Kippzonen BSRN Scientific Solar Monitoring System Manual do Utilizador descarregar pdf (Página 21)

Uncertainty is defined as a parameter associated with the result of a measurement, that characterizes the

dispersion of the values that could reasonably be attributed to the measurand. Former BSRN publications

have termed this accuracy. The terminology has been changed to follow the ISO guidelines. Accuracy is a

more general terminology that expresses a variety of ideas, many of which cannot be quantified.

ISO, 1993: Guidelines for the Expression of Uncertainty Measurement. First Edition.

nip (lowercase) is used as an acronym and is not to be confused with NIP™ of Eppley Laboratories.

2.1.2 Sampling Frequency of Ancillary Measurements

At stations where the ancillary m easurem ents are under the control of an independent agency,

such as a national weather service, the frequency of the various measurements cannot often be

altered. The higher the frequency the greater the usefulness of the data, up to the sampling rate

of the radiation measurements. BSRN station scientists should encourage any independent

collection agency to sample and record data following standard WMO procedures at the very

minimum.

When automatic data logging is employed to record such variables as pressure, temperature,

humidity, wind speed and wind direction, providing these data at the same frequency as the

radiation data is beneficial. Stations are encouraged to obtain these observations coincidentally

with the radiation measurements using a one-minute sample rate to aid in understanding the

energy balance of the radiation instruments and the infrared component of the radiation balance.

At a very minimum, all stations should record air temperature at the same location and at the

same sampling frequency as the radiation measurements.

2.2 Uncertainty of Measurements

2.2.1 Uncertainty in Radiation Measurements

These accuracies are based upon state-of-the-art commercially available equipment. At the onset

of the BSRN programme, a table listing the uncertainties about individual flux measurements was

produced (Table 2.1) that included the uncertainties thought to be achievable by 1997. These

uncertainty values have been achieved using new sensors and methods of observation, some

being surpassed. Nevertheless, new methods of observation are continuing to develop that will

continue to decrease the overall uncertainty associated with instantaneous measurements.

Even as instrumentation and methods of observation have improved over the decade since the

inception of the network, the estimate of uncertainty has become more refined. The publication of

the International Organization for Standardization (ISO) Guide to the Expression of Uncertainty

Measurement (GUM) provides a standard method for the determ ination of uncertainty in

measurement. National Metrology Institutions (NMI) and industrial laboratories have adopted its

methodology and the BSRN recommends that all uncertainty calculations follow the procedures

of the guide.

To meet and exceed these target accuracies, the measurement of each quantity will require a

particular methodology of measurement. While these methodologies are not absolute in nature,

they will ensure a given level of uncertainty in the measurement if followed (assuming appropriate

on-site maintenance etc.). The BSRN is concerned more with meeting the target measurement

uncertainty however, than the manner in which the uncertainty is met. Methods of measurement

associated with these uncertainties were first published in WCRP-64, 1991. While many

methodologies have not changed significantly since the inception of the programme, several

measurem ent techniques have im proved. Those that have not changed are repeated verbatim in

this manual.

2.2.1.1 Direct Solar Irradiance

The target uncertainty for measurement of direct solar irradiance in the BSRN is 1% (or 2 W m

-2

as the minimum deviation from the "true" value as reflected in the uncertainty of the World

Radiometric Reference). For the continuous measurements used in providing the mean value

over one-minute, a normal incidence pyrheliometer (nip ) or similar is recommended.

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