The Vernier PAR Sensor is calibrated before shipping and should not need user calibration.
However, you can calibrate the sensor using the Clear Sky Calculator (www.clearskycalculator.com). The Clear Sky Calculator for Quantum Sensors reports the theoretical PPFD at any time of day at any location in the world on a cloudless day. The application is most accurate at solar noon in the spring and summer.
To determine the theoretical PPFD at your location, you will need to know your latitude, longitude, altitude, time of day, air temperature and humidity. Complete the required fields in the web application. Record the Model Estimate of PPFD.
Please note that for best accuracy the sky must be completely clear if you are going to calibrate your Vernier PAR Sensor using the Clear Sky Calculator.
Take the PAR Sensor outside on a sunny and cloudless day. Make sure that you have calculated the theoretical PPFD for your date and location.