Kippzonen BSRN Scientific Solar Monitoring System Uživatelský manuál stáhnout pdf (Strana 52)

(iii) the deviation of the temperature compensation circuit of the instrument over the

temperature range (-10° to +10° of the local range in temperature) or if not compensated

the required temperature correction of the instrument

(iv) the opening angle and the slope angle of the instrument

(2) Checks should be made 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 climatic regim e

of the area in which the instrument is to be installed.

(3) The BSRN accuracy guide indicates that the case temperature of the instrument should be

monitored. If the instrument is fitted with a thermal measuring device, the wiring should be

checked and the reduction algorithm tested at known temperatures. In the case of pyrgeometers,

all thermistors should be tested.

(4) The impedance of the instrument should be checked against the manufacturer’s values.

(5) Some instruments require desiccant. If so, the desiccant should be fully activated. It is

recommended that the desiccating material be of the bead type and not one which easily

powders.

(6) All connectors must be waterproof and should be appropriate for the clim atic conditions in

which the sensor will be deployed. For example, in marine environments care must be taken

against using connectors that are prone to corrode. It is recommended that keyed connectors

be used for greater safety in maintaining instrument polarity.

4.3.3 Mechanical Installation

The primary obstacles in obtaining quality direct beam radiation measurements is the difficulty in pointing

the instrument toward the sun. This is not so much a problem in mounting the sensor as correctly installing

and operating the tracking device (Section 4.4).

The mechanical installation of the pyrheliometer or cavity radiometer must ensure that the instruments

are firmly attached to the tracker on which they are to be mounted. Care must be taken that the instrument

will not shift position throughout the day as the centre of gravity shifts with respect to the mounting

brackets. Figures 4.5, 4.6, 4.8 and 4.9 illustrate typical mountings of pyrheliometers on tracking devices

When installed on a correctly pointing tracker, the combination tracker and instrument should work

as an integrated unit with the sight of the instrument acting as the primary sight for the tracker. When

using an active tracker, care must be taken to ensure that the pyrheliometer or active cavity radiometer

sights are aligned with the respect to the positioning of the active eye. Trackers that use a combination

of active and algorithm tracking (an algorithm that calculates the location of the sun based on location

and time) depending on solar intensity, must be set up in a manner that the tracker does not ‘jump’

to a different position when the solar intensity drops below the active-eye threshold When both a

pyrheliometer and a cavity radiometer are mounted on the same tracker, the pointing of the cavity

radiometer should take precedence over the pyrheliometer.

It should be noted that in aligning direct beam radiometers, the field of view of the sighting diopter

is significantly smaller than the field of view of the instruments. Nominally, pyrheliometers have a field

of view of approximately 5°, while the sighting optic subtends a maximum 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 major

types of pyrheliometers used within the network. Annex D details the type of measurement errors

associated with the incorrect pointing of pyrheliometers based on model calculations.

4.4 Installation of Solar Tracking and Pointing Devices

Several types of solar tracking devices exist, from the single-axis synchronous motor tracker to the

computer-controlled dual-axis active-sensor tracker. Each type of tracker has advantages and

disadvantages which must be balanced by the individual researcher before installing the device of

choice. Table 4.2 indicates the advantages and disadvantages of some of the more common types

of trackers. It goes well beyond the scope of the manual to provide the installation and maintenance

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