Advertisement
However, if the upward and downward radiation components are to be measured separately it is necessary
to record the individual housing temperatures to calculate the radiation values.
Using the combination of a net pyrgeometer (two CGR 3 or CGR 4 instruments) and a CMA 6 or CMA 11
albedometer the net total radiation (energy balance) can be calculated with high accuracy from thefour
component values. Problems with dew deposition, frost, etc, can be minimised by using the Kipp &
Zonen CV 2 ventilation unit with optional heating.
This has many advantages over conventional net total radiation sensors with plastic (polyethylene)
windows. These cannot provide individual short and long-wave radiation values and cannot separate
upward and downward contributions. The soft plastic domes do not fully protect the sensor from the
thermal e ects of wind and rain, are easily soiled, are di cult to clean and require regular replacement.
2.3. Electrical installation
As standard the CGR is supplied with a waterproof connector pre-wired to 10 m cable with a number of
leads and a shield covered with a black sleeve. The number of connector pins and cable leads depends
upon the type of temperature sensor that is fitted. The colour code of the wires and the connector pin
numbers are shown on the instruction sheet. Longer cables are available as options.
Preferably, secure the pyrgeometer with its levelling screws or mounting rod to a metal support with a
good connection to earth (e.g. by using a lightning conductor).
The shield of the cable is connected to the aluminium radiometer housing through the connector body.
The shield at the cable end may be connected to ground at the readout equipment. Lightning can induce
high voltages in the shield but these will be led o at the pyrgeometer and data logger.
Kipp & Zonen pyrgeometer cables are of low noise type, but bending the cable produces small voltage
spikes, a tribo-electric and capacitance e ect. Therefore, the cable must be firmly secured to minimise
spurious responses during stormy weather.
The impedance of the readout equipment loads the temperature compensation circuit and the thermopile.
It can increase the temperature dependency of the pyrgeometer. The sensitivity is a ected more than 0.1%
when the load resistance is less than 100 kΩ. For this reason we recommend the use of readout equipment
with an input impedance of 1 MΩ or more. The solar integrators, data loggers and chart recorders from
Kipp & Zonen meet these requirements.
Long cables may be used, but the cable resistance must be smaller than 0.1% of the impedance of the
readout equipment. It is evident that the use of attenuator circuits to modify the calibration factor is not
recommended because the temperature response will also be a ected.
A high input bias current at the readout equipment can produce several micro-Volts across the impedance
of the pyrgeometer and cable. The zero o set can be verified by replacing the pyrgeometer impedance at
the readout equipment input terminals with a resistor.
The pyrgeometer can also be connected to a computer or data acquisition system. A low voltage analogue
input must be available. The resolution of the Analogue-to-Digital Converter (ADC) must allow a system
sensitivity of about 1 bit per W/m². More resolution is not necessary during outdoor measurements,
because even the best pyrgeometer (the CGR 4) exhibits o sets greater than 2 W/m² due to lack of
thermal equilibrium.
CGR 4 Manual
Page 10
Advertisement
