Kippzonen BSRN Scientific Solar Monitoring System Bedienungsanleitung PDF herunterladen (Seite 51)

This method works well if the instrument is on a vertical post attached to the boom extending

from the tower. The pyranometer is levelled while the post is vertical in an upright position.

The measurement of the angle of the post can be accomplished to within 0.1° using a high

quality carpenter’s level.

(2) The second procedure requires the construction of a levelling jig. This consists of a flat planed

parallel piece of metal attached to a circular ring whose diameter is such that it will sit around

the outside dome of the pyranometers to be inverted. The ring must have known parallel ends.

The metal flat (which can be reversed, side-to-side) is attached to one end of the ring, while

the other end of the ring sits on the ring surrounding the pyranometer outer dom e. To improve

the performance of this tool, three small ‘feet’ may extrude from the instrument end of the

ring for positive placement on the pyranometer. On the far end of the metal plate an adjustable

circular spirit level is attached for the ultimate levelling of the pyranometer (pyrgeometer)

to be used in the downfacing position.

The pyranometer is first levelled in its normal position following radiometric levelling of the

instrument. The levelling tool is placed on the pyranometer and the adjustable level on the

plate set to conform to the instrument bubble level. The plate is then turned over so that the

bubble level will be upright when the pyranometer is inverted.

When attaching the pyranometer to its inverted position, spring loaded retaining bolts are required

to maintain a constant pressure against which the levelling feet can be adjusted. The level can be set

by holding the levelling jig against the instrument and adjusting the levelling feet of the pyranometer

in the normal manner.

4.3 Installation of instruments for the measurement of direct beam radiation

4.3.1 General Considerations

The original goal of the BSRN was to use a cavity radiometer with an open entrance aperture for the

measurem ent of direct beam radiation. This was later amended to include the use of a norm al incidence

pyrheliometer (or more simply pyrheliometer) to fill gaps in the data stream during those time periods

when the cavity radiometer was in calibration mode. Further amendments were made to the original

concept when concerns about protecting the open cavity radiometer against the elements were brought

forward. Further investigations have found that calcium fluoride or sapphire optical flats provide good

protection from the elements while transmitting virtually the full solar spectrum. The ideal configuration

for the measurement of direct beam radiation remains the use of either an open all-weather cavity

radiometer or all-weather cavity radiometer with the appropriate optical flat used for protection along

with a pyrheliometer preferably capable of measuring the same spectral range, that can be used to

complete the data set during the periods the cavity radiometer is in calibration mode. Thus the cavity

radiometer is the primary instrument with the pyrheliometer being used to fill the ‘calibration gaps’ by

correlation with the observations obtained during the cavity measurem ents periods preceding and

following the calibration time period. The actual frequency and length of time required for the self-calibration

period depends upon the type of the cavity radiometer. Lesser alternatives, however, are acceptable.

In rank order of preference these are: (1) The use of a pyrheliometer as the primary instrument while

an open cavity radiometer is used in tandem at all times weather conditions permit. In this manner,

the pyrheliometer is calibrated against the cavity radiometer nearly continuously. (2) same as (1), but

with a cavity radiometer with a quartz flat covering the entrance aperture. This cavity in turn is to be

calibrated against an open aperture cavity radiometer to account for the effect of the flat. (3) The use

of two pyrheliometers measuring on a routine base, with an open aperture cavity radiometer checking

the calibration on a periodic basis during high solar radiation conditions.

4.3.2 Pre-installation checks and service

(1) If not provided by the manufacturer, the instrument should be calibrated so that the following

information is available:

(i) the responsivity of the instrument to radiation

(ii) the linearity of the instrument between 0 and 1500 Wm

-2

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