Have you ever wondered how, quantitatively, we can determine the thermal protective rating of protective materials and garments? These quantifiable ratings wouldn’t be possible without something called the Stoll Curve.
The Stoll Curve is one of the great determinants for pass/fail criteria across the board for personal protective equipment, and was developed as a basic threshold for predicting burns. Consider the Stoll Curve as a speed limit. If a driver is texting while driving under a certain speed and wrecks, the car’s collision capacity would be able to protect the driver; if the driver is texting while driving over a certain speed and wrecks, the car’s collision capacity may not ultimately protect the driver. Exceeding the speed limit doesn’t guarantee bodily damage, and similarly, crossing the Stoll Curve does not guarantee a burn. There are many other factors to consider; in the driving example, distractions (like texting), a police officer preemptively stopping the speeder, etc., will influence the outcome. Rather, the Stoll Curve identifies a point at which more protection should be considered to avoid injury.
The Stoll Curve determines the rating of the transfer of heat energy (calories) based on the time of transfer and the level of heat energy produced. Ideally, protective garments and equipment will delay the transfer by absorbing the heat energy at increased heat fluxes. Standards determine ratings as the amount of energy absorbed in cal/cm2 before the time of transfer to human tissue and the result of a predicted crossing of the Stoll Curve criteria (which, for short, standard writers and test houses call a “burn”). For example, per NFPA 2112: Standard on Flame-Resistant Garments for Protection of Industrial Personnel against Flash Fire, Section 7.1.1: Heat Transfer Performance, a garment is required to absorb 6.0 cal/cm2 of heat energy before the time to second degree burn in a ‘spaced’ exposure using the Thermal Protective Performance method.
Where did the quantifiable data of the Stoll Curve come from?
In a study performed by Alice Stoll and Maria Chianta, the women lined up and physically burned a number of young sailors on their forearms! Science! They recorded the temperature and time at which the men responded to the pain. This was performed across a range of radiant heat fluxes, and the burns that developed blisters after 24 hours were recorded as ‘second degree burns.’ This is actually how second degree burns became medical terminology, as they previously weren’t quantified. Other work was done on pig flesh and rats to further correlate the information at deeper levels.
The graph of the Stoll Curve represents the reaction of human tissue to low heat flux for a long period of time and high heat flux for a short period of time. However, these fluxes are nothing like an arc flash–they are much lower. Today, standards use the Stoll Curve to determine the time and energy at which a garment or piece of equipment will prevent the influx of pain and second degree burn. This is represented by comparing a graph of the heat flux exposure time with the Stoll Curve. Their point of intersection is recorded as the heat flux at the time to second degree burn, and is reported as a rating of the transfer response through a material.
In arc flash testing, copper calorimeters behind the exposed fabric panel report the energy which passed through the material, and other uncovered calorimeters record the energy incident on the fabric. A conversion of conductivity from copper to the thermocouple is calculated to determine the expected human tissue reaction of the exposed energy and is compared to the Stoll Curve to determine the level of survivable burn.
ASTM F1959, the arc test portion of ASTM F1506 that is used for clothing meeting NFPA 70E, considers a ‘survivable burn’ as only a 50% probability of second degree burn. This means the heat flux only has a 50% probability of crossing the Stoll Curve, indicating the onset of second degree burn. The actual area the arc hits is very small and would likely amount to less than 25% of the body receiving the onset of second degree burns. The arc flash standard is very conservative compared to flash fire standards like NFPA 2112, which allows 50% of the entire body to get second and third degree burns in a flash fire exposure.
Arc directionality, sweating in the garment, and contaminants (dirt, lint, etc.) in a garment can lower protection, but these are rarely life-threatening. The Stoll Curve gives us an excellent suggested safety benchmark to protect workers in a comfortable manner without increasing other hazards or hampering the work.