Over the last three to five years, the fire service has been very fortunate to see a large number of new research projects brought about by a number of agencies such as NIST and UL. This research has challenged many established fire service standards and has brought to light a considerable amount of new information. One area that this research has ventured into is the use of thermal imaging in structural firefighting.
There is no doubt that this research has brought a number of valuable issues to the forefront. This article will look at what that research has brought to light, specifically as it applies to using thermal imaging for size-up in structural firefighting. It will review the key concepts that are critical for firefighters to use thermal imaging both safely and effectively on the fireground. After reviewing this material, firefighters should have a better understanding of both the pros & cons when it comes to using thermal imaging for size-up.
Research Projects
There have been a number of recent research projects that have looked at the effectiveness of thermal imaging for structural firefighting. Some of this research looked directly at the capabilities and limitations of thermal imaging for some specific fireground applications. Other research projects used thermal imaging indirectly as a means of evaluating conditions in structural firefighting experiments.
Some of the most recent research projects that included thermal imaging are:
- NIST Special Publication 1040 – Thermal Imaging Research Needs for First Responders: Workshop Proceedings – Amon, Bryner, and Hamins
- NIST Technical Note 1709 – Examination of the Thermal Conditions of a Wood Floor Assembly above a Compartment Fire – Kent and Madrzykowski (For NIST publications visit www.nist.gov/fire.)
- UL File NC9140 – Structural Stability of Engineered Lumber in Fire Conditions Overview – Berhinig, Izydorek, Samuels, Smyser, and Zeeveld
- UL – Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction – Kerber (For UL publications visit http://ulfirefightersafety.com.)
Fireground Concepts
For firefighters to both safely and effectively use thermal imaging in size-up, and insure that they have a full understanding of both the pros and cons, there are some key concepts that they must be able to apply on the fireground. These concepts include heat/energy transfer principles:
- Conduction: The transfer of heat/energy through direct contact with a solid medium. In terms of thermal imaging and firefighting, this can be most often seen when the fire heats up elements within the structure. Good conductors within the structure will readily transfer the heat from the fire. Poor conductors within the structure will not easily transfer heat from the fire.
- Convection: The transfer of heat/energy through a liquid or air medium. In terms of thermal imaging and firefighting, this is often seen with fire gases, smoke and the heating of air within the structure.
- Radiation: The transfer of heat/energy via electromagnetic waves. In terms of thermal imaging and firefighting, this can be most often seen when heat/energy is transferred in a straight line from the fire and contacts a solid surface heating it.
Exterior Size-Up
When beginning the size-up process from the exterior of the structure, it is important for firefighters to understand both the capabilities and limitations of thermal imaging. Breaking the process down by using the three heat/energy transfer principles can make it easier.
Conduction: Some types of building construction will conduct and show heat from the exterior, and others will act as very good insulators and show little to no heat from the exterior.
The image below, from UL Research, shows how various types of windows can act as either a good conductor or a good insulator. Older “legacy” single-pane glass windows will readily conduct the heat from the inside to the outside showing an advanced heat/fire condition. Newer “modern” multi-pane synthetic windows are very good insulators and can mask the conditions inside.
In the thermal image below, the windows in the structure are older single-pane glass, and they are allowing heat from the fire to be conducted through them so the heat/fire conditions may be identified from the exterior. In this case the fire had started on the lower ground level, but it is also extending up the back stairwell, as indicated by the hot windows.
Firefighters must be able to fully interpret the images that the thermal imager is producing. In the thermal image there is also a large amount of heat showing on the side of the structure; however, this heat is not from a fire condition in the house, but is from direct sunlight heating that side of the structure.
Convection: This will not be as prevalent on the exterior as it will be on the interior, but firefighters should still make an effort to identify and evaluate it. By combining smoke reading with thermal imaging, firefighters are more likely to get a better idea on the location and extent of the fire conditions.
Radiation: This is typically an issue that will be noted with exposures, as heat from the fire is radiated outward and impacts nearby structures. By using the thermal imager firefighters will be able to identify how much heat is impacting the nearby structures and if their efforts to cool them are finding success.
The windows, doors, and foundation of the structure (top image p. 41) all show signs of heat, but it is not necessarily clear what is causing it. If the image is from a cold winter day, the heating could simply be from the HVAC system. By comparing the conditions and images from this house, to the one next to it (assuming they are the same construction), firefighters may be more likely to interpret those images and figure out what they actually have in terms of interior conditions.
