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an instrument for measuring direct solar radiation incident on a surface perpendicular to the solar beam. In Russian usage, a pyrheliometer is an absolute instrument used primarily to test secondary instruments—actinometers. In Western Europe and the USSR the Ångstrom compensation pyrheliometer, which is illustrated in Figure 1, has been adopted as the standard. Two very thin identical manganin strips that have been blackened on top are used as the detecting unit of the instrument. One strip (A) is heated by the solar radiation; the other (B) is shielded from the radiation and is heated by an electric current from an outside source (E). When the temperatures of the two
Figure 1. Angström pyrheliometer: (a) external view, (b) schematic of the instrument; (G2) and (G2) galvanometers, (R) rheostat regulating the strength of the compensation current
strips are equal, the themocouple attached to the strips does not generate a current, since there is total compensation. In such a situation, the amount of heat generated by the current in the second strip is equal to the amount of heat received by the first strip from solar radiation.
The water-flow pyrheliometer of C. Abbot with design corrections by the Soviet scientist V. M. Shul’gin has been adopted as the standard in the USA. Internally blackened chambers washed by streams of water are housed in two identical tubes. One chamber is exposed to radiation, and the other is screened and is heated by a current of a strength such that the temperatures of the streams of water emerging from the two chambers are identical. The heating is monitored by themocouples. The radiation intensity (calories/cm 2-min) is calculated from the amount of heat released in the chamber and the area of the chamber’;s receiving aperture.
The readings of the American pyrheliometer and the “American” pyrheliometric scale based on the readings are 3.5 percent higher than the “European” scale. A new single scale, the International Pyrheliometric Scale (I.P.S.), was adopted in 1956 and came into effect in 1957. In it, the data of all observations made on the European scale are increased by 1.5 percent, and data based on the American scale are decreased by 2.0 percent.
REFERENCESAverkiev, M. S. Meteorologiia, vol. 1. Moscow, 1951.
Sivkov, S. I. Melody rascheta kharakteristik solnechnoi radiatsii. Leningrad, 1968.
Ianishevskii, lu. D. Aktinometricheskie pribory i metody nabliudenii. Leningrad, 1957.
T. V. EVNEVICH