Kipp&Zonen CHP 1 Pyrheliometer - Manual (Page 7)

7
1.4 Electrical installation
As standard the CHP 1 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 model of
pyrheliometer and whether a temperature sensor is fitted (and which type). The colour code of the wires and the
connector pin numbers are shown on the instruction sheet. Longer cables are available as options.
The shield of the cable is connected to the aluminum 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 off at the pyrheliometer and data logger.
Kipp & Zonen pyrheliometer cables are of low noise type, but bending the cable produces small voltage spikes, a
tribo-electric and capacitance effect. Therefore, the cable must be firmly secured to minimize 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 pyrheliometer. The sensitivity is affected more than 0.1 % when the
load resistance is less than 100 k
Ω. For this reason we recommend the use of readout equipment with an input
impedance of 1 M Ω 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 affected.
A high input bias current at the readout equipment can produce several micro-Volts across the impedance of the
pyrheliometer and cable. The zero offset can be verified by replacing the pyrheliometer impedance at the readout
equipment input terminals with a resistor.
The pyrheliometer 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/m
2 . More resolution is not necessary during outdoor solar radiation measurements, because
pyrheliometers exhibit offsets up to ± 2 W/m 2 due to lack of thermal equilibrium.
For amplification of the pyrheliometer signal Kipp & Zonen offers the AMPBOX signal amplifier. This amplifier will
convert the micro-Volt output from the pyrheliometer into a standard 4 – 20 mA signal. The use of the AMPBOX
amplifier is recommended for applications with long cables (> 100 m), electrically noisy environments or data
loggers with a current-loop input. The AMPBOX can be factory adjusted to suit the sensitivity of an individual
radiometer to produce a defined range, typically 4 – 20 mA represents 0 – 1600 W/m
2 .