S-probe

Heat and smoke generated at a fire are transported by the fire’s buoyant forces; ultimately creating the potential for fire spread or toxic exposure at locations other than where it started.  Therefore, flow measurements are critical to fire research experiments to quantify the transport of heat and smoke generated by a fire.  For decades the bi-directional probe has been used to measure flow in fire research experiments and fire safety testing.  Based on the principle of the pitot-static probe, which measures the pressure differential between two ports to determine flow velocity, it was designed with large openings to resist clogging from soot and withstand high temperatures.  Despite being a practical and robust instrument for fire experiments, the bi-directional probe never saw commercial development. Thus, fire researchers must manufacture their own probes and rely on dated performance characterizations from the literature. Hence an instrument critical to the progress of fire research exists without reliable manufacturing or calibration standards.

The S-type pitot probe (S-probe) is the primary tool for auditing flow measurements for smokestack emissions.  Operating on the same principle as the pitot-static probe, it has proven effective as a flow measurement device in high temperature, sooty, and particle-laden conditions.  Like the bi-directional probe, the S-probe is practical and robust for use in harsh conditions. However, the S-probe is commercially available, with established manufacturing and calibration standards for better consistency and reliability, making it another tool to consider for advancing fire research.

This work is a comparative study of the S‑probe and bi‑directional probe to evaluate their performance for wind speeds in the range of typical fire‑induced flows (1 m/s to 10 m/s).  A well‑characterized wind tunnel is utilized to confirm each probe’s calibration coefficient, and a centrifugal fan is used to provide a flow field with turbulence and intermittency similar to the fire environment. In all cases, a pitot-static probe provides the reference measurement for evaluating probe performance.  Multiple probes are tested to introduce slight variations in probe geometry due to manufacturing and use (fouling). Results from the wind tunnel show that the S-probe and bi-directional probe have similar performance.  The probe calibration coefficients are in good agreement with values reported in the literature for the bi-directional probes and S-probe coefficients agree with manufacturer calibrations and standard test methods.  At the core region of the wind jet generated by the fan, differences in flow speed are within the uncertainty limits for each probe.  Overall, the S‑probe demonstrates better agreement to the reference measurement.  Since it is a commercial product that can be easily acquired, fire researchers have another tool to better quantify heat and smoke transport in fire experiments.

Related Publications

• Di Cristina Torres, G. and Bryant, R. (2024), Comparison of two flow measurement devices for use in fire experiments, Proceedings of the Combustion Institute, [online], https://doi.org/10.1016/j.proci.2024.105557,  https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=958471
• Di Cristina Torres, G. and Bryant, R. (2024), Comparison of Flow Measurement Devices for Large Fire Experiments, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2285, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=957463