Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 52)

40
(iii) the deviation of the tem perature com pensation circuit of the instrum ent over the
tem perature range (-10° to +10° of the local range in tem perature) or if not compensated
the required tem perature correction of the instrum ent
(iv) the opening angle and the slope angle of the instrum ent
(2) Checks should be m ade of all wiring to ensure that there are no nicks in the sheathing nor
stress on the connections. The wire should be of a variety that will withstand the clim atic regim e
of the area in which the instrum ent is to be installed.
(3) The BSRN accuracy guide indicates that the case tem perature of the instrum ent should be
m onitored. If the instrum ent is fitted with a therm al m easuring device, the wiring should be
checked and the reduction algorithm tested at known tem peratures. In the case of pyrgeom eters,
all therm istors should be tested.
(4) The im pedance of the instrum ent should be checked against the m anufacturer’s values.
(5) Som e instrum ents require desiccant. If so, the desiccant should be fully activated. It is
recom m ended that the desiccating m aterial be of the bead type and not one which easily
powders.
(6) All connectors m ust be waterproof and should be appropriate for the clim atic conditions in
which the sensor will be deployed. For exam ple, in m arine environm ents care m ust be taken
against using connectors that are prone to corrode. It is recom m ended that keyed connectors
be used for greater safety in m aintaining instrum ent polarity.
4.3.3 Mechanical Installation
The prim ary obstacles in obtaining quality direct beam radiation m easurem ents is the difficulty in pointing
the instrum ent toward the sun. This is not so m uch a problem in m ounting the sensor as correctly installing
and operating the tracking device (Section 4.4).
The m echanical installation of the pyrheliom eter or cavity radiom eter m ust ensure that the instrum ents
are firm ly attached to the tracker on which they are to be m ounted. Care must be taken that the instrum ent
will not shift position throughout the day as the centre of gravity shifts with respect to the m ounting
brackets. Figures 4.5, 4.6, 4.8 and 4.9 illustrate typical m ountings of pyrheliom eters on tracking devices
W hen installed on a correctly pointing tracker, the com bination tracker and instrum ent should work
as an integrated unit with the sight of the instrum ent acting as the prim ary sight for the tracker. W hen
using an active tracker, care m ust be taken to ensure that the pyrheliom eter or active cavity radiom eter
sights are aligned with the respect to the positioning of the active eye. Trackers that use a com bination
of active and algorithm tracking (an algorithm that calculates the location of the sun based on location
and tim e) depending on solar intensity, m ust be set up in a m anner that the tracker does not ‘jum p’
to a different position when the solar intensity drops below the active-eye threshold W he n both a
pyrheliom eter and a cavity radiom eter are m ounted on the sam e tracker, the pointing of the cavity
radiom eter should take precedence over the pyrheliom eter.
It should be noted that in aligning direct beam radiom eters, the field of view of the sighting diopter
is significantly sm aller than the field of view of the instrum ents. Nom inally, pyrheliom eters have a field
of view of approxim ately 5°, while the sighting optic subtends a m axim um angle of between 1.4 and
2.0°. Figure 4.7 illustrates how the pointing accuracy of a tracker affects the output signal of the m ajor
types of pyrheliom eters used within the network. Annex D details the type of m easurem ent errors
associated with the incorrect pointing of pyrheliom eters based on m odel calculations.
4.4 Installation of Solar Tracking and Pointing Devices
Several types of solar tracking devices exist, from the single-axis synchronous m otor tracker to the
com puter-controlled dual-axis active-sensor tracker. Each type of tracker has advantages and
disadvantages which m ust be balanced by the individual researcher before installing the device of
choice. Table 4.2 indicates the advantages and disadvantages of som e of the m ore com m on types
of trackers. It goes well beyond the scope of the m anual to provide the installation and m aintenance