On 10/27/2014 03:08 PM, Jed Rothwell wrote:
All hot objects radiate heat the same way, and they all turn the same
incandescent color at a given temperature.
Everything I read tells me this is modified by a materials 'emissivity'
factor.
"The amount of thermal energy an object will radiate is not only a
function of temperature, but depends on the material itself. Emissivity
describes a material’s ability to emit or release the thermal energy
which it has absorbed. A perfect radiator-known as a ‘black body’-will
emit the entire amount of absorbed energy. A real body will always emit
less energy than a black body at the same temperature. Emissivity ε is
the ratio of radiation emitted of a given object (real body) Φ_r and a
black body Φ_b at the same temperature."
http://www.keller-msr.com/temperature-pyrometers/emissivity-definition-and-influence-in-non-contact-temperature-measurement.php
"Emissivity is a modifying factor used in single color thermometry to
achieve a correct temperature
reading. Emissivity, or radiating efficiency, of most materials is
function of surface condition,
temperature and wavelength of measurement."
http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf
Likewise, aluminum oxide (alumina) has an emissivity coefficient of 0.8
according to this reference:
http://www.gphysics.net/emissivity-coefficient
and 0.75 according to this reference:
http://www.coe.montana.edu/me/faculty/sofie/teaching/me360/Pyrometry%20Emissivity%20Notes.pdf
So, as I understand it the emissivity factor must be applied to an ideal
black-box foruma as follows:
"The radiation energy per unit time from a *blackbody* is proportional
to the fourth power of the absolute temperature
<http://www.engineeringtoolbox.com/temperature-d_291.html> and can be
expressed with *Stefan-Boltzmann Law * as
/q = σ T^4 A/ / (1)/
/where/
/q/ /= heat transfer per unit time (W)/
/σ/ /= 5.6703 10^-8 (W/m^2 K^4 ) - *The* *Stefan-Boltzmann Constant*/
/T/ /= absolute temperature Kelvin (K)/
/A/ /= area of the emitting body (m^2 )/
For objects other than ideal blackbodies ('gray bodies') the
*Stefan-Boltzmann Law* can be expressed as
/q = ε σ T^4 A / /(2)/
/where/
/ε/ /= emissivity of the object (one for a black body)/
http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html
So, alumina, with an observed value of 950C and an emissivity factor of
0.75, would actually be at 1250C - 1350C, considering the conversion
from C to K back to C.
Craig
