Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 9)

CMP/CMA series manual
Page 8
Figure 2 Mast construction
5. Level pyranometer
Accurate measurement of the global radiation requires proper levelling of the thermopile surface. Level
the instrument by turning the two levelling screws to bring the bubble of the spirit level centrally within
the marked ring. For easy levelling, first use the screw nearest to the spirit level. When the
pyranometer is placed horizontally using the bubble level, or when it is mounted with its base directly
on a horizontal plane, the thermopile is horizontal within 0.1
o
.
6. Secure
pyranometer
Secure the pyranometer tightly with the two stainless steel bolts. Ensure that the pyranometer
maintains the correct levelled position!
7. Fit cable and sun shield
Locate the cable plug correctly in the radiometer socket (it only fits one way) and screw the plug
locking ring hand-tight. Finally, clip on the sun shield to prevent excessive heating of the radiometer
body. The bubble level is visible through the top of the sun shield for routine checks.
2.2.2. Installation for measurement of radiation on inclined surfaces
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
o
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.
2.2.3. Installation for measurement of reflected radiation
In the inverted position the pyranometer measures reflected
global radiation. According to the WMO the height should be 1
m to 2 m above a uniform surface covered by short grass.
The mounting device should not interfere significantly with the
field of view of the instrument. The upper plate prevents
excessive heating of the pyranometer body by the solar
radiation and, if large enough, it keeps the lower screen free of
precipitation. The lower glare screen prevents direct
illumination of the domes by the sun at sunrise and sunset and
is available as an accessory kit for the CMP series.
Offset signals generated in the pyranometer by thermal effects
are a factor of 5 more significant in the measurement of
reflected radiation due to the lower irradiance level. The mast
shown in Figure 2 intercepts a fraction D/2
πS. of the radiation
coming from the ground. In the most unfavourable situation
(sun at zenith) the pyranometer shadow decreases the signal
by a factor R
2
/H
2
.
A rule of thumb is:
A black shadow with radius = 0.1 H on the field below decreases the signal by1% and 99% of the
signal will originate from an area with radius 10 H.