Calibration results on a new integrating instrument for measuring daily values of total solar radiation

Abstract

Test results are reported for a newly develope instrument for accurately measuring daily solar radiation. The instrument consists of two components: the radiation sensor (a silicon-cell pyranometer that is mounted outdoors in the same way as the conventional Eppley or Kipp pyranometer), and the integrator (a conventional household style d-c ampere-hour meter). No electric power is needed to operate the instrument, which is easily portable, reasonably robust, and inexpensive.Calibrations were done in Barbados (latitude 13° 12′ N) over an eight-month period, and consisted of intercomparison of a number of the new instruments among themselves, as well as calibration against conventional Kipp and conventional Eppley solar radiation measuring systems.Test results led to the following conclusions: 1.1-Scatter in the daily values of solar radiation as measured by the new instruments, due to the inherent characteristics of the new instruments themselves, was within ±2 percent.2.2-When compared to a carefully maintained reference (Kipp or Eppley) system, the scatter of the calibration data was within ±3 percent.3.3-Because of the difficulty of accurately integrating the solar-radiation record that is obtained with conventional (Kipp or Eppley) systems in the coastal tropics (where cumulus cloud is prevalent), scatter of the calibration data over many months was as high as ±10 percent, but more than half of this scatter was attributable to the reference system and not to the new instruments.4.4-Although developed mainly for use in the tropics, the new instrument is suitable for year-round use at latitudes up to 35 or 40 degrees, and for summer use at even greater latitudes.5.5-Being portable and inexpensive, and at the same time surprisingly accurate and robust, the new instrument seems well suited for measuring solar radiation in such situations as irrigation control, estimating evaporation from dams, horticultural and agricultural research, and for meteorological and climatological purposes.