to +50C or better.

A beneficial feature in PIRs is temperature compensation. This feature adjusts sensitivity in line with increases in environmental temperature in the target area taking into account that some locations may approach or exceed the temperature of the human body. When this occurs, an uncompensated sensor may have a detection range of just a few metres.

Once the temperature hits +35C, any PIR is going to be seriously disadvantaged even if temperature compensation is winding up that tiny pyro signal through a specially designed amplification circuit. The answer is never to challenge a PIR’s long-range capability in hot climates and, at the risk of being thrown to the wolves, we’d recommend halving the quoted range when designing your system. In Australia, especially in the north, installers putting in PIRs should think overlap and dual technology.

Additional features in a quality sensor include silent alarm relay, multi-facted reflectors, low voltage signal; enhanced processing linked to rate and rise of threshold, duration of zone disturbance and intruder presence in both elements; a tamper contact, site-adjustable sensitivity and first-to-alarm memory.

Features of importance in PIRs include:

* Zone pattern and range
* Latching LED

*Window type
* Fresnel or mirror lens
* Recess options
* Walk test capability
* Mounting or tilting bracket
* Factory test EMI/RFI resistance
* Operating voltage, current draw
* Maximum operating temperature
* Temperature compensation
* Modular electronics
* Anti-tamper, anti-mask
* Preferred mounting height.