Comparing Techniques for Estimating Flame Temperature of Prescribed Fires

Abstract

A variety of techniques that estimate temperature and/or heat output during fires are available. We assessed the predictive ability of metal and tile pyrometers, calorimeters of different sizes, and fuel consumption to time-temperature metrics derived from thick and thin thermocouples at 140 points distributed over 9 management-scale burns in a longleaf pine forest in the southeastern US. While all of these devices underestimate maximum flame temperatures, we found several to be usehl for characterizing other rnetrics of fire behavior. While the degree to which thermocouples underestimated maximum temperatures was based on thickness, metrics derived from thermocouple data that inte~ated time and temperature minimized this discrepancy between thin and thick thermocouples. Thermocouples also provided the most detailed spatial and temporal data of the devices tested. Pyrometers underestimated maximum temperatures relative to tl~ermocouples, but due to their low cost, can be useful for examining spatial variation in temperature during fires. Use of calorimeters is disadvantageous given their lack of precision and high labor cost. Simple fire behavior observations taken during burns and indicators of fire severity observed post-burn were inexpensive to estimate and revealed useful differences among fires. Due to the wide variation among these techniques in cost, labor, accuracy, and level of detail of results, their suitability for a particular project will vary according to research objectives and available resources. Researchers should ensure that the fire behavior parameter measured has a logical relationship to the effect of interest, is measured at an appropriate level of detail, and is reported with attention to the limitations of the measuring devices used. Estimating flame temperature and duration has become increasingly common in studies of the ecological effects of fire. Researchers have found estimates of fire temperatures and their durations correlate well with fire effects on specific plant parts (seeds, roots, cambium) or soil components (e.g., Auld and O'Conner 1991; Dichnson and Johnson 2001). Time-temperature curves are most commonly measured using thermocouples deployed with data loggers. However due to the high cost of data loggers, researchers often rely on less expensive tools such as pyrometers or calorimeters to provide an index of flame temperature or heat release. Pyrometers, namely pellets, paints, or crayons manufactured to melt or change colors at specific temperatures, are commonly used to estimate fire temperatures in ecological studies (Fonteyn et al. 1984;Hobbs et al. 1984;Gibson et al. 1990;Cole et al. 1992; Franklin et al. 1997 (Beaufait 1966; Knight 1981;Moreno and Oechel 1989;Perez and Moreno 1998).This suite of techniques commonly used by fire ecologists vary considerably in cost, level of detail of results, and most importantly, accuracy. Notably, few of these instruments measure actual flame temperatures (or heat release in the case of calorimeters) as is sometimes er...

Effects of prescribed fire and other plant community restoration treatments on tree mortality, bark beetles, and other saproxylic Coleoptera of longleaf pine, Pinus palustris Mill., on the Coastal Plain of Alabama

“…Thin plus burn plots were treated the same as thin only plots but were then burned. Specific details on the prescribed burns can be found in Outcalt (2003) and Kennard et al (2005). Herbicide plus burn plots received a fall application of 4.5% Garlon 4 TM (triclopyr; Dow AgroSciences, Calgary, Alberta) with Timberline 90 TM (UAP Timberland, Monticello, AR) surfactant added, and then burned 3 months later.…”

Delayed conifer mortality after fuel reduction treatments: interactive effects of fuel, fire intensity, and bark beetles

“…These tiles provided a measure of the maximum instantaneous temperature during the passage of the flaming front within the constraints of the 25 different lacquer spots, although they provided no indication of duration. Work with similar tile pyrometers indicated that this method may underestimate maximum temperatures relative to those recorded by thermocouple, yet is sufficiently precise and accurate for characterizing fire behavior (Iverson et al 2004, Kennard et al 2005. Heat sensitivities of the lacquer spots ranged from 408 to 9008C with about 208C intervals between 408 and 4008C and 758C intervals .4008C.…”

Changes in fuelbed characteristics and resulting fire potentials after fuel reduction treatments in dry forests of the Blue Mountains, northeastern Oregon

“…These tile pyrometers provided a measure of the maximum instantaneous temperature during the passage of the flaming front within the constraints of the 25 different lacquer spots, but provided no indication of duration. Work with similar tile pyrometers has indicated that this method may underestimate maximum temperatures relative to those recorded by thermocouple, yet is sufficiently precise and accurate for characterizing fire behavior (Iverson et al, 2004;Kennard et al, 2005). Heat sensitivities of the lacquer spots ranged from 40 to 900 8C with about 20 8C intervals between 40 and 400 8C and 75 8C intervals >400 8C.…”