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Annex F Suppliers of Data Acquisition Systems (Partial Listing)
F 1. Data Acquisition Types
Although the requirements for observing the basic radiation quantities associated with the BSRN are
relatively simple in principle, the need for high accuracy, high resolution observations to be obtained
once per second will tax many data acquisition systems if more than a few channels are to be sampled
and the BSRN uncertainty requirement in irradiance observations of 0.01% of the reading or ±1
:V.(whichever is greater based on an unamplified signal) is to be met.
Three types of data acquisition systems are normally considered for such measurements.
• Type 1 is a bench system consisting of a combination of digital multimeter and a multiplexer. These
can be either housed in a single unit or be connected externally. Programming and final data storage
are via a desktop computer. These types of systems are highly programmable, are capable of
scanning nearly an unlimited number of channels and can normally measure voltage, resistance
and current. Type 1 systems are the most accurate and are highly configurable, but must be kept
in laboratory conditions and are the most expensive of the three types. Instrument manufacturers
such as Keithley Instruments, Fluke or Agilent Technologies are the most common suppliers of
this type of instrumentation.
• Type 2 are all-weather data acquisition systems that can be battery operated. Most are only capable
of measuring voltage, so bridges must be used to convert resistance measurements. These systems
can provide multiple measurement ranges and have on-board computational capabilities. Data
can be downloaded from their internal storage by a variety of means, including direct connections
to the internet. While most are capable of measuring only 10 or 12 channels, some are expandable,
or can be chained to similar systems so that they can behave as a single unit. Units manufactured
by Campbell Scientific, Climatronics or Vaisala are in this category.
• The third type of system is installed directly into a desktop computer or is connected to the computer
through USB or similar interfaces. Similar to Type 2 systems, these data acquisition cards normally
only sample voltages. Software, either provided by the manufacturer or purchased separately is
used to program the card. At present, the highest resolution of Type 3 systems is 16-bits. Data
storage is directly to the hard-drive of the computer. The major advantage of these cards over
either Type 1 or Type 2 systems is the sampling speed is based on a different method of analog
to digital conversion that allows speeds up to mega-samples per second. Newer software allows
time averaging of such samples to increase the resolution of the measurement.
Table F 1 provides a comparison of the typical capabilities of these data acquisition system types with
respect to voltage measurements and whether or not they a capable of measuring resistance directly
(e.g. for the measurement of the temperature thermistors in pyrgeometers). Actual specifications and
options are dependent on the manufacturer and the model and are continually changing. The table
is provided only to assist in understanding the capabilities of the various types of data acquisition systems.
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