In designing ARMAC some key goals needed to be addressed.
Early warning is an important component of defence against fire. Decisions must be made about when to set the system on, and off, and need to take into account the minimum requirements of the system to operate, different modes of applying cooling, and available water in the face of the combustion threat posed by the approaching fire. In the past the official wisdom had been that if an owner was prepared and equipped to defend a property against fire, then they should stay and defend. If they were not they should leave early. However, following a series of major fires that is no longer the belief. Very often it is simply wisest not to be where the fire is. Hence two capacities are needed:
- A remote control system able to be used anywhere where internet/mobile network capabilities are available.
- An autonomous control system able to run the system according to a pre-determined decision matrix, if remote communication with the system becomes unavailable.
Both of the above require an adequate and practical set of sensors which can inform decision making whether remotely or on-site.
Primary awareness of vulerability to a fire is provided by emergency warning from fire authorities and in particular warning in real-time of significant fires which pose an on-going threat. That is available from the VicEmergency App, and its warnings are built into the system control panel.
The next threat is the approach of wind bearing embers – sometimes called firebrand attack. Flame detectors are available, but the range tends to be limited and resolution either too good or not good enough. The crucial question is whether there is an incoming wind carrying embers of sufficient thermal output to pose a threat to wooden structures or ignite grass or other flamable materials which can create such a threat. The system here is designed to greatly reduce the impact of such an attack, but can only do so if ember attack at dangerous levels has commenced. In this system the approach taken has been twofold:
- to create some direct measurements of threat by setting up “ember traps” filled with flamable material, but sheltered from rain, and equipped with simple thermal sensors which will report when either radiation from a glowing ember is detected, or the flamable “bed” within the trap is set alight. The choice of flamable material in this system is cloth soaked in hardened paraffin wax which is resilient not only to the elements in normal days (eg rain) but also to the early temperature rise in an on-coming fire, but highly flamable. These are maintained in an open but roofed metal box (the normal metal chimney cowls which are cheaply available). The wires are above ground, since once a trap has detected on-coming embers it and its connected wires has served its purpose. The relevant detection signals are available both to the autonomous module of the defence system, and on the remote control panel.
- To create a single infrared flame sensor in the 720 nm frequency range, which although only short range (1 m) can detect with considerable sensitivity small burning embers, supplemented by detection of small particles.
A video camera is also helpful for as long as it lasts, to survey the area of fire approach. Most of these operate in both the visual and infrared frequencies and a moveable camera is provided with its input able to be viewed in the remote control panel.
Later, as the fire front itself arrives, this is determined by both the external temperature, and the external temperature rise, since in bushfires, the temperature rises very rapidly in the later stages of the fire-front’s approach.