Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 51)

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This m ethod works well if the instrum ent is on a vertical post attached to the boom extending
from the tower. The pyranom eter is levelled while the post is vertical in an upright position.
The m easurem ent of the angle of the post can be accom plished 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 m etal attached to a circular ring whose diam eter is such that it will sit around
the outside dom e of the pyranom eters to be inverted. The ring m ust have known parallel ends.
The m etal 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 pyranom eter outer dom e. To im prove
the perform ance of this tool, three sm all ‘feet’ m ay extrude from the instrum ent end of the
ring for positive placem ent on the pyranom eter. On the far end of the m etal plate an adjustable
circular spirit level is attached for the ultim ate levelling of the pyranom eter (pyrgeom eter)
to be used in the downfacing position.
The pyranom eter is first levelled in its norm al position following radiom etric levelling of the
instrum ent. The levelling tool is placed on the pyranom eter and the adjustable level on the
plate set to conform to the instrum ent bubble level. The plate is then turned over so that the
bubble level will be upright when the pyranom eter is inverted.
W hen attaching the pyranom eter to its inverted position, spring loaded retaining bolts are required
to m aintain a constant pressure against which the levelling feet can be adjusted. The level can be set
by holding the levelling jig against the instrum ent and adjusting the levelling feet of the pyranom eter
in the norm al m anner.
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 radiom eter with an open entrance aperture for the
m easurem ent of direct beam radiation. This was later am ended to include the use of a norm al incidence
pyrheliom eter (or m ore sim ply pyrheliom eter) to fill gaps in the data stream during those tim e periods
when the cavity radiom eter was in calibration m ode. Further am endm ents were m ade to the original
concept when concerns about protecting the open cavity radiom eter against the elem ents were brought
forward. Further investigations have found that calcium fluoride or sapphire optical flats provide good
protection from the elem ents while transm itting virtually the full solar spectrum . The ideal configuration
for the m easurem ent of direct beam radiation rem ains the use of either an open all-weather cavity
radiom eter or all-weather cavity radiom eter with the appropriate optical flat used for protection along
with a pyrheliom eter preferably capable of m easuring the sam e spectral range, that can be used to
com plete the data set during the periods the cavity radiom eter is in calibration m ode. Thus the cavity
radiom eter is the prim ary instrum ent with the pyrheliom eter being used to fill the ‘calibration gaps’ by
correlation with the observations obtained during the cavity m easurem ents periods preceding and
following the calibration tim e period. The actual frequency and length of tim e required for the self-calibration
period depends upon the type of the cavity radiom eter. Lesser alternatives, however, are acceptable.
In rank order of preference these are: (1) The use of a pyrheliom eter as the prim ary instrum ent while
an open cavity radiom eter is used in tandem at all tim es weather conditions perm it. In this m anner,
the pyrheliom eter is calibrated against the cavity radiom eter nearly continuously. (2) sam e as (1), but
with a cavity radiom eter with a quartz flat covering the entrance aperture. This cavity in turn is to be
calibrated against an open aperture cavity radiom eter to account for the effect of the flat. (3) The use
of two pyrheliom eters m easuring on a routine base, with an open aperture cavity radiom eter 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 m anufacturer, the instrum ent should be calibrated so that the following
inform ation is available:
(i) the responsivity of the instrum ent to radiation
(ii) the linearity of the instrum ent between 0 and 1500 W m -2