Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 56)

5.0 Data Acquisition

5.1 Introduction

Installing and maintaining the network data acquisition system(s) is crucial if consistent high quality

radiation data is to be sent to the archive. Within this manual data acquisition system (DAS) means

those electronic devices (including the controlling software) and the connectors, which carry out the

process of measuring the signals emanating from the radiation and ancillary measurement devices

(transducers).

A DAS designed for operating and recording data from automated laboratory equipment is generally

suitable for radiation measurement. Usually, these systems consist of four components:

(1) the m ultiplexer sequentially switches across a number of input channels, each of which is

connected to one of the transducers that are to be measured.

(2) the analog-to-digital converter (ADC), that converts the analog signal (e.g., voltage, resistance)

into a digital signal.

(3) the recording system, which may be a combination of internal and external, buffers and

permanent storage locations

(4) the controlling computer(s), both internal and external, that handle sending control signals

to the multiplexer, the ADC and the storage based upon the user’s commands.

These may be combined into a unit, may be separate units connected, for example, by a General Purpose

Interface Bus (GPIB) instrument bus (e.g., HP, Fluke), or may be on a card that plugs into a PC that

is used as the overall control unit. Each of these arrangements has their own advantages and

disadvantages. The combined system is more compact and the programming may be easier. A system

with a separate computer may allow for easier data analysis by allowing it to be accomplished on the

same computer. Those that are totally separate normally have higher accuracies than combined or

PC card systems and can be more easily updated if required.

Although the number of data acquisition or data logging products on the commercial market is enormous,

m any do not meet the exacting specifications required by the BSRN. Annex F lists the nam es and

addresses of companies that can provide systems that will meet the general requirements imposed

by the BSRN. The most stringent of these requirements is the accuracy requirement of 1 :V on a 10

mV signal (0.01%) and the need to make 60 measurements per channel per minute. Within the system,

the two major components that must be carefully considered with respect to accuracy and timing are

the multiplexer and the ADC.

Multiplexing is accomplished either by magnet operated relay contacts or by semiconductor switches.

Relay multiplexing is better for radiation measurement because the relays contribute very little noise

(1-2 :V). Unfortunately, some relay equipped systems are slow. Conversely, semiconductor multiplexing

systems are much faster, but the noise or offset voltage may be greater than 15 :V. In either case,

settling time is required before the measurement can be made.

In considering the ADC, the type and time of integration, the number of bits of resolution and the linearity

must all be considered. High-end, bench-type digital multim eters (DMM) are now capable of 24-bit

resolution and uncertainties of 10's of ppm under stable operating conditions. More rugged systems

usually consist of either 12 or 16 bit ADC. The former does not provide the resolution, without regard

to the accuracy, required for the BSRN, while the latter still may not meet the accuracy requirements.

In cases where the resolution is fixed to a 5 V scale, but the accuracy is adequate with respect to full

scale, the addition of a high quality instrument pre-amplifier at the transducer end of the signal can

increase the m agnitude of the signal to a level where the DAS can meet the BSRN requirem ent. In

such cases though, the overall accuracy of the system is a linear combination of the uncertainties of

both the DAS and the pre-amplifier. (A cautionary note: resolution does not equal accuracy).

Another means of reducing uncertainty for systems that otherwise meet the resolution and timing

requirem ents is by calibrating individual data acquisition system s and then correcting for any nonlinearity

etc. found within the DAS. This process, while possibly saving capital funds, can be labour intensive

and requires that the DAS be calibrated under the conditions associated with the measurement regime.

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Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario Pagina 56

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