How Do You Measure Flammability?

Making a s’more is a small lesson in flammability.

A marshmallow doesn’t burn the moment it gets near a fire. First, it heats up, melts and starts to brown. If you do everything just right, it turns crispy on the outside and gooey on the inside. But if you hold the marshmallow a little too close or too long, it ignites and burns.

A marshmallow on fire will break down and release combustible gases. How easily that marshmallow ignites, how intense the fire burns and how much heat is released is what's known as flammability.

Flammability can tell you how quickly a material — whether it’s a marshmallow or a mattress — contributes to fire growth. Flammability research is about moving materials farther away from that tipping point — so instead of igniting quickly, they heat more slowly, release less fuel, or resist ignition long enough for people to avoid a fire.

One widely used tool for measuring flammability is the cone calorimeter.

The Cone Calorimeter

NIST researchers developed the cone calorimeter in 1982. It measures a material’s flammability by burning a material and determining how much oxygen it consumes, a method known as oxygen consumption calorimetry.

The cone calorimeter is considered the gold standard for flammability measurements. The fire research community uses it to select and develop materials that meet fire safety regulations and standards for different products.

But how does it work exactly? Researchers create a puck-sized sample of a flammable material, such as a wood composite. They placed the sample under a cone-shaped electric heater, the tool’s namesake. The heater ignites the material, which breaks down and releases fuel. When there is enough fuel, the sample starts to burn.

Cone Calorimeter

The material sits on a scale, allowing researchers to measure the sample’s loss of mass during combustion. The sample’s loss of mass is related to its heat release rate and the amount of oxygen it consumes.

The fire’s byproducts flow up toward an exhaust system that rests above the cone-shaped electric heater. Researchers can then measure different properties of the fire, such as levels of toxic gases, using instruments connected to the exhaust system.

The cone calorimeter allows scientists to measure the flammability of small samples of material in a compact laboratory space. This includes a wide range of materials, including solid plastics, wires and cables, aerospace composites, carpeting, fabrics, furniture, electronics and construction materials.

However, to understand the real-world behavior of a flammable material, full-scale fire testing is needed. Researchers conduct these kinds of tests in NIST’s National Fire Research Laboratory. While the cone calorimeter provides critical data on a material’s flammability, full-scale tests reveal additional information, such as how a fire spreads in different environments.

There are currently two cone calorimeters on NIST’s campus, and the original design has been commercialized.

The tool’s impact is significant, to say the least. It has led to the development of new fire-resistant materials for products, such as mattress coverings, and was instrumental in the development of new safety standards and codes, such as for smoke alarms, all of which have ultimately reduced fire-related deaths.