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To measure radiant heat transfer, such as night-sky radiation or the sun hitting a solar thermal system, use a pyrgeometer. A pyrgeometer is a passive device used to measure far infrared radiation. A thermopile sensor detects infrared radiation, but a coated silicon shield over the sensor blocks the shortwave radiation. Shortwave radiation is measured with a pyranometer. Pyrgeometers output a voltage signal that is proportional to the radiation exchange between the sensor and the sky. They provide an averaged measurement across the field of view, which would include effects of partial cloud cover, etc.

An example of a pyrgeometer (left) and a schematic of the main components of a pyrgeometer (right)
Courtesy of Kipp & Zonen

The temperature of the sensor is needed to calculate the net incoming or outgoing infrared radiation, so an integrated temperature sensor measures body temperature of the sensor. Depending on the pyrgeometer manufacturer, the temperature compensation may automatically be taken into account or is a separate calculation to be done by the researcher. In the later case, the pygreometer will output a voltage signal from the thermopile and from one or more thermistors used to measure sensor temperature. A manual for two common pyrgeometers, the Eppley PIR and the Kipp Zonen Pyrgeometer, are included in the related documents section below.

Pyrgeometers are an important field test instrument when a measurement of the available thermal radiation is needed, such as when a solar thermal system is installed. Typically, one pyrgeometer will be affixed to the solar thermal collector to measure the infrared radiation in the plane of the collector. Additionally, the pyrgeometer should be mounted in an unshaded location, away from any heat sources. Among other things, these measurements can be used to calculate the installed thermal efficiency of solar thermal systems.

Related Documents that include installation and calculation instructions:

  1. Eppley PIR Pyrgeometer
  2. KippZonen_Manual_Pyrgeometer_CGR4_V1401.pdf