What is a Stoll Curve?
As a woman-owned company, Tyndale is dedicated to celebrating and supporting women in the industry. Our popular Women in Power Series shines a light on the growing number of women working in the field today. But whether in the spotlight or behind the scenes, women have played an integral role in industry all along. Have you ever heard of the Stoll curve? This important calculation got its name from the groundbreaking work of Alice Stoll – a pioneer in developing the arc-rated and flame resistant (AR / FR) fabrics countless workers, like you, trust with your lives each day.
In fact, Alice Stoll’s work was essential to determining an AR / FR fabric’s ability to protect workers against flash fire, electric arc, and other workplace hazards. The Stoll curve, which determines how much energy is required to cause a second-degree burn on human skin, is named for her. Listen in as Scott Margolin, Tyndale’s Vice President of Technical and Corporate Strategy, explains how the Stoll curve establishes the basis for the arc flash testing and flash fire testing of fabrics:
Who was Alice Stoll?
Alice Stoll (1917–2014) was born in Long Island, NY and received a Bachelor of Arts degree in chemistry and physics from the Hunter College of the City of New York in 1938. She joined the US Navy, serving as a lieutenant during WWII, and was released to inactive duty in 1946. She remained in the reserves until 1966 when she retired as a Commander. In 1948, Alice received her Master of Science degree in physiology and biophysics from Cornell University’s Medical College, where she served as a research and teaching assistant. During her time at Cornell, she obtained a patent and developed the Thermistor Radiometer, a scientific instrument used for the measurement of skin temperature and radiant environmental temperature.
Alice furthered her research during her employment with the Federal Government, conducting experiments in thermal tissue damage while administering a laboratory of approximately 70 personnel. Coincidentally, the Naval Air Development Center, where she did much of her thermal laboratory research that contributed so significantly to our industry, was located not too far from Tyndale’s headquarters in Bucks County, PA.
In the 1960s, she became Head of the Biophysics and Bioastronautics Division, studying thermal radiation and the effects of flame contact with human skin and protective materials. She was active in numerous technical societies and chaired the Heat Transfer in Biotechnology Technical Committee of the American Society of Mechanical Engineers. In 1969, Alice Stoll was recognized with an Achievement Award from the Society of Women Engineers “in recognition of her significant contributions in the field of the development of fire-resistant fibers and fabrics, based on her pioneering studies of heat transfer by flame contact.” Thanks to the work of Alice Stoll, it is possible to rate different materials for their ability to protect against thermal hazards.
What is Heat Flux?
Alice Stoll’s work in the 1950s and 1960s with fellow researcher Maria Chianta, which studied the reaction of human skin to thermal energy, contributed significantly to our understanding of heat flux. Their research focused on determining the amount of energy necessary to cause a blister, known as a second-degree burn, on human skin. Burn research was conducted on various live, anesthetized test subjects to record the exposure time and skin temperature at which the onset of pain occurred, producing minimal blistering within 24 hours. These experiments indicated that temperature, the surface area of exposed skin, and the duration of exposure are combined factors – known as heat flux – that determine the onset of second-degree burns.
As Scott explains, heat flux is different than temperature. Heat flux is the amount of energy (heat flow) over time over a given surface area. Therefore, the hotter something is, the less duration is required to cause a second-degree burn. The cooler something is, the longer the exposure time necessary to cause a second-degree burn.
What is the Stoll Curve, and How is it Used in Fabric Testing?
The Stoll curve represents the energy, surface area, and time required for the onset of second-degree burns to human skin. Alice Stoll’s work demonstrated that about 1.2 calories per centimeter squared (1.2 cal/cm2) per second produces the onset of a second-degree burn.
This formula is still applied today – over 60 years later – in computer programs used in arc flash and flash fire laboratories. These programs measure heat transfer from an arc or fire through a PPE fabric to a calorimeter behind the fabric and translate the data to determine expected second-degree burns. A fabric’s arc rating is the calorie level it can “block” before the onset of second-degree burns. For example, CAT 2 fabrics have a minimum arc rating of 8.0 cal/cm2 using second-degree burn criteria based on the Stoll curve. Essentially, all relevant AR and FR fabric standards and tests use the Stoll curve to predict body burn, including ASTM F1930 (manikin test) cited in NFPA 2112 and ASTM F1959 (arc rating test) cited in ASTM F1506.
We can thank Alice Stoll for her important contributions to our field, which led to the development of AR / FR fabrics. Her research provided the benchmark that underpins the AR / FR fabric testing used today to keep workers safe.
