Measurement error due to radiation exchanges within pyranometers for surface radiation

Abstract

The Eppley pyranometer is widely used to measure broadband shortwave irradiances on the earth's surface. Measurements obtained using these instruments are known to be influenced by infrared radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. Described is an effort to model the energy exchanges within the instrument to describe the measurement error as a function of external conditions. A finite element method (FEM) analysis simulates heat diffusion in the instrument, and a Monte Carlo ray-trace (MCRT) code models radiative exchange in the instrument. An FEM analysis models heat diffusion in the outer dome of the instrument and produces the dome temperature distribution resulting from specified external boundary conditions. An MCRT code is used to model the influence of radiative exchanges between the domes and the sensor surface on the sensor signal. The code confirms offsets expected from a variety of ambient conditions. The goal of the effort is to create a working MCRT model of the pyranometer that will be combined with the existing FEM model. The completed tool will allow an accurate study of the signal sensitivity to various external conditions.