Firebrick Emissivity

[jamescw]

What is the thermal emissivity (correction for a laser infrared thermometer) of the firebrick in my Pompeii 110 kit?

I have just about completed construction and am into the curing process. I've seen several different figures posted on the Internet for the emissivity of firebrick ranging from 0.70 to 0.90. It makes a big difference in my temperature readings. I imagine the amount of soot on the brick (dome ceiling vs oven floor) would also make a difference.

I suspect 0.70 is closer than 0.90 to the right figure. Firebrick is supposed to absorb heat--act as a heat sink--not reflect it. Or do I have that backwards?

Any thoughts?

[altamont]

Re: Firebrick Emissivity

Gosh, trying to bring details up to the active lobes after they've been retired to the sleeping portions of my brain for soooo long.
I think you will actually obtain pretty accurate readings without factoring in emissivity. Calibration of the infrared thermometer will probably give you a greater variance from the actual temperature.
IF I recall (capital IF) the infrared thermometers will be reading the overall color or spectra being emitted - emissivity should not affect that much. The 'amount' of radiation off of the surface will be affected more by the properties of the material (but, again, not the spectra). And varying 'amounts' of radiation or light is different from varying 'colors'. As the temp increases, the wavelengths get shorter and that is what the infrared thermometers are reading.
I might be wrong (ref: the capital IF).
Now, considering that my electric oven has to be set to @ 275 F in order to actually be at 325 F (50 degree adjustment) I can live with any consistent variation my thermometer from Harbor Freight gives me.
Just so long as it is consistent. If I get a reading of 850 F time after time while the oven is actually at 907 F I will be happy. I will just adjust as I have had to with my old electric oven.

[FritzNeumann]

Re: Firebrick Emissivity

James
If the brick is covered by soot the emissivity of the surface will approach 0.96. Any retained heat in the brick will be radiated back into the oven. When the soot is burnt off the surface of the brick turns a nice matt cream which also has a high emissivity factor approaching 0.93. Emissivity is mainly a function of surface finish, please refer to the attached URL file that will make this clearer. You are correct with the radiant heat , we want the bricks to absorb energy and then continue to radiate heat back into the oven and that is why a insulating layer and reflective silver barrier is laid over the outside of the bricks to reflect any heat conducted through the brick back towards the internals of the oven. Same principal as a solar oven, shiny reflective surfaces have very low emisivity and generally reflect energy, what the oven is doing is using the bricks as a heat sink and the matt black /cream surface allows the stored energy to be radiated back out of the heatsink. With your thermal gun, always try to point to a perpendicular matt surface to measure the temp to get a true reading, angled measurements always will give you combinations of different radiant sources, also get as close as possible, the cone effect of the detector means you will be measuring an averaged 100mm circle temperature at a distance of 1000mm from the source. The closer you are, the smaller the circle, the more accurate the temperature.
Regards Fritz

http://www.omega.com/temperature/z/pdf/z088-089.pdf

[casper]

Re: Firebrick Emissivity

any table i look up states alumina firebrick to have an emissivity of 0.68
Emissivity Materials