Tim Snell, Director at Industrial Monitoring and Control, examines what site operators need to know when purchasing thermal imaging technology.
Thermal imaging technology is now a widely accepted tool for the early detection of fires and potential fire hazards.
Permanently installed cameras, operating in parallel with conventional detectors, offer complementary technology with several key benefits over traditional methods.
Thermal cameras are able to ‘see’ heat build-up, meaning thermal imaging is able to detect potential fire threats earlier, even before the fire can start. This gives operators the ability to stop a fire from occurring or being able to lessen its impact, making the site safer and reducing business costs.
Thermal imaging cameras are less susceptible to issues with airborne dust and particulates, which can cause traditional smoke detectors to fail or inhibit operation of visual camera systems.
Thermal imaging cameras are now manufactured by many companies and released under numerous brands. However, the imaging detectors that are used within these cameras, known as microbolometers, are only manufactured by a handful of firms globally.
To ensure a fire detection system operates effectively, it is important to understand some of the basic properties of thermal imaging detectors. These properties are common across all makes, models and brands that use common microbolometer sensors.
When considering cameras, it is important to ensure they offer temperature measurement or radiometry. Thermal imaging cameras capture thermal radiation in the long wave infrared portion of the electromagnetic spectrum, far beyond what the eye can see.
A false colour palette is applied, which gives each pixel a colour based on the level of radiation reaching the detector, providing brightly coloured thermal images.
A radiometric thermal imaging camera goes one step further. For every pixel within an image, a temperature can be calculated for the object being viewed. This is critical for fire detection application as it enables temperature anomalies to be identified based on temperature thresholds, differentials or rates of change.
It’s important to understand the accuracy of the radiometric measurement. Cameras will usually specify the accuracy within a range of temperatures or give a percentage above or below the measured temperature.
Some cameras provide better accuracy, within one or two degrees above or below the target, though fire detection applications often don’t require extremely accurate thermal imaging cameras, because the difference between 100 and 102 degrees Celsius is of little consequence.
Cameras with an accuracy of more or less five per cent tend to work best for fire detection purposes, as they have a high enough accuracy to avoid false alarms when compared to a less accurate sensor.
For example, the ignition temperature for paper is around 230 degrees Celsius. With a camera that offers plus or minus 10 per cent accuracy, the alarm threshold would need to be set at 198 degrees Celsius. Having an alarm threshold set this low dramatically increases false alarm rates caused by hot objects.
INSTANTANEOUS FIELD OF VIEW
Most cameras will require three by three pixels on a target to measure temperature accurately. Some cameras may require an even larger spot, which can dramatically limit the maximum working distance of the camera.
The size of the pixel of the camera is known as the instantaneous field of view (iFoV). If a hot object is smaller than this minimum size, then the temperature reading will be lower than the objects true temperature as the reading is averaged with the image background.
If the installer attempts to use a camera with a lens angle that is too wide, or a lower resolution, the temperatures measured could be much lower than the actual temperatures. This can make a fire detection system ineffective, as hotspots or even small fires may not be detectable by the system.
Using wide angle of view lenses is an effective way to get broad coverage and reduce overall system costs. There is a significant trade-off when taking this approach and the installer needs to be certain the iFoV at the target distance is small enough to accurately detect fire risks.
An example of how iFoV impacts what a camera can see and detect can be seen in the images below. The images show the top of a soldering iron at one, three and six metres distance. Maximum temperatures for those distances are 415, 379 and 280 degrees Celsius respectively.
The temperature reading reduces by less than 30 per cent from one to six metres and the further out you go, the more it reduces.
Every site will have different requirements and operating conditions, meaning there is no one-size-fits-all approach to alarm configurations. The goal is to make the thermal camera alarming as sensitive as possible while minimising any false alarms.
The most basic form of alarming is an over temperature alarm. If a temperature within the image exceeds a certain level, then an alarm is raised.
These are very effective but are prone to false alarms set off from machinery. Time delays can be placed to reduce the number of false alarms but can reduce the effectiveness.
Temperature differential alarms trigger when the difference in temperature between two points exceeds a defined threshold. For example, the points could be located on multiple conveyor rollers. If one is significantly hotter than others, then the alarm can be raised.
In stockpiling situations, this method can be used effectively by setting the alarm to trigger if any point within the stockpile is greater than the average stockpile temperature. Differential alarms are also susceptible to false triggers on exhausts and other hot items.
Rate of change alarms are based on the rate of temperature rise. This may be useful for monitoring a static stockpile for temperature build up that is a precursor to combustion. On dynamic sites with lots of vehicle movements, this type of alarm can reduce false detections while still offering an effective way to monitor for ignition, typically identifying a fire significantly before a traditional detector.
Alarm configurations should be flexible and offer multiple tiers of alert threshold and a hierarchy of actions based on the potential risk level. Systems should allow conditional operating modes for different thresholds based on different site operating states.
LINE OF SIGHT
A common misconception with thermal cameras is that they can see through or into objects. Thermal cameras, just like normal cameras, can only see the surface of what they’re viewing, meaning line of sight is required between the camera and the monitoring areas.
Dynamic waste management sites often include a lot of moving vehicles and infrastructure. Camera mounting locations must not be impeded by this equipment or the camera will be ineffective. Roof mounting cameras to provide a top down view reduces the risk of a camera’s view being impeded.
Sometimes, the cameras are able to detect heat below the surface due to the thermal differentials at the surface level. In stockpile monitoring applications, the cameras are often able to see heat build up below the surface.
The science behind radiometric thermal imaging can be complex and any integrator should have a comprehensive understanding of the fundamentals.
Many sites have thermal imaging inspections carried out periodically on mechanical and electrical switchboards.
People offering this sort of inspection work should be qualified and certified with a professional body such as the Australian Institute for Non-Destructive Testing (AINDT) or the Australian Professional Thermography Association (AUSPTA).
There are currently no legislated requirements for individuals assessing sites for thermography-based fire detection, so selecting an assessor with professional thermography experience is recommended.