Interior Size-Up
Just like with exterior size-up, firefighters need to be aware of the positives and negatives of thermal imaging, and can apply the three heat/energy transfer principles for assistance in making their judgements.
Conduction: Firefighters should evaluate the interior conditions as soon as the structure is opened up and they can see them with a thermal imager. Per the NIST & UL research many floor assemblies can act as an insulator and will show little of the heat/fire below them.
Given that floor can be a good insulator, it can impact the effectiveness of the temp measurement feature, which can only measure the surface temp of the upper materials. It is important to note the limitations of the temperature measurement feature, which can only measure the surface temperature of the upper floor materials and not the true conditions below. As shown in that research, the temperature below the floor assembly can be over 1,000 degrees F, while the thermal imager may only register temperatures just above 100 degrees F.
While a floor assembly may be a good insulator, most floor assemblies will have openings in them where heat/fire conditions may be identified via conduction and convection. In the second image on p.41, the heat/fire conditions are clearly visible where openings exist in both the floor and lower wall area. While firefighters can’t count on getting an accurate temperature reading of the heat/fire conditions, and they may not be able to identify the integrity of the floor assembly, they will in many cases be able to identify a hidden heat/fire condition that they would not be able to detect without the aid of a thermal imager.
Convection: A thermal imager will not normally “see” air or smoke; however, if they become dense and hot enough, a thermal imager may detect them. This convected heat energy can provide firefighters with some very valuable information– if they know how to interpret it. A convected thermal layer that has filled the upper half of a compartment is a warning sign that fire conditions are progressing significantly and a hostile fire event may be imminent. By identifying the movement and flow of the layer, firefighters may be able to follow the flow back to the seat of the fire, and observe if an obvious flow path exists.
With training and experience firefighters will be able to build an understanding of how well their thermal imager will identify convected heat. Some thermal imagers will detect and display it well, others will not.
While a thermal imager may be able to identify the build-up of a heavy thermal layer just prior to flashover, the time between recognition and flashover may only be seconds, so firefighters must take great care to not over rely on this potential warning sign.
Radiation: While it can’t be readily identified as it moves away from the fire in a straight line, radiation may be identified when it strikes a surface and heats it. Firefighters should look for this heating, especially if it is significant in the lower areas of the room. This can be an indicator that the upper surfaces in the compartment, and the thermal layers, are fully saturated with heat and are radiating heat downward into the lower levels of the compartment. As the heat levels reach saturation in the upper levels of the compartment, and the lower levels are heated, the potential for a rapid hostile fire event go up significantly.
In the thermal image below, radiated heat from the convected heat layer at the ceiling is being transmitted downward onto the floor, where some of it is being absorbed and some of it is being reflected. If the floor was showing a high level of heat, without a fire having a straight line of sight or a thermal layer above, then the heating may be via conduction from below.
Valuable Tool
As proven through research and real world fireground operations, thermal imaging is a valuable tool.However, it isn’t the perfect tool for every situation. But with proper training and understanding, firefighters will be able to take advantage of the value it provides.
Understanding thermal imaging principles and how they interact with the fireground is key for success. Effective size-up is critical for safe and effective fireground operations, and a thermal imager can provide firefighters with information that they may not be able to obtain through any other means.
Sidebar – Exterior Size-Up Tips
Building Construction: Remember newer energy efficient construction, which is a poor conductor, will more readily hide conditions.
Comparisons: If multiple structures of the same type of construction are close by, remember to compare what the imager is showing for the various structures.
“False Positives”: Things like direct sunlight, active heating systems and reflections can easily trick firefighters into believing that they have something when they don’t.
Sidebar – Interior Size-Up Tips
Convection: An extremely valuable tool for judging conditions, but with some imagers, and in some situations, it may not be readily visible. Firefighters must learn through training and experience what they can realistically expect to achieve with their units.
“False Positives”: Items like glass, mirrors and highly polished surfaces can produce reflections which can easily trick firefighters into believing that they are seeing something when it is not really there but is actually somewhere else.
Structural Integrity: While a thermal imager may be a very good tool for identifying heat/fire conditions in areas that are critical to structural integrity, they cannot be relied on as a sole indicator of whether an area is structurally stable.
