Large-scale calorimetry time response characterization and correction

Recent fire experiments testing 4 m to 6 m tall Douglas fir trees pushed the limits of the time response of the National Institute of Standards and Technology (NIST) large fire calorimeter due to their fast fire growth. The fires obtained peak heat release rates from 7 MW to about 42 MW within 7 s to 10 s. The calorimetry system is dependent on multiple instruments each with their own time response. Calibration experiments with imposed square wave thermal pulses have characterized the system time constant as approximately 8 s. Consequently, the time response becomes a significant source of uncertainty in the transient results. Utilizing measurements from fast-responding mass load cell and far-field radiometers as models for the heat release rate (HRR) response, a methodology is developed to rescale the transient HRR to correct for the calorimetry system’s time response. The results from each correction method are compared to each other and the oxygen consumption HRR. Although both methods have different limitations, their respective results agree within 15% of each other, on average. This study provides insight on the accuracy and uncertainty of oxygen consumption calorimetry systems.