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

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

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

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

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

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

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?

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 !

How accurate is the absolute value? Measured offset to validated simulations of Fehlmann: 1334 ± 300 ppm (k = 1) Calculated offset: 1290 ppm Good agreement between measured and calculated offset to simulations  Periodical recalibration ± 150 - 300 ppm (k = 1) depending on measurement conditions 2. wind and heat accumulation problem for outdoor measurements

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

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

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

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

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

Thermally-stable environment Measurement on the WSG sun tracking platform ≈ 0.6 K ≈ ± 100 ppm uncertainty =115 ppm mean(tint) = 0.9269 ≈ ± 100 ppm

Thermally-stable environment Measurement on the WSG sun tracking platform ≈ ± 100 ppm uncertainty =165 ppm mean(tint) = 0.9269 ≈ ± 100 ppm

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 150 - 300 ppm (k = 1) 3. Overall uncertainty: 250 - 350 ppm (k = 1)

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

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

Cryogenic Solar Absolute Radiometer Measurement procedure: Cleaning the windows One day for cooling CSAR to 20K Cavity characterization Electrical measurements every 10-25 min Sampling rate = 3 s MITRA Helium pipes

Cryogenic Solar Absolute Radiometer IPC-XII DAY: 2015-09-30 CSAR measures about 0.2 - 0.3% lower than WRR

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 ...

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 ≈ 250 - 350 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!