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the diffusometer has been fitted to the pyrheliometer. Table C 3.4 provides information on the original
length of the diffusometer arm and the optimal arm-length fitted for the pyrheliometers used for these
calculations.
The measures of fit are:
(1) the standard deviation
(2) and the maximal absolute value of DE.
Diffusometer Fitted to HF Fitted to CH-1 Fitted to NIP Original
Austral CM 885 785 695 795
German CM 790 690 595 687
SciTec CM 630 600 505 577
Canadian CM 940 910 730 750
Austral PSP 885 820 700 795
USA PSP 845 745 580 603
Table C 3.4. Original and “optimal” arm length values (mm) for existing BSRN diffusometers.
The second step was the variation of the radii of shading devices. These results are given in Table
C 3.5.
Diffusometer Fitted to HF Fitted to CH-1 Fitted to NIP Original
Austral CM 31 34 39 34.8
German CM 27.3 29 34.5 30
SciTec CM 23.7 24 29 25.4
Canadian CM 29.5 31.7 37.5 37.5
Austral PSP 29 33 39 34.8
USA PSP 24.3 27 37.5 37.5
Table C 3.5. Values of original and “optimal” radii (mm) of shading disk or sphere for the existing
BSRN diffusometers.
C 3.2.4 Optimal systems
The third step was to look for the arm length and shader radius for each pyranometer – pyrheliometer
pair that gives the best possible fit. Since the full glass dome must be shaded (this is the common
sense feeling contrary to the statement made by Kees van den Bos of Kipp & Zonen at the 1998 Budapest
BSRN Workshop), therefore the shading disk/sphere must be a little larger than the glass dome. For
three pyranometers (the exception is the Eppley 8-48) the lower limit value of the shading disk/sphere
is the optimal one (Table C 3.6).
(1) In the case of optimized arm lengths or shading disks/spheres (let us say half optimization)
the bias is zero, the mean standard deviation is 0.5 W m , the maximal difference is 1 W m .
-2 -2
(2) For the fully optimized pairs the mean (for the 12 pairs) of standard deviations is 0.2 W m ,
-2
the maximal difference is 0.5 W m .
-2
(3) The NIP fits best to each pyranometer having one third of the values mentioned in the previous
conclusion. The weakest of the NIP’s fit is to the PSP, this is due to the large dome size –
sensor size ratio at PSP.
(4) The 8-48 could be fitted best to any pyrheliometer, since its dome is only slightly larger than
its sensor.
(5) It is suggested to make individual optimization for each station (solar elevation, aerosol, radiation
and sunshine conditions) taking into account the available instrumentation (pyranometer,
pyrheliometer, tracker). This way the best fit could be achieved within the available financial
possibilities and the actual differences can be determined for the system to be realized.
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