NIST - National Institute of Standards and Technology

02/13/2026 | Press release | Distributed by Public on 02/14/2026 03:09

Demonstration of Near-Infrared Open-Path Absorption Spectroscopy for Water Vapor and Temperature Monitoring During Structure-to-Structure Fire Spread Experiments

Table 1.

H 2 O Transitions of Interest from ν1+ν3 Vibrational Band

Label Frequency, cm−1 Line Intensity, cm−1/(molecule⋅cm2) at 296 K Lower State Energy, cm−1
I 7175.4930 2.81 × 10 − 23 1360.2353
II 7175.9220 2.26 × 10 − 24 2129.5992
III 7175.9873 2.68 × 10 − 22 206.3014

Table 2.

Review of Data Throughput and Overall Experimental Results

Observed Change within Ignition Windowb Observed Change within Vent Activation Windowb
Test ID Peak HRR (MW) Time of Peak HRR (min)a Vent Closure (Yes, Partial, No) Time of Fire Suppression (min)a Attic Data Throughput Eaves Data Throughput Eaves Path Attic Path Attic Path
e9 1.1 12.68 Partial 25.02 90 % 25 % Yes No No
e10 1.4 12.85 Partial 15.15 90% 30% Yes Yes Yes
e11 1.4 11.02 Partial 12.47 95% 42% Yes Yes No
e12 0.75 15.45 No 25.03 90% 21% Yes Yes N/A
e13 1.1 8.88 No 25.03 90% 17% Yes Yes N/A
e14 1.3 16.15 No 25.00 50% 17% Yes Yes N/A
e15c 1.7 16.55 Partial 18.93 90% 25% Yes Yes No
e17d 1.4 18.00 Partial 25.02 69% 31% Yes N/A No
e18 1.9 17.98 Partial 18.85 98% 36% Yes No Yes
e19 1.8 15.53 Partial 16.85 84% 31% Yes No Yes
e20 1.4 16.07 Yes 40.00 70% 40% Yes No No
e21 1.4 14.72 Yes 50.10 23% 29% Yes Yes No

Times calculated as minutes since ignition time.
Observable changes which indicate a clear increase or decrease in temperature, water vapor mole fraction, or noise level of the data within specific time windows (see Section 5.C for further discussion and Supplement 1 for additional figures). Ignition window is defined here as one minute prior to and three minutes after fuel ignition. The vent activation window is defined as two minutes before and after the determined vent activation time.
Excessive steam during fire suppression engaged the laser safety shutdown procedure due to prolonged loss of signal at the detector.
Realignment of receiving optics in the attic path prior to this experiment resulted in saturation of the detector and a lower throughput around ignition.
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