You can follow the procedure in the following paper:
Ahmad Fudholi, Kamaruzzaman Sopian, Mohammad H. Yazdi, Mohd Hafidz Ruslan, Mohamed Gabbasa, and Hussein A. Kazem, “Performance analysis of solar drying system for red chili”, Elsevier-Solar Energy SE (ISSN: 01425919), Vol. 99, January 2014, pp 47–54.