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3.
Levelling
Accurate measurement of the global radiation requires proper levelling of the
thermopile surface. Level the instrument by turning the levelling screws to
bring the bubble of the spirit level within the
marked ring.
For easy levelling first use the screw nearest to the spirit level.
When the CM 11 is placed horizontally using the spirit level, or when it is
mounted with its base on a horizontal plane, the thermopile is horizontal within
0.05°. This causes a maximum azimuthal variation of + or – 0.5% at a solar
elevation of 10°. By radiometrically levelling, the pyranometer can be placed
more accurately horizontal (See Appendix II).
Finally, the pyranometer should be secured tight with the two stainless steel
bolts. Ensure that the pyranometer maintains the correct levelled position!
3.2.2 Installation for measurement of solar radiation on inclined
surfaces
See also 'installation for measurement of global radiation'.
It is advised to pre-adjust the levelling screws on a horizontal surface for easy
orientation of the instrument parallel to the inclined surface. Because the
temperature of the mounting stand is expected to rise considerably (more than
10° C above air temperature), the body must be thermally isolated by the
levelling screws from the stand. This will promote a thermal equilibrium
between domes and body and decrease zero offset signals.
The CM 11 pyranometer shows no significant tilt effect up to irradiances of
2
1000 W/m
.
INSTALLATION
25
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