Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 134)

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Figure C 2.5. Dependence of the HUNI/HUNIII on direct radiation.
C 2.4 Reduction of measurements to standard geometry
The em pirical form ulae in section C 2.4.2 are valid for HunIII. Since
H u n III
RATIO = HunI/HunIII ~ Standard/HunIII,
therefore the reduction of m easurem ents of HunIII to standard geom etry can be m ade by using any
of the two form ulae. The correction of other diffusom eters are connected to that of HunIII in Table C
2.3 by the norm alized deficit.
The norm alized deficit of other diffusom eters can be estim ated by the following equation:
N O R M
DEFICIT = a*SLOPE + b*LIMIT + c
where a = 0.425
b = 0.496
c = - 2.270
These em pirical constants derived from the data of the 10 instrum ents found in Tables C 2.1 and C
2.3. The correlation is 0.991. This way any diffusom eter can be “placed “ into the last colum n of Table
C 2.3 if its slope and lim it angles have been calculated for “vertical position”. Since
1 + DEFICIT = Standard/Diffuse
where “Diffuse” = the output of the diffusom eter so that the “Standard” value can be derived.
C 2.5 Results
(1) The following equipm ents have alm ost identical geom etry: the two Australian diffusom eters, the
Germ an BSRN station diffusom eter and the SciTec-Kipp diffusometer, this way they could be regarded
as a geom etrically standard group of diffusom eters.
(2) The relation of the HunIII diffusom eter to the standard (in practice to the SciTec-Kipp) is described
by two em pirical equations derived from 18 m onths m easurem ents m ade in Budapest.
(3) The relation of any diffusom eter to HunIII is described by an other em pirical equation (based on
calculated circum solar irradiances) that requires the radius of the shader, the radius of the sensor
and the distance between them .