TIR100-2 Measurement of Thermal Emissivity

Transcription

1 TIR100-2 Measurement of Thermal Emissivity Dr. Thomas Meisel INGLAS GmbH & Co. KG Introduction Some basic physics Principles of measurement pren Working with the TIR100-2 Practical course INGLAS GmbH & Co. KG 1

2 TIR100-2 History 1987 First Emissiometer developed and patented by Dornier System GmbH Friedrichshafen. Application: solar collector coating. Analog Instrument, semiconductor sensor. Range e: 0,2.. 0, Emis01 by Dornier System, Table top, microcontroller, thermopile Sensor. Range e: 0,10.. 0, TIR100 by INGLAS GmbH & Co. Application: glass and solar collector coating. Table top, thermopile Sensor. Range e: 0,07.. 0, TIR100-2 by INGLAS GmbH & Co. Handheld instrument, thermopile Sensor. Range e: 0,02.. 0,98 INGLAS GmbH & Co. KG 2

3 TIR100-2 Reference List 2/2010 INGLAS GmbH & Co. KG 3

4 Stefan-Boltzmann s law Some Basic Physics Every body with a temperature above absolute zero radiates thermal heat. The quantity of heat (power) is given by Stefan-Boltzmann s law P = s * e * T 4 P = Radiative Power [W/m 2 ] T = Temperature [K] e = Emissivity s = Planck s Constant [W/K 4 m 2 ] Rule of thumb: A body with a surface of high conductivity (much electons) possesses a low emissivity (e.g. polished aluminum e ~ 0,012) A body with low conductivity (few electrons possesses a high emissivity (e.g. glass e ~ 0,837) Emissivity e is the degree of radiative energy compared to that of an ideal blackbody: e =

5 Radiation W/m 2 µm Wien's displacement law Wien's displacement law states that the wavelength distribution of radiated heat energy from a black body at any temperature has essentially the same shape as the distribution at any other temperature, except that each wavelength is displaced, or moved over, on the graph. The wavelength of maximum radiative energy could be calculated by l max = 2,9*10 3 /T µm Black Body Radiation at 100 C 3E+07 3E C 2E+07 2E+07 1E+07 5E+06 0E INGLAS Wavelength GmbH µm & Co. KG 5

6 Radiative power (W/(m 2 *nm*sr) Black Body - Spectral distribution Spectral distribution of thermal radiation at different temperatures 1E+10 1E+9 l max =2,7 µm Spectrum T = 800 C = 1073 K Spectrum TIR 100 T = 100 C = 373 K l max = 7,8 µm 1E+8 1E+7 1E+6 Spectrum DIN EN 673 T = 10 C = 293 K l max = 10,3 µm 1E+5 0,0 5,0 10,0 15,0 20,0 25,0 30,0 35,0 40,0 45,0 50,0 Wavelength (µm) 6

7 Radiative Solare Strahlung power [W/m 2 /µm] Transmission [%] Solar and thermal radiation in everyday life Solar radiation Solare Strahlung Glass transmission Glas Thermal radiation at 100 C Thermische Strahlung 300 K VIS UV NIR IR Wavelength Wellenlänge nm [nm] 0 INGLAS GmbH & Co. KG 7

8 Principles of emissivity measurement Conditions a + r + t = 1 Kirchhoff's law of thermal radiation a l = e l Spectral measurement Wavelength dependant measurement of reflectivity with ir-spectrometer. Mathematic evaluation of emissivity: Integral measurement Integral measurement with a black body radiator. Computing of emssivity from reflectivity measurement: e l = 1 - r l if t = 0 e = (1- r l )S l d l S l d l e = 1 - r DIN EN 673 a = absorption r = reflexion t = transmission e = emissivity S l = Black body radiation at wavelength l. INGLAS GmbH & Co. KG 8

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11 Normal and effective Emissivity Emissivity depends on factors such as temperature, wavelength and emission angle. The emissivity of flat metal surfaces tends to be lower in normal direction as hemispherical. The TIR100-2 measures the normal emissivity. If the hemispherical (effective) emissivity shall be calculated for a flat metal surface, correction factors have to be applied. The EN673 lists a table of correction factors. Rough metal surfaces, coated textiles and non conductive materials may not be adjusted Measured normal emissivity e , Relation e eff / e INGLAS GmbH & Co. KG 11

12 Emissivity depends on surface roughness Example: r = 0,8 r = 0,8 * 0,8 = 0,64 e = 1 - r => 0,2 e = 1 - r => 0,36 Structures equal or larger than the dominant infrared wavelength (~10 µm at room temp.) increase emissivity: To get low emissivity surfaces, they must be polished to a roughness of < 1/10 of dominant infrared wavelength! INGLAS GmbH & Co. KG 12

13 TIR100-2 Features Integral reflectivity measurement Nondestructive measurement of temperature sensitive samples at room temperature Black Body (semi-)sphere at 100 C Homogenous, diffuse irradiation Allows one-click measurement of smooth and rough surfaces Thermopile-sensor with Fresnel - optic integral ir broad band measurement angle of measurement 12 : comparable to measurements under normal incidence Ø10 mm spot measurement area for small samples INGLAS GmbH & Co. KG 13

