BY JONATHAN F. BASTIAN
Fire departments across North America are rapidly adopting thermal imaging technology, recognizing all of its inherent benefits. Although a number of features and benefits are available on each brand of thermal imaging camera (TIC), no feature is more misunderstood than the temperature measurement device. If the data that the temperature measurement devices provide are misunderstood or misapplied, that can lead firefighters to make poor and potentially dangerous decisions. Proper application of this feature, however, can benefit the firefighter in certain circumstances.
THE TECHNOLOGY
TICs detect heat, convert this information electronically, and display it on a video screen for the user to interpret. Temperature measurement is usually displayed as a number (e.g., 146°F) or as a bar graph that either changes as the temperature increases or displays an indicator line that moves across a fixed scale.
For the TICs to function, infrared energy must be focused onto a focal plane array (FPA), also known as a detector or sensor. Electronics connected to the FPA create an “engine” that senses the energy, calculates relative differences, and prepares the data for display. Three FPA technologies are currently available to the fire service. Barium strontium titanate, or BST, is the most common and is known for its proven performance in the fire service. Vanadium oxide (VOx) is a type of microbolometer (which refers to how the FPA registers changes in infrared energy) that is popular for its good image quality. The newest fire service technology is amorphous silicon (aSi), a type of microbolometer known for its small size and relatively low cost.
BST imagers equipped with temperature indicators include a pyrometer, a separate device that measures surface temperatures. The pyrometer and the BST engine are electronically integrated so that information from both components is shown on the TIC display. This type of TIC has two lenses on its front, one for the BST and one for the pyrometer. Micro-bolometers can estimate surface temperatures directly from the FPA using radiometry. This is commonly called “through the lens” measurement, since the temperature is calculated through the one lens on the front of the TIC.
Regardless of whether the TIC uses radiometry to do “through the lens” measurement or measures the temperature using a pyrometer, it is critical that firefighters understand the limitations of the device.
A COMMON MISCONCEPTION
The most common misconception about temperature measurement is that it estimates air temperatures. Thermal imagers do not read air temperatures; they read surface temperatures. TICs operate solely based on differences in surface temperatures. Although occasionally a TIC may show superheated or cryogenic gases, in general, TICs do not “see” or measure gases. In a practical application, this means that temperature measurement de-vices will not predict a flashover. Flashovers occur because the radiant heat flux from the ceiling layer raises the temperatures of all materials in an area to their respective ignition temperatures. Firefighters should not be lulled into a mistaken sense of security because the temperature measurement on the TIC seems relatively low or has not reached its scale maximum.
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This is not to say that a TIC cannot be used to predict a flashover. Firefighters can identify preflashover conditions through proper image interpretation. Each FPA technology will display thermal layering and extreme heat differently, so firefighters must practice with the exact type of TICs that they will use at actual incidents. Depending on the FPA technology, it can take a significant amount of practice to interpret thermal layers correctly (photo1).
THE SCIENCE
Once firefighters understand that the TIC measures only surface temperatures, they need to understand why a temperature indicator can be inaccurate. The explanation for this lies with the actual science of noncontact temperature measurement (NCTM).
Outside the fire service, it is relatively common for industry to use pyrometers or radiometry to determine surface temperatures. One of the primary uses is for predictive maintenance, because equipment that fails or goes out of service unexpectedly can cost a business tens of thousands of dollars a minute. An industrial thermographer (the term for an industrial TIC user) uses NCTM to identify mechanical and electrical equipment that is overheating, which enables the plant to plan for scheduled replacement before failure. Industrial thermographers successfully use NCTM because they can adjust calculations to take into account any factor that may cause an error in the reading. In short, they are using the technology in controlled environments on controlled and known surfaces.
The fire service, on the other hand, does not have the luxury of controlled environments, known surfaces, and the time to make detailed calculations. Emergencies are uncontrolled events, and the fire department is trying to return a level of control to the situation. It is impractical for firefighters to make calculation adjustments to account for the variety of environmental changes inherent in a fire environment.
