Many of us in the fire service have said repeatedly that thermal imagers (TIs) are not thermometers, yet for some reason when a firefighter looks through the display of his TI, he thinks that the digital number he sees is the temperature of the object he is looking at.
The digital temperature measurement feature on any fire service TI should not be used for exterior and interior structural firefighting as the use of this feature MAY CAUSE ERRORS IN JUDGMENT, WHICH MAY RESULT IN SERIOUS INJURY OR DEATH.
National Fire Protection Aassociation (NFPA) 1801, Standard on Thermal Imagers for the Fire Service (2021 ed.), removed the Spot Numeric Temperature Indictor (digital number) from the TI’s Basic Mode but does allow it to return when the TI is switched to TI Basic Plus mode.
Older model fire service TIs that are not NFPA-compliant will be equipped with a spot numeric temperature measurement feature. NFPA-compliant TIs will use a heat color reference bar indicator and digital readout in Basic Plus Mode only as they will have both features in their displays, which will show an approximate surface temperature of a targeted surface.
THE MYTH
The temperature measurement zone on any make and model of a fire service TI is either a small green box or a set of crosshairs in the center of the TI’s display screen. This area measures a noncontact solid (relative) surface temperature that is not 100% accurate. Different materials or the same materials with different composition, surface textures, color, and gloss will not register temperature readings in the same way, resulting in variations in the temperature readings that can vary from ±5° to ±41° (specifications); the reality is plus or minus several hundred degrees.
Another factor affecting temperature measurement is based on the distance to the area being observed, which is called the distance-to-spot (DTS) ratio. The DTS ratio is roughly proportional to the width of the pixel array (640 or 320) divided by the field of view (FOV). So, more pixels (307,200 vs. 76,800) give a higher ratio; larger FOV gives a smaller ratio. Depending on the make/ model of TI, the DTS could range, as an example, from 37:1 (1’×1’×37’) to 73:1 (1’×1’×73’). Understand the TI’s DTS ratio, which tells you about the effective distance from which the TI can accurately measure temperature. Ideally, you should be as close as practical to the object, as the surface temperature area will be larger. Obviously, the farther away you are, the smaller the temperature area gets.
1 An NFPA-compliant TI in Basic Plus Mode showing digital spot temp measurement. Would you trust 119°F (48.3°C) on the window? Look at what this image is telling you. (Photos courtesy of Bullard.)
2 Checking for hot spots using spot temperature measurement.
Several factors may affect the temperature reading, such as the following:
- How much heat.
- The material being measured and its ability to absorb or reflect heat (emissivity).
- The object’s temperature.
- The distance from the object being measured as well as the angle at which the object is being viewed.
- The cleanliness of the front germanium lens because of steam, smoke, and soot.
- If the object does not fully fill the center target area, a false reading ma