Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 57)

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Of secondary im portance in the selection of the DAS is its program m ability. While the minim um requirem ent
for the DAS is to m easure a set of signals with a 0.01% accuracy at 1 Hz, the output to be archived
is the one m inute m ean, m inim um , m axim um , and the standard deviation. Thus one can store the second
data and post process the results or use the features associated with the DAS. In overall storage
requirem ents and operator ease, the program m able DAS is the m ore attractive option.
5.2 Set-up Considerations
Depending upon the location of the instrum ents, the accessibility of laboratory space and the overall
clim atic conditions, the installation of the DAS m ay be near the instrum ents (within 5 m ) or som ewhat
distant inside a laboratory. The ideal site would have a com pletely clim ate-controlled building within
a few m etres of the instrum ents (e.g., rooftop m easurem ents with a laboratory below). W hen this does
not occur, the decision m ust be m ade as to what will provide the higher quality data; the use of a robust
data logger near the instrum ents or increased cable length to reach a bench m odel DAS.
Having a data system that functions in all environm ental conditions elim inates the problem s of signal
loss along cables and the potential of electrical interference. However, the design and/or operation
of a robust outdoor system usually involves som e sacrifice in m easurem ent frequency and/or data
availability and/or accuracy.
For increased quality control it is recom m ended that the DAS be one that has the capability of displaying
m easured data (either in raw or engineering form ) in near real tim e (e.g., within 2 m inutes), preferably
graphically. T he station operator should be able to exam ine, anytim e, both the instantaneous
m easurem ents and the data acquired during the preceding few hours or even the last day. W ith recent
developm ents in serial com m unications and scientific display software for PC’s, this can be easily
accom plished with both types of DAS.
W hatever the choice, internally or externally housed, the DAS m ust be both secure and easily accessible -
secure against inadvertent reprogram m ing or physical harm and accessible for easy m aintenance
and changes in software or signals. It is highly recom m ended that a secondary external power supply
be used as insurance against loss of prim ary power. This m ay include an uninteruptable power supply
(which will also reduce the possibility of dam age from power surges) for bench and com puter-style
acquisition system s and a secondary battery or battery charger connected to the m ain battery of system s
developed to operate on DC power.
The operator m ust consult the m anuals provided by the DAS m anufacturers for the specific set-up
requirem ents of the system . W hen several different products are purchased, it is preferable that they
are bought from the sam e supplier as a package to alleviate the problem of attem pting to determ ine
which one of several products is in conflict during the initial installation.
5.3 Standard Practices
(1) Each signal should be connected as a differential input to ensure the greatest m easurem ent
accuracy.
(2) The m inim um integration tim e for radiation signals should be one power line cycle to elim inate
powerline frequency noise.
(3) Every two years: Calibrate the ADC in an accredited standards’ laboratory. This is im portant
even if constant voltages and resistance are input into the system on a continuous basis (see
below). Check the associated m ultiplexer for changes in noise levels and settling tim e and
repair or replace as required.
5.4 Suggested Practices
(1) Test the pyranom eter input to the data acquisition system .
(i) Measure the electrical zero. This test should be done near the location where the
pyranom eter is to be placed to ensure that the cabling is free from induced signals.
Disconnect the pyranom eter and replace it with a resistor of about the sam e value as
the pyranom eter resistance. Check that the voltage reading is zero to the accuracy
specified for the data acquisition system . If not, determ ine whether the output is due