Flashover Recognition

  Several weeks ago, I happened to be out at a fire department training center as the department trained with thermal imagers. The only training they planned to conduct was to get the flashover container hot and then go in and see what it looked...


  Several weeks ago, I happened to be out at a fire department training center as the department trained with thermal imagers. The only training they planned to conduct was to get the flashover container hot and then go in and see what it looked like with their thermal imager. The thought...


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Pre-Flashover With A Thermal Imager

Pre-flashover is where your thermal imager can help and will pay for itself many times over. The fact remains that the only sure-fire way to guarantee that you survive a flashover is to not be there when it happens. You must recognize the signs of a pending flashover and then react to these signs so that you are safely out of harm's way (off the train tracks) when the flashover occurs (the train comes through).

Your thermal imager cannot predict a flashover. It will not tell you or alert you that a flashover is pending; however, it can give you a visual indication of warning signs you would otherwise not see. Without a thermal imager, thick smoke acts as an effective visual barrier to what is going on at the ceiling. Convective velocity, thermal layering and even rollovers are often hidden inside the smoke and are difficult or impossible to detect. The thermal imager has no problem visualizing these events. With the thermal imager, you can often get a good read as to how rapidly the fire gases are moving across the ceiling (indicating they have someplace to go rather than being confined to the room you are in).

Thermal layering is also very often visible to the thermal imager, if not directly (visualization of the fire gases themselves), then indirectly (by looking at vertical wall surfaces for indications of temperature differences). When either of these two things change, convective velocity slows or thermal layers descend, it can serve as early indicators of potential flashover conditions.

You can use your thermal imager to scan a room prior to entry. Look for signs of excessive heat buildup, particularly near the ceiling, or indication of high heat closer to the floor where you may not otherwise expect it. If your imager is equipped with high-heat colorization, you should make sure you are familiar with the corresponding temperatures associated with your particular color system. Use your thermal imager to locate potential vertical or horizontal vent points in case you need them and always use your thermal imager to know where the secondary means of egress are located.

Again, all of these tactics are directed at recognition and avoidance. This is where the thermal imager is effective. You must get out of the way if you have no other means of control (ventilation, hose stream, etc). If you wait until the flashover is taking place, it is too late for the imager, or anything else for that matter, to be of much help.

A Word of Caution

There is one other cautionary note that I want to convey clearly. The temperature-sensing function of your thermal imager is not a reliable indicator of flashover or pre-flashover conditions. (For a more detailed explanation of the capabilities and limitations of temperatures sensing, please see "The Truth Behind Temperature Sensing — Part 1 and Part 2" in the April and May 2009 issues of Firehouse® Magazine.) Although much has been written about what temperature range(s) commonly exist during a flashover, do not be tempted to rely on the temperature measurement function of your thermal imager to determine safe or not safe. Even though the imager is equipped with a three- or four-digit number that tells you the temperature, these devices simply are not reliable enough to bet your personal safety on. This is true for all manufacturers of thermal imagers. No one imager's ability here is any better or worse than any other.

The first concern is that the temperature-sensing capability of your imager cannot accurately detect the temperatures of gases, which is where the greatest threat usually lies in the growth stage of a fire. Thermal imagers are designed to detect surfaces, but not gases. The second concern is that, as temperatures rise, the imager's temperature-sensing margin of error grows correspondingly. A 10% margin of error in ambient temperatures only indicates a seven-degree accuracy question; however, at high temperatures, many thermal imagers will increase to a 20% or higher margin of error, which can represent a 200 degree or greater measurement error at 1,000 degree temperatures in controlled conditions.