Measurement of thermal and net radiation

1
Measurement of thermal and net radiation
Sd - Solar Diffuse
Sb - Solar Beam
La Sky Thermal
Loe Emitted Outgoing Thermal
Sr - Solar Reflected
Lg Ground Thermal
2
Infrared Thermometer (IRT)

• Useful for non-contact measurement of surface T
• IRTs measure F (radiant flux density)
• If IRT sensor surface (generally blackened
  thermopile) has e close to 1, then

3
Infrared Thermometer (IRT)
Ideally.
4
IRT absorption spectrum

• Most IRTs have filters that only pass 8-13 µm
  radiation
• Why?

Atmospheric Emittance (clear sky)
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Measuring T with IRTs

• IRTs measure radiant flux density (F, W/m2)
• If radiating surface is not a black body, F is
  made up of emitted component and reflected
  component
• Only emitted component affected by T

Total Fseen by IRT Femitted by surface F
emitted by surroundings and reflected
(error)
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Errors in IRT from reflected radiation
From previous slide
What happens if Ti Ts?
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Errors in IRT from reflected radiation
From previous slide
What happens if es 1 ?
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Magnitude of error when e?1 and Ts ? Ti

• Generally less than 1 C under clear skies
• Much less under cloudy skies or indoors
• Correction not needed except in extreme
  situations
• Well create one of those situations in lab

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Determining emissivity of a surface
Solving our favorite equation for es
So if you can measure TT, Ts, and Ti, you can
calculate es
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Pyrgeometers

• Measure only thermal radiation
• Dome is specially designed to reflect solar and
  transmit thermal radiation
• Hemispherical view (unlike IRT)

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Measurement of Total Radiation

• Total radiation solar thermal
• Now we need both wavelengths to reach sensing
  surface
• Still need protection from convection..
• Can we still use a glass dome?
• Can we use black and white sensor?

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Total radiometer
solar in
thermal in
thermal emitted
conduction to base
If you assume as al e 1
C Calibration constant V mV output Need to
know Tb!
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Measurement of Net Radiation

• Rn total incoming total outgoing total
  radiation absorbed by surface
• Two ways to measure
• Summation of radiation components measured
  independently
• Net radiometers

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Component Summation

• Most accurate method
• Expensive several thousand for each sensor

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Net Radiometers

• Less Expensive, Less accurate
• Measure temperature difference between upward and
  downward facing surface

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Net Radiometers
Rn Sd Ld Su - Lu
Only need to measure mV output to get Rn! (all
emissivity, absorptivity, and Tbase terms cancel
out)
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Calibration of total and net radiometers

• Short wave calibration
• Use a known standard (Eppley or Kipp pyranometer)
• Shade simultaneously on a clear day
• Long wave altered only slightly
• Direct short wave altered equally
• From standard pyranometer you know ?Sb
• From radiometer you know ? mV
• Radiation incident on opposite side must remain
  constant

Calibration ?Sb/ ? mV
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Calibration of total and net radiometers

• Long wave calibration
• Alter long wave radiation with a long wave
  emitter (radiator)
• Need to know
• ? radiator temperature
• Emissivity of radiator
• Plate view factor
• Radiation incident on opposite side must remain
  constant

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Calibrating the radiometer for todays lab
?Tradiator measure with IRT or
thermocouple Emissivity radiator painted with
optical black paint, e 0.98 View Factor (F)

d distance between source and sensor L length
of the radiator edges
calibration ? F/?mV
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Todays Lab (contd.)

• Use IRT to measure longwave energy budget for a
  person
• Pay attention to view factors
• Look at thermal emissivities of different surfaces