Kipp & Zonen CGR 4 Instruction Manual

Kipp & Zonen CGR 4 Instruction Manual Manual pdf 17 page image

The solar blind filter is opaque to radiation below 4.5 μm, known as the cut-on wavelength. Currentlymost pyrgeometers have their cut-on at a lower wavelength. Problems may occur in the case of clearsunny days with low humidity. In the solar spectrum between 2.5 and 4.5 μm there can still be anamount of infrared solar radiation up to 10 W/m², which should not be included in the measurement.This unwanted fraction is blocked in the CGR 4 by the filter coating.CGR 4 uses a specially designed pure silicon dome. Although the dome is not hemispherical, CGR 4 hasa 180° field of view with good cosine response. A big advantage of the meniscus shaped dome over thetypical hemispherical dome is the ability to deposit more uniform coatings on the window surface.Deposition of a uniform filter coating on a strongly curved surface is very difficult and unpredictableprocess. To avoid these problems Kipp & Zonen developed a dome with excellent optical quality due tothe optimised shape and coating uniformity.The solar radiation absorbed by the window is conducted away very effectively by a unique constructionin the CGR 4. Even in full sunlight the window heating effect is very low compared to that of otherpyrgeometers on the market. This allows accurate daytime measurements without the need for ashading disk. It also eliminates the need for window heating compensation by using the correctionformula and window temperature sensors.3.2. DetectorThe thermopile sensing element is made up of a large number of thermocouple junction pairsconnected electrically in series. The absorption of thermal radiation by one of the thermocouplejunctions, called the active (or ‘hot’) junction, increases its temperature. The differential temperaturebetween the active junction and a reference (‘cold’) junction kept at a fixed temperature produces anelectromotive force directly proportional to the differential temperature created. This is a thermoelectriceffect. The sensitivity of a pyrgeometer depends on the individual physical properties of the thermopileand construction. The sensitivity of each thermopile is unique and therefore each radiometer hasunique calibration factor, even with the same radiometer model.On the top surface of the sensor a black paint is deposited which has a very rough structure containingmany micro-cavities that effectively ‘trap’ more than 95% of the incident radiation in a broad spectralrange. Furthermore, the spectral selectivity is less than 3%. This means that within the spectral rangeof the pyrgeometer, the absorption for each wavelength is equal to within 3%. The black paintedsensing element forms the detector. Considering the long-term stability of the instrument, the blackpaint is one of the most crucial and delicate parts of the pyrgeometer. Kipp & Zonen black paint givesthe best possible stability over a long period of time under all meteorological circumstances.3.3. Temperature sensorThe housing temperature sensor is a crucial part of a pyrgeometer and is needed to calculate thedownward long-wave radiation component. The body temperature sensor represents the ‘absolute’temperature of the detector surface and therefore it is mounted close to the cold junctions of thedetector. A housing temperature sensor is fitted as standard to the CGR pyrgeometers.Page 17CGR 4 Manual