Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 163)

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Annex F Suppliers of Data Acquisition Systems (Partial Listing)
F 1. Data Acquisition Types
Although the requirem ents for observing the basic radiation quantities associated with the BSRN are
relatively sim ple in principle, the need for high accuracy, high resolution observations to be obtained
once per second will tax m any data acquisition system s if m ore than a few channels are to be sam pled
and the BSRN uncertainty requirem ent in irradiance observations of 0.01% of the reading or ±1
:V.(whichever is greater based on an unam plified signal) is to be m et.
Three types of data acquisition system s are norm ally considered for such m easurem ents.
• Type 1 is a bench system consisting of a com bination of digital m ultim eter and a m ultiplexer. These
can be either housed in a single unit or be connected externally. Program m ing and final data storage
are via a desktop com puter. These types of system s are highly program m able, are capable of
scanning nearly an unlim ited num ber of channels and can norm ally m easure voltage, resistance
and current. Type 1 system s are the m ost accurate and are highly configurable, but m ust be kept
in laboratory conditions and are the m ost expensive of the three types. Instrum ent m anufacturers
such as Keithley Instrum ents, Fluke or Agilent Technologies are the m ost com m on suppliers of
this type of instrum entation.
• Type 2 are all-weather data acquisition system s that can be battery operated. Most are only capable
of m easuring voltage, so bridges m ust be used to convert resistance m easurem ents. These system s
can provide m ultiple m easurem ent ranges and have on-board com putational capabilities. Data
can be downloaded from their internal storage by a variety of m eans, including direct connections
to the internet. While m ost are capable of m easuring only 10 or 12 channels, som e are expandable,
or can be chained to sim ilar system s so that they can behave as a single unit. Units m anufactured
by Cam pbell Scientific, Clim atronics or Vaisala are in this category.
• The third type of system is installed directly into a desktop com puter or is connected to the com puter
through USB or sim ilar interfaces. Sim ilar to Type 2 system s, these data acquisition cards norm ally
only sam ple voltages. Software, either provided by the m anufacturer or purchased separately is
used to program the card. At present, the highest resolution of Type 3 system s is 16-bits. Data
storage is directly to the hard-drive of the com puter. The m ajor advantage of these cards over
either Type 1 or Type 2 system s is the sam pling speed is based on a different m ethod of analog
to digital conversion that allows speeds up to m ega-sam ples per second. Newer software allows
tim e averaging of such sam ples to increase the resolution of the m easurem ent.
Table F 1 provides a com parison of the typical capabilities of these data acquisition system types with
respect to voltage m easurem ents and whether or not they a capable of m easuring resistance directly
(e.g. for the m easurem ent of the tem perature therm istors in pyrgeom eters). Actual specifications and
options are dependent on the m anufacturer and the m odel and are continually changing. The table
is provided only to assist in understanding the capabilities of the various types of data acquisition system s.