Identifying the Thermal Layer?

In many fire behavior/thermal imaging classes, identification of the thermal layer is taught to assist in identification of overall severity of conditions. 

However, if we don’t know/understand our TIC this can cause us great harm as this is two TIC’s looking at the same fire.

The thermal layer is defined as the line or space between the superheated areas and the cold area below. This is not to be confused with the concept known as a neutral plane.

The neutral plane is the area between the exhaust and the intake at an opening and can only be identified once an opening is made. 

The neutral plane is based on differences in pressure whereas the the thermal layer is based on temperature.

However, the thermal layer isn’t always easily identifiable nor is seen at the same location in the same fire when viewed through various brands of thermal imaging cameras.

NFPA 1801 (the standard in thermal imaging cameras for the fire service) did an outstanding job of quantifying the specific requirements of thermal imaging cameras for the fire service.

Unfortunately, they did not standardize when the TIC does the following:

• At what temperature and overall pixel percentage does the TIC switch from High Sensitivity to Low Sensitivity 
• When colorization engages: high and low sensitivity or only low sensitivity and at what specific temperatures?

Firefighters need to understand that the TIC ‘sees’ the thermal layer based on its individual programming. For example, the following TIC’s show colorization at these varying levels:

• FLIR shows colorization beginning at 300 Degrees Fahrenheit (beginning with yellow)
• Leader/Tempest series TIC’s show colorization beginning at 392 Degrees Fahrenheit in Low Sensitivity. 
• Scott X380 shows colorization beginning at 300, 400, and 600 depending upon which sensitivity mode it is in (High, Medium, or Low Sensitivity)
• Bullard 500 Degrees Fahrenheit (beginning with yellow)
• Drager UCF 9000 shows colorization at 270 Degrees Fahrenheit in High Sensitivity and 570 Degrees in Low Sensitivity
• MSA 6000 Shows colorization at 270 Degrees Fahrenheit in High Sensitivity and 1000 Degrees in Low Sensitivity

The issue becomes even more complex when you have different brands of TIC’s operating within the same fire department. A firefighter operating without proper training could easily misinterpret this data. As shown in the photo below of two different brands of TIC’s (newer vs and older); the thermal layer is shown completely differently.

Many previous thermal imaging instructors preferred gray scale as their primary color palette. This complicates the identification of the severity of the environment even further with high resolution TIC’s.

Firefighters have seconds to make a decision and many TIC’s use as many as 246 shades of grayscale whereas the human eye can only identify 30 shades of gray and only 4 shades of gray under stress.

By adding colorization at earlier temperatures, firefighter’s can identify superheated areas to prevent further thermal insult to the victim, to their property, and prevent rapid fire progression.

We hope these short articles are helpful in understanding thermal imaging but they are not meant to be a substitute for hands on training with live fire.

Keep up the great work,

Instructor Andy Starnes

Insight Training LLC

Level II Thermography Certified