Campbell Scientific CHP1 Pyrheliometer (from Kipp & Zonen) - Manual (Page 10)

10
4 PRINCIPLE COMPONENTS OF PYRHELIOMETERS
The detector of the Kipp & Zonen CHP 1 pyrheliometer is based on a passive thermal sensing element called a
thermopile.
The thermopile responds to the total power absorbed by the black surface coating, which is a non-spectrally
selective paint, and warm up. The heat generated flows through a thermal resistance to the heat-sink (the
pyrheliometer body). The temperature difference across the thermal resistance of the detector is converted into a
voltage as a linear function of the absorbed solar irradiance.
A drying cartridge (dessicator) in the radiometer housing is filled with silica gel and prevents dew on the inner sides
of the windows, which can cool down considerably on clear windless nights.
4.1 Window
The window material of the pyrheliometer defines the spectral measurement range of the instrument. In general
about 97 – 98 % of the solar radiation spectrum will be transmitted through the window and will be absorbed by the
detector.
4.2 Detector
The thermopile sensing element is made up of a large number of thermocouple junction pairs connected electrically
in series. The absorption of thermal radiation by one of the thermocouple junctions, called the active (or ‘hot’)
junction, increases its temperature. The differential temperature between the active junction and a reference (‘cold’)
junction kept at a fixed temperature produces an electromotive force directly proportional to the differential
temperature created. This is a thermoelectric effect. The sensitivity of a pyrheliometer depends on the individual
physical properties of the thermopile and construction. The sensitivity of each thermopile is unique and therefore
each radiometer has unique 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 containing many micro-
cavities that effectively ‘’trap’’ more than 97 % of the incident radiation in a broad spectral range. Furthermore, the
spectral selectivity is less than 2 %. This means that within the spectral range of the pyrheliometer, the absorption
for each wavelength is equal to within 2 %. The black painted sensing element forms the detector. Considering the
long-term stability of the instrument, the black paint is one of the most crucial and delicate parts of the
pyrheliometer. Kipp & Zonen black paint gives the best possible stability over a long period of time under all
meteorological circumstances.
4.3 Housing
The radiometer housing accommodates all fundamental pyrheliometer parts. The anodized Aluminum parts are
light weight and give a high mechanical and thermal stability to the instrument. Due to its fine mechanical
construction all pyrheliometers are virtually sealed and comply with the international standard IP 67.
4.4 Drying cartridge
In case moisture enters the radiometer body the silica-gel desiccant regulates the humidity level inside the
pyrheliometer. Initially the desiccant will have an orange color. After some time it becomes saturated with moisture
and the colour will change to become clear (transparent). At this time the contents of the drying cartridge should be
replaced with fresh, unsaturated orange colored desiccant as soon as possible. Replacement desiccant is available
through Kipp & Zonen distributors.
4.5 Cable and connector
For ease of installation and replacement during recalibration of the radiometer, the CHP 1 is provided with a
weather proof signal cable connector.
Kipp & Zonen radiometers use a custom-made cable that is selected as a low noise type particularly suited to handle
the low voltage output of the thermopile or of a temperature sensor.
The shield of the cable is connected to the metal body of the connector and preferably should be connected to
ground at the readout equipment. Cables come pre-wired to the connector plug in a range of lengths.