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A number of factors affect the accuracy of NCTM, including the material being measured, its temperature, the humidity of the room, atmospheric interference (such as smoke or steam), the cleanliness of the sensor lens, the distance to the material, the angle at which the surface is measured, and other conditions. The two most important factors influencing the accuracy of NCTM are emissivity and distance-to-spot ratio.
Entire textbooks have been written about emissivity, covering more information than any firefighter might need. Emissivity is a very complicated topic, and condensing it to a few paragraphs oversimplifies it. However, the explanation that follows should help you to understand the concept.
Consider a brick and a T-shirt. When the brick and the T-shirt are placed on a driveway on a sunny summer day, they both absorb infrared energy from the sun and get hot. Neither item generates heat by itself; they are only absorbing what the sun radiates. Once the sun sets, however, the brick and the T-shirt do not cool immediately. They each radiate the heat absorbed during the day and slowly cool off. However, the brick cools much more slowly than the T-shirt. The idea of emissivity is that different materials radiate heat at different rates. Basically, emissivity is the ratio of heat radiation compared with a universal reference standard called a “black body.”
Some materials are excellent absorbers of heat, while others are good reflectors of heat. (Good absorbers are always poor reflectors, and vice versa.) For practical purposes, this rating is listed as a percentage, comparing its radiation to the standard “black body.” Concrete, for example, has an emissivity of 0.95, which indicates that it is an absorber of radiant heat. Aluminum foil, however, has an emissivity of 0.04, indicating that this material is a reflector of radiant heat. A steel door painted in gloss paint might have an emissivity of 0.45. And this is exactly the problem for firefighters: Different materials have different emissivities.
Because firefighters cannot halt operations to reference a manual on a material’s emissivity, TIC manufacturers have made an assumption on the firefighters’ behalf. The result is that the pyrometer or radiometer is preset, usually with an emissivity of 0.95. Most common construction materials, such as brick, drywall, and wood, have emissivities close to 0.95, so the measurement can be somewhat accurate much of the time. If, however, the firefighter is attempting to estimate the temperature of a metallic surface, such as a steel fire door or a pressurized container, the measurement can be drastically inaccurate.
Materials with emissivities below 0.95, when measured with a TIC preset to 0.95, will generally receive an artificially low reading (or false cold). The greater the emissivity varies from 0.95, the greater the level of inaccuracy in temperature measurement. Thus, a steel door with an actual surface temperature of 300°F (149°C) might read only as 100°F (38°C) using NCTM. Obviously, the way a firefighter might open a 300°F (149°C) door is significantly different from the way he might open a 100°F (38°C) door (photo 2). Note that because emissivity values are affected by everything from the amount of oxidation to the type of paint on a surface, there is no firm rule on how to convert the TIC temperature measurement to a reliable estimate. (If you know the exact emissivity of an item, you can work a formula backward to calculate the temperature; because the formula involves taking temperatures in degrees Kelvin to the fourth power, it is impractical to make these calculations quickly at an incident.)1
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The second primary factor causing temperature mismeasurement is the distance-to-spot ratio, or DTS. The DTS of a given pyrometer or radiometer indicates the size of the area from which the tool is attempting to receive information. The greater the ratio is, the “tighter” the point of surface measurement. The effectiveness of temperature measurement is comparable to that of a flashlight beam trained on a wall. A flashlight near a wall will have a tight, effective beam. As you back away from the wall, the light beam becomes wider, more diffuse and less effective. In short, the area being “viewed” gets bigger and is not seen as effectively (Figure 1).
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The temperature the TIC displays is calculated from the energy received from a cone that projects out from the lens. Just as the flashlight will shine on anything in the cone of light it projects from its lens, the pyrometer (or radiometer) will estimate temperatures from everything within the cone. On the fireground, the DTS means that despite the presence of “crosshairs” or a “target diamond,” the area actually being measured is dependent on the accuracy of the measurement device as well as the distance to the object being measured. Training a TIC on an object is not like aiming a gun, where the “crosshairs” show the exact spot the bullet will hit. Just 15 feet from a wall, a 10:1 pyrometer is estimating the temperature of almost two square feet! If a desk or other piece of furniture is between the user and the wall, the furniture temperature will be averaged into the wall temperature, further reducing the accuracy of the measurement (Figure 2).
