Delta Electronics SS1-UM-1.05 PAR calibrations, Factory light calibration, Recalibrate option

Page 51

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

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Contents SS1-UM-1.05 SunScanCE conformity CopyrightAcknowledgements TrademarksContents Technical Reference section Menus and Screens More Psion and file handling notesMeasurement options LAI theoryIndex AppendicesOrganisation of this manual How to use the manualsIntroduction SunScan Canopy Analysis SystemData Collection Terminal Field accessoriesSunScan probe Beam Fraction sensorGetting Started Preliminary checksWorkabout and SunScan probe Checking the Workabout hardware What the s/w does Installing the SunData s/w in your PCInstalling the s/w SunData s/w DisketteRunning SunData Setting up your PCs COM portCommunication checks SunScan probe to PCWorkabout to PC Running SunData in Windows SunScan Tutorial About this tutorialWorking with the Workabout Starting SunData in the Workabout What to do if you get lostStarting the PC software Using a PC instead of the WorkaboutThis page is intentionally blank Working through the menu options Setting up a measurement sessionHot Keys File SavingTaking readings Using the Emulator modeWithout a Beam Fraction Sensor Connecting the SunScan probeConnecting the Beam Fraction Sensor Measuring Leaf Area Index without a Beam Fraction Sensor Averages Reviewing your data fileTransferring the data file to your PC From the WorkaboutTo the PC Initiating the file transfer from the Workabout Meanwhile, on the PC Conclusion of the Tutorial RS232 communication problemsSunData Screens on the Workabout Settings Menus and ScreensFile Utils ContdQuit Navigating the Psion directories and screens More Psion and file handling notesNavigating Psion directories and screens Workabout User GuidePsion subdirectory usage in file select dialogs Re-installing the SunData application icon Deleting unwanted Workabout filesFlashcards reformatting Configuration and data file handling SunData Configuration filesData memory management Data files Default .cfgCreating a configuration file Restoring a configurationDisplaying data files on your PC .PRN file.CSV file Data file layouts and data groups Page Measurement options Experiment designAbove-canopy reference requirements Canopy Sampling volume Canopy type and BFS practicalitiesCanopy type and LAI estimates Planning for the sun’s position Preferred light and weather conditionsAdvice on Absorption and Eladp values Setting EladpAbsorption Estimating Eladp in the field Relationship between Mean Leaf Angle and EladpWorkabout setup SunScan System Measurement modesLAI, PAR and All displays Autolog function Levelling the probe Measurement procedures in the fieldProbe handling in the field Probe GO buttonWorkabout Using the tripodBFS handling in the field Use of the tripodFinding North, and setting the shade ring Levelling the BFSExtension cables, and the location of the BFS Recalibrate option PAR calibrationsFactory light calibration Checking the probe/BFS matchingComparing the calibration with other PAR sensors Routine maintenance and cleaningRestoring the factory calibration Effect of the shade ring on the BFSSunScan probe and Beam Fraction Sensor Environmental and moisture protectionIngredients of the LAI computation method LAI theoryDerivation of Wood’s SunScan canopy analysis equations Major assumptionsTheory versus reality Beers law for canopy absorption Campbells Ellipsoidal LAD equationsTransmission of Diffuse Light Transmission fraction τ Is given by I/I0 Diffuse light transmission cosine corrected sensor Modelling the canopy transmissionDetail Diffuse light hemispherical response sensor Functions used to model canopy transmissionAccuracy of LAI calculations Diffuse light cosine response sensorExp 0.1 . x . atan 0.9 Atan L Q Spher Modelling incomplete PAR absorption and scatteringScientific references Calculating zenith anglesSummary Jones, Hamlyn G Plants and Microclimate second edition. CUP SunScan probe batteries Maintenance and repairTechnical Reference section Checking the batteriesChecking the desiccant Re-setting the factory calibration Checking the PAR calibrationFactory calibration method While running SunData TroubleshootingProblems running the SunData application Psion WorkaboutInsufficient power to write data reported Technical Support Data Collection Terminal type DCT1 Psion Workabout SpecificationsSunScan Probe Type SS1 Beam Fraction Sensor type BF1Logging Cables Carrying Case type SCC1Telescopic Tripod type BFT1 Spares Kit type SPS1Spectral response PAR PerformanceSunScan system cosine response Cosine responses of probe and BFSLogging the probe as a Linear Quantum Sensor AppendicesLogging the Beam Fraction sensor Revision History Upgrading the SunScan systemSunData PC s/w Workabout SunData s/wWorkabout Remote Link File transfer between Workabout and PCChoice of different programs PsiWinSlink and Rcom Example using SlinkDrives on the PC are referred to as REMA, Remc REM = remote Documentation of Rcom and Slink Example using RcomSending a file to a serial printer Using the Workabout Comms applicationAlternative file transfer mechanisms Sending a file to a Communication programGlossary Page Accuracy IndexIndex Field use 39, 46, 48, 50, 51, 58

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