How to Check the Accuracy of Your Solar Radiation Measurements

by Bruce Smith | Updated: 05/29/2015 | Comments: 0

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CS300-L Apogee pryanometer

Solar radiation is an important measurement for many researchers and those who are monitoring their evapotranspiration values for irrigation purposes. If you have been wondering how accurate your solar radiation measurement is, Apogee Instruments (an associated company of Campbell Scientific) has a Clear Sky Calculator mobile app for that, and an online Clear Sky Calculator to help you find out. 

For best results, perform the comparison on a clear day with no haze or clouds. The big, fluffy clouds scattered through the sky can reflect sunlight and will affect the solar sensor measurement. Doing the comparison close to noon, within 1 hour, also works best.

For the online version of the Clear Sky Calculator, enter the appropriate Input Parameters for Estimating Solar Radiation. You will notice that the time of day value for this online version must be entered in a decimal format. For example, noon is equal to 12.0 in decimal format. But if it is 11:50 a.m. when you are doing your comparison, you need to convert the Time of Day into decimal format for use in the online calculator.

The following is an example of how to convert the time of day to a decimal format:

  1. First convert the hour of day to minutes. 
    If the time is 11:50, take the 11:00 as 1100 and multiply by .6 to convert the hours to minutes:
                    1100 x .6 = 660
  2. Then add the :50 as 50 minutes to the 660:
                    660 + 50 = 710
  3. Then divide that number by 60 to get the decimal hour:
                    710 / 60 = 11.8333

So, 11:50 a.m. becomes 11.8333 in decimal format.

Compare the Watts per meter squared (Wm²) value from your pyranometer measurement with the calculated value.

  • If you are viewing kWm² with Toro weather station software, move the decimal point three places to the right to convert the kWm² to Wm². For example: 0.996 kWm² becomes 996 Wm².
  • If you are viewing Langley's with Rain Bird weather station software, you can convert Langley to Wm² by multiplying the Langley value by 8373:  Langley x 8373 = Wm². For example:  0.119 Langley x 8373 = 996 Wm².

So now that you know how well your pyranometer is calibrated, what’s next?

If the accuracy of your solar radiation data is critical to your application, and the sensor is out of calibration by more than 5%, you may want to consider sending your pyranometer to Campbell Scientific for calibration services.

Recommended for You: Visit our Repair and Calibration page for additional information. We also have available the 30008 CS300, CS305 or SP230 Recalibration with Documentation service.


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About the Author

bruce smith Bruce Smith is an Application Engineer for the Environmental Group at Campbell Scientific, Inc. He works primarily to provide support for weather stations used in the commercial irrigation industry.

View all articles by this author.


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