Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 11)

CMP/CMA series manual
Page 10
2.3. Electrical installation
As standard the CMP/CMA 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 radiometer 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.
Preferably, secure the radiometer 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 off at the pyranometer and data logger.
Kipp & Zonen pyranometer 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 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 pyranometer. 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 pyranometer and cable. The zero offset can be verified by replacing the pyranometer
impedance at the readout equipment input terminals with a resistor.
The pyranometer 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 pyranometers exhibit offsets up to ± 2 W/m
2
due to lack of
thermal equilibrium.
For amplification of the pyranometer signal Kipp & Zonen offers the AMPBOX signal amplifier. This
amplifier will convert the micro-Volt output from the pyranometer 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
. The CMA series have two independent signal outputs, so two AMPBOX
amplifiers are required.