The bench test 1: Wider and higher

It is housed in a distinctively-shaped, tri-cornered enclosure with a fresnel lens at each corner. Each lens can be individually vertically adjusted to provide optimum coverage of the protected area, particularly useful where efficient coverage of split-level areas is required, or where centre mounting may not be convenient. The DS939 has been designed for ease of installation with totally enclosed electronics and a separate detachable mounting base. PIR to base and base to ceiling tamper protection is provided: it would need a well-equipped and determined intruder to reach it! Despite the manufacturer's statement in the technical sales information, regarding the signal processing method used, that "By adjusting its sensitivity based on signal amplitude, polarity, slope and timing, first step processing eliminates the need for the installer to select the sensitivity level for the application", high and low sensitivity settings are provided along with guidance notes for choosing the appropriate setting in the installation instructions.

We carried out our usual performance and stabil-ity tests with the high sensitivity setting selected. A highly visible LED is included for walk test and latch memory indication. The alarm output is given by NC-C-NO contacts with a 5 Ohm resistor in series; the C-NC connections are used with EC compliant systems. The contacts are rated at 28v DC, 125 mA, 3W maximum for DC resistive loads.The power requirements are a bit higher than usual at 9-15v DC, 22 mA standby and 39 mA in alarm.

Construction: The plastic enclosure consists of an inner base and cover which contain the PCB, lenses and lens adjusters, and a mounting base. The inner base and cover are not opened during installation so there is no risk of contamination of the sensing elements with dust or fingerprints. The single PCB (75 x 81mm) carries all the electronic components including three pyroelectric sensors, terminal connector block, LED on/off and sensitivity selector jumpers, form 'C' relay (change-over contact type) and tamper switch. An outer metal shield is secured to the PCB for electromagnetic interference shielding purposes, shaped to leave only the three sensors and the LED exposed. The sensors are set so that they are each depressed through an angle of approximately 35 degrees, and 120 degrees apart. The quality of PCB construction is very good and it is well supported on the inner base. The connector block, jumpers and tamper switch are exposed through openings in the inner base, which are sealed with sponge gaskets. Openings are also provided for the fresnel lens adjusters.

The cover carries the lenses, which are held in guides by clip-on covers which allow vertical lens movement. Each clip-on cover also provides a support for the adjuster and a shield around the lens and its corresponding sensor to prevent false alarms due to air circulation within the enclosure.

The rotary lens adjusters engage directly with the lenses for accurate vertical adjustment, a very neat design, simple and effective. There is a deep recess around the edge of the cover, which fits neatly onto a flange on the inner base. The design and quality of construction of the PCB enclosure is such that while not being completely airtight, it will deny access to the vast majority of insects.

Installation: The detachable mounting base has a centre cable entry hole, two sets of keyhole securing slots and a post for optional base to ceiling tamper. We pondered which securing slots to use: either the curved 4mm wide slots, requiring a rotation of the base before clamping in position or the straight 3mm wide slots, requiring sideways movement of the base before clamping.

The curved slots are 15mm long and permit subsequent rotation of the PIR for coverage pattern adjustment. This may appear an odd statement for a PIR having 360-degree coverage, but the DS939 is in effect three 120-degree PIRs each having separate tilt adjustment. Such decisions are made by installers, usually dictated by site conditions and, in this instance, influenced by the 4mm mounting base fixing screws which are provided, along with plastic masonry fixing plugs. The manufacturer recommends mounting on a solid, vibration-free ceiling, using a reinforcing backing plate behind removable tiles in dropped ceilings. After securing the mounting base, the PIR unit is easily clipped onto hinge pins, wiring is connected and the lens vertical adjusters set in accordance with the Mounting Height/ Maximum Range table in the instructions. While hanging on the hinge pins the PIR unit is perfectly secure, leaving both hands free for wiring. The PIR unit (cover) is swung and clipped into place, walk testing carried out and the cover finally secured with the anti-tamper screw.

Performance: The PIR was mounted at the usual height of 2m for stability assessment, considering this to be the "worst case" environment. With the high sensitivity setting selected, the DS939 did not respond to a variety of provocations. All the written instructions are brief, clear and adequate.

• Retail and department stores
  
• Banks and financial institutions
  
• Manufacturing facilities
  
• Car dealerships
  
• Schools, colleges and universities
  
• Offices
  
• Atriums.

It features three independent PIRs, which can be individually adjusted for customised coverage. You can provide standard floor coverage or monitor the main floor plus an elevated loft area at the same time. You can also focus the detector for pinpoint coverage of areas such as cash registers and jewellery counters. First step processing (FSP) helps reduce false alarms from background environmental sources.

Besides offering great benefits for property owners, this detector is also the installer-friendly detector. It features a hinged design which acts as a second pair of hands during installation.

Simply flip open the detector and attach the base to the ceiling. The chassis stays in place, making connections easy and convenient.

Once installed, achieving optimum performance is simple. An innovative light pipe system channels light from the high-efficiency LED. This LED is visible throughout the entire coverage area. Installers can more accurately determine coverage without complicated testing procedures.