The 240-100 Net Radiometer contains a high output 60 junction thermopile with a nominal resistance of 4 ohms and linear calibration. It generates a millivolt signal proportional to the net radiation level. The thermopile is mounted in a glass reinforced plastic frame with a built-in level. A ball joint is supplied on the stem to facilitate leveling. Thermopile surfaces (or sensor surfaces) and surrounding surfaces are flat black and the frame is black to reduce internal reflections within the instruments thus providing more uniform performance over reflective and non-reflective surfaces. The black paint used on these radiometers has far superior absorption properties and durability than that used on previous versions.
Sensor surfaces are protected from excessive convective cooling by a hemispherical polyethylene windshield. Polyethylene is used for the windshield material because it is transparent to both long and shortwave energy. The windshield is heavy duty and is self supporting so no pressurization is required. O-ring seals are included to enhance windshield replacement. A desiccant supply is contained in the support arm to keep air spaces inside the windshield dry. The instrument and its support arm also contain purge ports. A mounting bracket is supplied for mounting to horizontal or vertical pipes.
Net radiation values determined by the 240-100 may not compare well with those calculated using pyranometer and pyrgeometer combinations if the other radiometers do not include proper windshield temperature correction algorithms. Extensive research has been conducted to better understand the effects of windshield temperatures on radiation measurements. The 240-100 Radiometers are designed and calibrated to minimize errors caused by windshield temperatures.