SR05-D1A3-PV manual v1801 56/83
7 Making a dependable measurement
7.1 The concept of dependability
A measurement with a pyranometer is called “dependable” if it is reliable, i.e. measuring
within required uncertainty limits, for most of the time and if problems, once they occur,
can be solved quickly.
The requirements for a measurement with a pyranometer may be expressed by the user
as:
• required uncertainty of the measurement (see following paragraphs)
• requirements for maintenance and repairs (possibilities for maintenance and repair
including effort to be made and processing time)
• a requirement to the expected instrument lifetime (until it is no longer feasible to
repair)
It is important to realise that the uncertainty of the measurement is not only determined
by the instrument but also by the way it is used.
See also ISO 9060 note 5. In case of pyranometers, the measurement uncertainty as
obtained during outdoor measurements is a function of:
• the instrument class
• the calibration procedure / uncertainty
• the duration of instrument employment under natural sunlight (involving the
instrument stability specification)
• the measurement conditions (such as tilting, ventilation, shading, instrument
temperature)
• maintenance (mainly fouling)
• the environmental conditions*
Therefore, ISO 9060 says, “statements about the overall measurement uncertainty under
outdoor conditions can only be made on an individual basis, taking all these factors into
account”.
* defined at Hukseflux as all factors outside the instrument that are relevant to the
measurement such as the cloud cover (presence or absence of direct radiation), sun
position, the local horizon (which may be obstructed) or condition of the ground (when
tilted). The environmental conditions also involve the question whether or not the
measurement at the location of measurement is representative of the quantity that
should be measured.
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