APPLICATIONS
Despite the problems and potential inaccuracies, a TIC’s temperature indicator does have limited applications in the fire service. Temperature readout can assist in some haz-mat incidents and some size-up operations.
Although the temperature displayed on the TIC may be inaccurate in measuring the temperature of a specific object, it will be relatively accurate when the user is comparing two similar objects. For example, imagine a haz-mat incident in which one tanker trailer is exposed to fire and another is not. At the start of the incident, each tanker appears gray on the TIC display, and temperature measurement reads 60°F (15.5°C) on each. Although this is inaccurate, at least the incident commander knows the exteriors of each tank are just about the same temperature (both by shade of gray displayed and by the temperature measurement device). As the incident progresses, one tank indicates a temperature of 180°F (82°C) while the other is only 120°F (49°C). Again, the temperatures are inaccurate, but the incident commander has obtained two pieces of information. First, the exterior temperature of both tankers has increased. Second, one tanker’s exterior is hotter than the other’s. Note that measurements in Fahrenheit cannot be compared as percentages (meaning that if a temperature goes from 60°F to 120°F, its actual temperature has not doubled); TICs that measure in Celsius can be compared as percentages.
The incident commander will not know the exact temperature of the tankers’ exteriors or the temperatures of their contents. However, he can use the information to draw conclusions about how effective cooling efforts have been and whether the risk of tank failure outweighs the benefit of cooling the tankers.
A similar approach can be taken when evaluating doors or windows at a structure fire. For example, at a fire at a manufacturing plant, arriving fire companies may know that there is a fire in the warehouse section, but they may not know which portion of the 150,000-square-foot warehouse section is at most risk. Temperature measurement of the steel fire doors on the exterior may show that one indicates 50°F (10°C) while another indicates a temperature of 90°F (32°C). Again, even though the temperatures are inaccurate, the company officer knows that one door is hotter than the other. He may not know why it is hotter, but this may be an indication of where the fire is.
Remember that metals will generally give a false cold reading to the TIC, so the officer needs to ensure that his company opens metal doors with extreme care and caution, regardless of the TIC’s temperature reading.
Overall, there is one very important tactical recommendation firefighters should take from this article: Never make a potential life-or-death decision based solely on the indicated temperature on a TIC. Potentially dangerous situations should be analyzed further by properly interpreting the thermal image itself and by using the natural senses of the firefighters and traditional, “low-tech” methods. Because firefighting TICs do not operate under ideal conditions, firefighters using them cannot rely on the accuracy of temperature measurement devices. The very nature of the science that allows firefighters to see through smoke makes it difficult for them to measure temperatures accurately.
In some ways, temperature measurement devices are not much different from any other firefighting tool. If firefighters understand how the tool works and how it can be useful, then they can operate more effectively and more safely. If they do not understand the risks and challenges, however, they can misuse the tool and place themselves at greater risk.
Endnote
1. Kelvin is a temperature scale in which zero occurs at absolute zero and each degree equals one kelvin. Water freezes at 273.15K and boils at 373.15K. (Source: Dictionary.com.TM)
JONATHAN F. BASTIAN is the thermal imaging training manager at Bullard and leads a team of firefighters that instructs the fire service in thermal imaging basics. Bastian served 12 years with the North Park (IL) Fire Department, the last three as a captain. As health and safety officer, he led the development and implementation of the department’s rapid intervention team SOG. A certified Fire Instructor I and Firefighter III, he served 12 years as an EMT-I/D. He has taught classes on thermal imaging, rapid intervention teams, and search and rescue operations. Bastian is certified as a thermal imaging instructor by the Law Enforcement Thermographers’ Association (LETA), an international public safety organization specializing in TIC certification and training.
