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LP NET 07
LP NET 07 net radiometer is designed to measure the Net radiation
passing through a surface, across the spectral range between the near ultraviolet
and the far infrared. The Net radiation is defined as the difference between the
radiation that strikes the top surface, and the radiation that strikes the bottom
surface of the net radiometer. The upward facing surface measures direct and
diffuse solar radiation plus long-wave irradiance from the sky (clouds), while the
downward facing surface measures the reflected solar radiation (Albedo) plus the
terrestrial long-wave irradiance.
LP NET 07 is designed for continuous outdoor use, and is suitable for all weather
conditions.
Although net radiometers are generally used in meteorology to measure radiation
balance, the LP NET 07 can also be used to measure indoor radiant temperature
(ISO 7726).
Working Principle
LP NET 07 is based on a thermopile sensor with one set of hot junctions in
contact with the upper surface and a set of cold junctions in contact with the lower
surface. The difference in temperature between the two receivers is proportional
to the net radiation. Through the Seebeck effect, the difference in temperature
between hot and cold junctions is translated into a Potential Difference. A
hemispheric Teflon
-coated dome protects the two receivers, and their particular
®
shape allows an optimal cosine corrected response. The Teflon
both a continuous outdoor use and a constant spectral response, ranging from
the near ultraviolet (200nm) to the far infrared (100μm) spectral regions.
Installing and mounting the net radiometer for total irradiance
measurements:
• To allow cleaning the two receiving surfaces regularly, LP NET 07 should be
mounted in easily reachable places. The surfaces can be washed with plain
water or pure ETHIL alcohol.
• Mount the instrument so that no shadow will be cast on it at any time of day
and of the seasons, from obstructions such as buildings, trees, or any other
obstacle.
• In the NORTHERN hemisphere, the net radiometer is normally oriented
towards the SOUTH, while it should be oriented NORTHWARD, in the
SOUTHERN hemisphere.
• The instrument should be mounted at least 1.5 meters above the ground
surface.
The flux on the downward facing sensor is representative for a
circular area having a radius of 10 times the height.
• While mounting the net radiometer, avoid touching both receiving surfaces.
Electrical Connections and requirements for electronic readout devices:
• LP NET 07 does not require any power supply.
• It is available with a 5 m. output cable
• It is supplied with a PTFE, UV resistant, braided shield and 2-wire cable. The
color code is as follows:
shield
→ connected to the housing
red
→ (+) positive pole of the signal generated by the detector
blue
→ (-) negative pole of the signal generated by the detector
Fig.1 shows the wiring diagram.
• It has to be connected to a millivoltmeter or to a data acquisition system with
input impedance higher than 4000kΩ. Normally, the output signal from the net
radiometer does not exceed ±20 mV. In order to grant the best performances
in measurements, the instrument resolution should be of 1μV.
Maintenance:
In order to grant the instrument's best performance, the two receiving surfaces
must be always kept clean; the cleaner you keep the instrument, the better
the accuracy in measurements will be. Washing can be made with water and
standard lens paper; in case this wouldn't work, use pure ETHIL alcohol. After
using alcohol, the domes must be washed with water only.
We strongly recommend checking LP NET 07 calibration every year. Calibration
can be checked directly in the field, by mounting another net radiometer (sample)
alongside, and comparing results. Calibration in the field is less precise than the
one carried out in a laboratory, but has the advantage of not having to remove
the probe from its holder.
Calibration and measurements:
Net radiometer sensitivity, indicated as S (or calibration factor), allows
determining the net radiant flux passing through a surface. S factor is
measured in μV/(Wm
).
-2
• Once the potential difference (DDP) has been measured at sensor ends, E
flux is obtained through the following formula:
where;
E
:
indicates the radiant flux expressed in W/m
e
DDP: indicates the potential difference expressed in μV and measured
by the multimeter,
S:
indicates the calibration factor expressed in μV/(W/m
on the net radiometer label (calibration factor is also mentioned in
the calibration report).
N.B. If the difference of potential (DDP) is positive, the radiation on the
upper surface is higher than the radiation on the lower surface (it
happens normally in daily hours); if DDP is negative, the radiation
coating allows
®
E
= DDP/S
e
,
2
) and shown
2
on the lower surface is higher that the one on the upper surface (it
happens in nightly hours).
Each net radiometer comes factory calibrated and has its own calibration factor.
Calibration is performed inside Delta Ohm Metrological Laboratory, and is carried
out by comparison with a reference net radiometer, using a solar simulator as a
light source. Calibration is performed using a parallel light beam.
Sensitivity to wind speed:
At the same radiant flux density, by increasing the wind speed, the net radiometer
output signal will decrease (by increasing the wind speed, sensitivity will
decreases).
Measurements taken inside the wind tunnel, have shown that S
related to the wind speed for LP NET 07, can be corrected by using the following
functions:
S
=S
(1-0.011×V)
v
0
S
=S
(0.95-0.006×V)
v
0
Where: S
sensitivity at zero wind speed
0 =
V = wind speed in m/s
Fig. 2 shows the calibration factor related with wind speed.
Once we know both the net radiation - calculated through the sensitivity at zero
wind speed (F
) - and the wind speed in (V) in m/s, the correct data is obtained
net_0
by using the following formula:
F
=F
/(1-0.011×V)
net
net_0
F
=F
/(0.95-0.006×V)
net
net_0
Cosine response/Directional error:
The radiation falling on a surface should be measured with a sensor, whose
response related to the light incidence angle, has to be a Lambertian Response.
A receiver is known as Lambertian when its sensibility (S
incidence angle between the light and the detector surface, has the following
behavior:
Where: S
is the sensitivity when light strikes perpendicular to the surface, ϑ is
0
the angle between the incident light beam and the line which is normal to
the surface.
Fig. 3 shows the typical behavior of the error related to the angle of incidence.
Technical specifications:
Typical sensitivity:
Impedance:
Measuring range:
Spectral range:
Operating temperature:
Weight:
Response time (95%):
������
Fig.1
1
0.98
0.96
e
0.94
0.92
0.9
0.88
0.86
0.84
0.82
0.8
0
2
4
Fig.2
V≤10m/s
10m/s<V<20m/s
V≤10m/s
10m/s<V<20m/s
S
=S
cos(ϑ)
0
ϑ
10 μV/(W/m
2Ω ÷ 4 Ω
±2000 W/m
0.2 μm ÷ 100 μm
-40 °C ÷ 80 °C
0.35 Kg
<75 sec
������ ���������
� ����
6
8
10
12
14
WindSpeed(m/s)
sensitivity,
v
), related to the
ϑ
)
2
2
� � � �
16
18
20
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