An isothermal pyranometer incorporating detection and elimination of offset error due to dome radiation

Concern about change in the global climate has spurred development of global climate models and world-wide networks of surface radiometers to measure down-welling solar radiation. Comparisons of these surface measurements with satellite data, however, indicate a discrepancy possibly due to errors in the measurement of down-welling solar radiation. This paper outlines a new approach to the measurement of solar radiation in which the energy received by the receiver surface is actively removed to maintain the surface at the same temperature as a thermal base. The effort required to pump the received energy is a measurement of the solar radiation. A Peltier device under closed loop digital control maintains the isothermal condition.

The design allows key sources of inaccuracy in pyranometers to be addressed directly. These include errors due to radiant exchange with the dome (night-time offset), non-ideal cosine response, variation of responsivity due to temperature, and non-linear response. Radiant heat exchange from dome to receiver is eliminated by sensing the thermal IR radiation from the dome and driving the temperature of the thermal base to null the IR signal. The receiver size and shape are adjusted to improve the cosine response. A digital controller automatically compensates for temperature sensitivity. Miniature resistors on the back of the receiver disk allow remote field checks of linearity and calibration by electrical substitution.

A mathematical model of the detector is developed to predict performance and guide the design of a prototype and digital controller. The design and fabrication of the prototype is discussed, and preliminary results presented. These include tests of error due to dome radiation, cosine response, time response, linearity and calibration checks by electrical substitution, and temperature stability.