1. The “Monitor to measure the Integral TRAnsmittance” (MITRA)
and the “Cryogenic Solar Absolute Radiometer” (CSAR)
IPC-XII Seminar, Davos, 12 October 2015
Benjamin Walter, Wolfgang Finsterle, Andre Fehlmann, Rainer Winkler, Ricco Soder, Markus Suter, Werner Schmutz

2. WRR SI MOTIVATION Direct Solar Irradiance measurements [W m-2]
traceable
(Fehlmann et al. 2012)
WRR
internal
stray
light
scale difference 0.34 % ± 0.09 % (k=1)
Intercomparisons: SI

3. WRR SI MOTIVATION Direct Solar Irradiance measurements [W m-2]
traceable
WRR
CSAR = LINK
directly
traceable SI
Intercomparisons
SI - cryogenic
radiometers

4. The MITRA Instrument Transmittance depends on solar spectrum which
changes with air
mass and
atmospheric
conditions
WSG
World
Standard
Group
CSAR = Cryogenic Solar Absolut Radiometer
entrance
window
Monitoring of
transmittance
entrance
window

5. Operation method & target uncertainty:
window
IPC-XI uncertainty ≈ 1 %
Previous uncertainty > 1 %
sensing
Measuring temperature
rise Δt at both cavities
reference
cavity
Target uncertainty for the CSAR:
0.01% (= 100 ppm)
spectrally integrated transmittance 
MITRA requirement:
uncertainty of tint < 100 ppm

6. Measurements in front of the sun
WSG tracker 
outdoor conditions
HELIOSTAT 
Laboratory conditions

7. Measurements in front of the sun
Laboratory: - instrument stability = 150 ppm = 0.015%
- heliostat windows: wavelength dependent reflectivity
Outdoor: - instrument stability = 900 ppm ≈ 0.1%
- Measurement influenced by:
wind / temperature variations
Two questions: 1. How accurate is the absolute value?
2. How to reduce wind and temperature variation problems in outdoor measurements?

8. How accurate is the absolute value?  Periodical recalibration !
Temperature differences must be very accurate:
no long term stability of parameters
Δt measurement uncertainty < 0.1 mk
no generally valid
thermometer
calibration possible
 Periodical recalibration !

9. How accurate is the absolute value?
Measured offset to validated simulations of Fehlmann: ± 300 ppm (k = 1)
Calculated offset: ppm
Good agreement between measured and calculated offset to simulations
 Periodical recalibration ± ppm (k = 1) depending on measurement conditions
2. wind and heat accumulation problem for outdoor measurements

10. WIND and HEAT ACCUMULATION:
10°C warmer relative to
ambient air temperature

11. WIND and HEAT ACCUMULATION: Thermally-stable environment
wind cannot enter the cavity covered by the window
wind influences both cavities similarly
Thermally-stable environment
wind shelter

12. Thermally-stable environment
No wind influences and
Heat accumulation problems anymore
electrical and thermal insulation
Indirect cooling
of MITRA
- 6 Peltier elements
- CPU fans
- PID - controller
Constant backplate
temperature ≈ ± 0.1 K

13. Thermally-stable environment
Measurement on the WSG sun tracking platform
≈ 0.6 K
Calibration of thermometers
< ± 1 mK

14. Thermally-stable environment
Measurement on the WSG sun tracking platform
≈ 0.6 K
≈ ± 100 ppm

15. Thermally-stable environment
Measurement on the WSG sun tracking platform
≈ 0.6 K
≈ ± 100 ppm
uncertainty =115 ppm
mean(tint) =
≈ ± 100 ppm

16. Thermally-stable environment
Measurement on the WSG sun tracking platform
≈ ± 100 ppm
uncertainty =165 ppm
mean(tint) =
≈ ± 100 ppm

17. Summary MITRA improvements:
≈ ± 150 ppm
Thermally-stable environment (Peltier cooling system)
 Outdoor instrument stability ≈ 150 ppm (k = 1)
Periodical recalibration of thermometers
 Absolute value uncertainty about ppm (k = 1)
3. Overall uncertainty:
ppm (k = 1)

18. Summary MITRA improvements:
≈ ± 150 ppm
Promising results
Future improvements to reach the 100 ppm goal:
Three cavity design to monitor the temperature drift
Liquid cooling of the instrument
Larger heatsink to provide more stable conditions

19. Cryogenic Solar Absolute Radiometer (CSAR)
Cryocooler (Helium)
20 K
reference
block
quartz window
10 mm
cavity
View limiting
aperture
apertures

20. Cryogenic Solar Absolute Radiometer
Measurement procedure:
Cleaning the windows
One day for cooling CSAR to 20K
Cavity characterization
Electrical measurements every min
Sampling rate = 3 s
MITRA
Helium pipes

21. Cryogenic Solar Absolute Radiometer
IPC-XII DAY:
CSAR measures about % lower than WRR

22. Cryogenic Solar Absolute Radiometer
( k = 2 )
IPC-XII
Good agreement with previous findings of difference between the
WRR and the SI scales.
Detailed values and uncertainties will follow soon ...

23. Thank you for your attention! SUMMARY
MITRA: Thermally-stable environment (Peltier system)
a.) No wind influences / convection / heat accumulation
b.) Strongly reduce temperature drift
c.) Remaining drift can empirically be corrected for
d.) Periodical recalibration of thermometers
d.) MITRA uncertainty ≈ ppm (k = 1)
CSAR: a.) Overal stable performance
b.) Preliminary results are in good agreement with previous
findings that the SI scale is about 0.2% - 0.3% lower than WRR
Thank you for your
attention!