Operation and investigation of a tilted bottom cavity for pyrgeometer characterizations

A new cavity for pyrgeometer characterizations has been built at the Infrared Radiometry Section of the World Radiation Center (WRC-IRS) at PMOD/WRC. The calculated effective emissivity of 0.99993 +/- 0.00033 was obtained from Monte Carlo simulations taking into account the geometry and the measured temperature distributions of the cavity. The cavity is operated in a temperature range of -30 degrees C to +30 degrees C and is initially flushed with nitrogen to reduce the relative humidity in the cavity. The estimated uncertainties of retrieved pyrgeometer parameters k(1), k(2), and k(3) are +/- 0.024, +/- 0.0008, and +/- 0.03, respectively. The relative uncertainty of pyrgeometer sensitivity C is 0.8%. The comparison with the cavity used since 1995 at PMOD/WRC gave average differences of 0.005, 0.00026, and 0.08 for k(1), k(2), and k(3), respectively. The pyrgeometer sensitivity retrieved with the new cavity is on average 1.0% higher than with the original cavity. (C) 2008 Optical Society of America.