Assessments - 4, GPA: 3.3 ( )

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At 200 metres it could add about 20 .mol.m-2.s-1 to the readings, which may need subsequent adjustment . 50 • •••• Measurement options Document code: SS1-UM-1.05 PAR calibrations This section describes the basis for the light calibrations used in the SunScan system, and explains when and how you might want to recalibrate the probe or restore its factory calibration. Factory light calibration The SunScan probe and Beam Fraction Sensor are calibrated to give PAR readings which match those of a standard PAR quantum sensor in typical bright daylight conditions. This matching cannot be made completely reproducible because an ideal PAR quantum sensor has perfect spectral and cosine responses whereas the SunScan probe and BFS can only approximate to the ideal. However, for most normal usage, the SunScan calibration will be perfectly satisfactory, but if you are working under strong artificial lights (for example) you may need to contact Delta-T for advice. SunScan readings of LAI and fractional interception depend for their validity on the ratio of the transmitted light to the incident light rather than their absolute values, so it is the matching between the probe and the BFS calibration that is important. Checking the probe/BFS matching It is good experimental practice to carry out this test in the field before (and after) taking a lot of readings. ¦ Mount the SunScan probe and your BFS horizontally in uniform sunlight. Place the shade ring horizontal as well. Make sure the probe and BFS diffusers are clean. ¦ In the SunData program, select the PAR display (. ....+F hot key), and take some readings. The display will show you the values of the SunScan probe, BFS Total and BFS Diffuse sensors. All three values should be approximately the same. Store these readings, and you will have complete results that you can refer back to later if need be. The BFS values should be within about 5% of each other. Mismatches of this magnitude have only a minor effect on the beam fraction calculations and hence the LAI. (There is no field method for re-matching the Total and Diffuse sensors. Contact Delta-T if you have a problem). The probe and BFS Total readings may be within 5-10% of each other without greatly contributing to errors in canopies where the transmission is below 50%. Errors from the mismatch are likely to be swamped by the magnitude of the variation in the samples. However, if you want to improve the matching, then proceed with the Recalibrate option. The Recalibrate option This option matches your probe to your BFS (you cannot recalibrate the probe if you do not have a BFS). ¦ Set up the probe and BFS as for the previous test. On the Workabout, access the Calibrate procedure (. ....+C) and select Recalibrate SunScan, then follow the instructions. SunScan User Manual v 1.05 Measurement options • •••• 51 You should not expect to have to do this very often. The photodiodes and light measurement circuits are very stable. The source of the apparent variability mentioned above is the not quite ideal cosine and spectral response of the sensors. Restoring the factory calibration At any time after carrying out the Recalibrate option you can restore the original factory calibration. You do not have to set up the probe or BFS in uniform light. ¦ On the Workabout, simply access the Calibrate procedure (. ....+C) and select Restore Factory calibration. You will briefly see a message confirming that this has been done. Effect of the shade ring on the BFS If the above matching tests are carried out with the shade ring elevated (but not actually casting a shadow on either BFS sensor), you will get different results. The shade ring blocks about 7% of the diffuse sky light, and none of the direct beam. In bright sun conditions it will have no significant effect on LAI readings. In completely overcast conditions it might contribute a small error of up to 0.1 to the LAI value. Comparing the calibration with other PAR sensors You can carry out matching comparisons between the probe and BFS and any other PAR quantum sensor. You cannot reset the probe values to it, but you can annotate the readings and retain the comparison information in the SunScan data files. Routine maintenance and cleaning The probe diffuser is Perspex. Clean it when necessary with a damp cloth, moistened with mild detergent, or with IPA (iso propyl alcohol). The BFS diffusers are also made of Perspex. Clean them very carefully, especially the groove around the diffusers. Use a fine soft brush to remove any dirt or dust from the groove - it could affect the accuracy of the cosine response of the sensors. The "spread" measurement The "spread" value is a measure of the relative variation of the light along the probe. This is a useful parameter in light profiling measurements: it is the value of the standard deviation of the 64 photodiode readings, divided by their mean. You can check the probe uniformity of calibration at any time by taking a reading in uniform light...

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