Experiment compartment at FSRI’s large-scale testing facility in Delaware County

Researchers Conduct Experiments Measuring Gravimetric Soot Mass and Soot Particle Size

September 16, 2025

Earlier this summer, the Fire Safety Research Institute (FSRI), part of UL Research Institutes, conducted experiments to better understand gravimetric soot mass and soot particle size. Following initial data analysis from the first phase of experiments conducted in 2024, FSRI has begun the second phase of experiments in May at FSRI’s large-scale research facility in Delaware County, Pennsylvania, and will wrap up later this fall.

The image shows the compartment set up to measure gravimetric soot mass and soot particle size for the research project, fire modeling development and validation — Experiment set up in the compartment used to measure gravimetric soot mass and soot particle size. On the floor of the compartment is the gas burner used as the fire source. On the walls and ceiling of the compartment are four removable panels (seen leaning against the wall) containing attached filter paper for soot deposition measurement. Also visible are two of the three laser obscuration devices (one more is along the ceiling). Temperature and gravimetric soot measurements are made at each laser location. Soot size distribution measurements are made at the upper laser location.

Using Soot Data in the Fire Dynamics Simulator

The Fire Dynamics Simulator (FDS) is a widely used computer fire modeling system developed by the National Institute of Standards and Technology (NIST). It predicts variables in the fire environment, such as smoke concentration, temperature, flame position, and heat release rate. Through the Fire Modeling Development and Validation research project, FSRI contributes to FDS’s ongoing development through software development and both bench- and large-scale experiments.

This series of experiments aims to improve the predictions of visibility made by FDS. Visibility is a key quantity in performance-based fire protection design due to its impact on egress times. The experiments are designed to collect a comprehensive set of soot-related data to support both model validation and improvements to user inputs for performance-based design. The experiments are designed to yield data on the relationship between soot mass and visibility, the relationship between soot size distribution and visibility, and deposition rates at prototypical scales. To achieve this, FSRI researchers designed a set of small-scale (hood) and full-scale experiments to collect measurements of gas temperature, gas concentration, laser obscuration, gravimetric soot mass, gravimetric soot particle size, and gravimetric surface deposition.

Hood testing took place under a small hood with oxygen consumption calorimetry. These tests were used to develop model inputs for the soot production in the propylene fire.

At-scale testing took place in a standalone compartment with both closed and open-door tests. Gravimetric soot mass and soot particle size were measured by pulling gas from the compartment through filters, and the surface deposition was measured through filter paper attached to the wall, ceiling, and floor.

The image shows pre-test weighing of the set of gravimetric filters used in a single test. The left tray contains the 15 filters used to measure the gravimetric soot mass. The right tray contains the 35 filters used in cascade impactors to measure the soot size distribution. — Filter paper is weighed pre-experiment.

The image shows the lower wall of the compartment in the at-scale experimental setup. On the wall is the filter paper used to measure surface deposition. The ceramic fiber wrapped boxes house the laser and photodiode used to measure obscuration. — Filter paper is placed on the compartment wall in the experiment setup to measure surface deposition. It’s placed adjacent to the laser path used to measure obscuration.

“Visibility during egress, fire patterns seen post-fire, and radiation from flames and hot layers are all tied to the behavior and properties of soot in a fire; however, there are few large-scale experiments where data for soot are collected beyond a simple obscuration measurement. This poses challenges for model validation and user guidance for model inputs.”

—Jason Floyd, principal research engineer, FSRI

Following data analysis, findings from this experiment phase will contribute to the continued development of the Fire Dynamics Simulator.

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