14 TIR100-2 requirements for precise measurement The TIR100-2 is easy to use, but some precautions should be observed to get precise results: As measurement of reflected power follows T 4, small changes of surface temperature of the blackbody, the sample and the calibration standard have a distinct effect on the results. Follow the instructions in the operation manual and the pren 15976: Temperature of sample and calibration standard must be equal Distance of black-body radiator to sample surface must be identical to standard - blackbody distance during calibration Use the same instrument orientation at calibration and measurement Avoid strong moves of the instrument Keep surrounding area free of draft Blackbody half sphere must be completely covered by sample (possibly use a sample holder) Flexible samples should be kept plain INGLAS GmbH & Co. KG 14

15 7.3 Preparation of specimens before testing The specimens should be kept for a minimum of 2 hours at a temperature of 23 +/-2 C and relative humidity of 50 +/-20%. Special precaution should be taken to ensure that the calibration standards, the specimens and the apparatus are equilibrated in the same standard climatic conditions. Air currents and draughts in the measuring area must be avoided. INGLAS GmbH & Co. KG 15

16 8 Procedure for measurement of specimens The apparatus should be switched on at least 2 hours before calibration and beginning measurements. The apparatus should be installed in a fixed position and must not be moved during measurement. The specimen is brought up to the apparatus in a vertical orientation, pressed firmly against the spacers around the measuring window of the apparatus and the apparatus is activated to begin measurement. In order to avoid that the specimen temperature changes during the measurement, the residence time of the specimen in the measuring position must be reduced to a minimum. Between specimen positioning and start of measurement, not more than 1 second shall pass. If this speed of measurement is not achieved, if the measurement is otherwise interrupted or if the measurement on a specimen is to be repeated, the specimen should be withdrawn from the apparatus for the time it needs to cool down to laboratory temperature. The higher the emissivity and/or the lower the specific heat capacity ( c ) of the material, the longer the specimen will need to cool down to laboratory temperature. In order to reduce measurement variability to a minimum (laboratory, specimen and apparatus related), after a time interval of maximum 1 hour, the apparatus shall be recalibrated using the two calibration standards. INGLAS GmbH & Co. KG 16

17 9 Expression of the results The emissivity of the specimen is directly indicated as a three decimal number. The emissivity mean value, all the single values per specimen and the standard deviation of the results from the tested product shall be included on the test report. The emissivity mean-value is to be rounded to two digits. All single measurements resulting in an emissivity <0,02 or >0,94 (measurement range of the apparatus) should be set to 0,02 or 0,94 respectively. INGLAS GmbH & Co. KG 17

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19 Working with the TIR100-2 Touch screen 230V / 115 V* RS232 On/Off INGLAS GmbH & Co. KG 19

20 Working with the TIR100-2 Getting into operation Switch on instrument. If Continue is displayed and no wait! you can start measuring It should warm up 1 hr minimum. The longer, the better The instrument consumes about 24W in operation (already heated) Place the instrument on a place with no draft, eventually use a working box Calibration standard and samples should be stored together at your working place to acclimate. Keep samples and standard away from the hot blackbody to avoid heating INGLAS GmbH & Co. KG 20

21 Working with the TIR100-2 Calibrating instrument Put calibration standard (black, ribbed surface) in front of the blackbody radiator. It should be in contact with the silicon distance pads of the instrument Press CALIBRATE HIGH immediately and wait until measurement is complete (sound) Remove standard immediately to avoid heating Repeat procedure with low emissive aluminum surface INGLAS GmbH & Co. KG 21

22 Working with the TIR100-2 Measurement Put sample in front of the blackbody radiator. It should be in contact with the silicon distance pads of the instrument Press MEASURE immediately and wait until measurement is complete (sound) Remove sample immediately to avoid heating! Tip Remeasure calibration standard every 10 min to ensure correct calibration INGLAS GmbH & Co. KG 22

23 Working with the TIR100-2 Measurement of glass and massive samples See also video on our Website: Put the sample in tight contact to the blackbody. Transparent samples should be kept dark on the rear side Do not move instrument between calibration and measurement. Avoid circulation around the instrument INGLAS GmbH & Co. KG 23

24 Working with the TIR100-2 Measurement of foils and textiles Use foil support and fix foils or textiles with reversible adhesive tape to keep it in good thermal contact with the support A magnetic fixture can be use if tape is not suited for adhesive fixture INGLAS GmbH & Co. KG 24

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26 Measurement of the emissivity of woven textiles and porous samples P 1 = textile P 1 = background TIR100-2 receives the reflected power of textile and background. The instrument computes: 1-e = A * r1 + (1-A) * r2 If background does not reflect (r2 = 0), textile reflects heat with r1 proportional to its fraction of area A. r2 -> 0; e = 1- (A * r1) Conclusion: Using a non ir- reflective background during measurement, the effective emissivity of the textile could be determined, but not the absolute emissivity of the textiles surface! INGLAS GmbH & Co. KG 26

27 Certificates INGLAS GmbH & Co. KG 27

28 Certificates INGLAS GmbH & Co. KG 28

29 TIR100-2 Measurement of Thermal Emissivity INGLAS GmbH & Co. KG Thank you very much for your attention! INGLAS GmbH & Co. KG 29