Pyrometry Emissivity Notes

7/27/2019 Pyrometry Emissivity Notes

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Emissivity

Electromagnetic radiation is either absorbed

(emitted), reflected, or transmitted through the surface.

Emissivity is ratio of absorbed vs.reflected/transmittedenergy.

An emissivity of 0.9 indicates that the surface willabsorb (emit) 90% of the energy and reflect (transmit)10%

Emissivity is a spectral parameter and as such thesame material will transmit heavily at one wavelengthand absorb heavily at another wavelength.


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General Emissivity ValuesPolished brass 0.03

Polished copper 0.15

Polished iron 0.2

Aluminum oxide 0.75

Sawdust 0.75

Sand 0.76

Black oxidized copper 0.78

Oxidized iron 0.8

Rubber Nat Soft 0.86

Wood Oak 0.9

Roofing paper 0.91Plastics 0.91

Rubber Nat Hard 0.91

Paper 0.92

Porcelain, glazed 0.92

Plaster 0.92

Paper 0.93

Quarts glass 0.93

Water 0.94Glass 0.94

Rubber, hard glossy plate 0.94

Lampblack 0.95

Water 0.95

Paint 0.96

Tile 0.97

Plaster 0.98Laboratory blackbody 0.99

Aluminum Commercial sheet 0.09

Aluminum Heavily Oxidized 0.25

Aluminum Anodized 0.77Aluminum Rough 0.07

Polished aluminum, 300 deg K 0.03

Polished aluminum, 1000 deg K 0.07

Steel Oxidized 0.79

Steel Polished 0.07

Stainless Steel, weathered 0.85Stainless Steel, polished 0.075

Steel Galvanized Old 0.88

Steel Galvanized New 0.23

Tungsten 500K 0.05

Tungsten 1000K 0.11

Tungsten 2000K 0.26Tungsten 3000K 0.33

Tungsten 3500K 0.35

*emissivity is wavelength dependent


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Explain the hot spot?

This can of brew is ice cold straightout of the fridge. When scanned with

an infrared camera you would expectthe entire image to be relatively evenin temperature and to appear "cold" inrelation to the background. Can youexplain the apparent "hot" spot in thecenter of the can. Hint: it's not a

fingerprint!

Live image reveals truth!

The paint on the outside of the can hasbeen scratched off in a small area. The

bare aluminum has a different emissivitythan the painted aluminum. Thecamera can only allow for oneemissivity setting at one time so to thedetector the bare aluminum "images"hotter than the rest of the can.


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Two cars sitting in the sun

1. Chrome plated steel bumper (e=0.04)

2. Black painted steel bumper (e=0.95)Which bumper is hotter? (assuming identical bumper design in identical conditions)

How many times have you been burned by the chrome seat belt buckle insideyour car on a very hot day

This question is not so easy to answer and involves time

dependency, surface/bulk temperature effects as well as

convection and conduction!

1. The painted bumper will heat up more quickly, but 95% of the absorbedheat is free to re-radiate.

2. The chrome bumper will heat up more slowly, but only 4% of theabsorbed heat is free to re-radiate, hence the internal steel structureunder the chrome bumper will eventually exceed that under the paintedbumper and ultimately the entire chrome bumper will be hotter than thepainted bumper.


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Carbon tape

Heat Transfer Effects


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Painted surface increases emissivity, thus improving heat

transfer and keeping the motor cooler and more uniform,

while allowing more precise temperature mapping

The high E coating allows theheat generated internally to bedissipated more effectively via

radiation


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A cylindrical black-body cavity is described, which will operate up to 1100 C, made ofstainless steel 15 cm long and 6 cm in diameter and with a working aperture 3 cm indiameter. Internal baffles and grooved surfaces are used to attain an emissivity of0.997 while keeping the dimensions of the cavity small. The mounting of the cavity ina horizontal Nichrome-wound furnace is also described along with experiments whichconfirm that the emissivity is as high as 0.997 (internal emission and re-absorption by thewalls nullifies surface affects)

Emitted energy is dispersed thuslowering emissivity

Emitted energy is concentratedthus increasing emissivity


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How to Overcome Emissivity Paint the object with a carbon paint so that the

surface mimics a black body

Bore a deep narrow hole in the object and imagedirectly in the black body cavity

How to Determine Emissivity Emission spectrometer - $$$$ process typically

done in a laboratory

Put a black body object in the proximity of the

unknown material. Obtain a temp. value for thematerial with a known emissivity (black body).Focus the pyrometer on the unknown object andadjust emissivity value until correct temperature is

measured

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Pyrometry Emissivity Notes