Fire ember pyrometry using a color camera

Abstract

Firebrands can dramatically increase the hazards of wildland fires. While embers have been extensively studied, little is known about their temperatures. To address this an imaging ember pyrometer is developed here using an inexpensive digital color camera. The camera response was calibrated with a blackbody furnace at 600–1200 °C. The embers were smoldering cylindrical maple rods, 6.4 mm in diameter and 2 cm long. Temperatures were obtained from ratios of green/red pixel values and from grayscale pixel values. Ratio pyrometry is more accurate when ember emissivity times ash transmittance is below unity, but grayscale pyrometry has signal-to-noise ratios 18 times as high. Thus a hybrid pyrometer was developed that has the advantages of both, providing a spatial resolution of 17 μm, a signal-to-noise ratio of 530, and an estimated uncertainty of ±20 °C. The measured ember temperatures were between 750 and 1070 °C with a mean of 930 °C. Comparing the ratio and grayscale temperatures indicates the ember's mean emissivity times ash transmittance in the visible was 0.73. Temperatures were also measured with fine bare-wire thermocouples, which were found to underpredict the ember's mean temperature by 230 °C owing to ember quenching and imperfect thermal contact.