1. New Approach to Pyranometer Calibrationby
M. K. Gupta
Director
India Meteorological Department

2. Methods of Pyranometer Calibration
With reference to a standard pyrheliometer (Shading-unshading method)
With reference to a standard pyrheliometer and reference diffuse pyranometer
With reference to a secondary standard pyranometer

3. Pyranometer Calibration With reference to a standard pyrheliometer.
Princlple: G = D + S sinθ
Method:
Alternately shading and unshading the pyranometer to measure outputs proportional to global(G) and diffuse(D) radiation,
and simultaneousely measuring direct(S) using a standard pyrheliometer, we have
vg/k –vd/k = S. sinθ

4. Sources of errors
The output during shaded condition is not an actual value rather it is the mean of two consecutive measurements of pyranometer outputs in shaded position
The pyranometer is never operated under steady state condition

6. New Method of Pyranometer Calibration
Two pyranometers under calibration are mounted horizontally, side-by-side, one for measurement of global irradiance and the other for measurement of diffuse irradiance.
Both the measurements are made as frequently as possible, say every five minutes, from sunrise to sunset.
Contd…

7. Contd…
The next day, pyranometer-functions are inter-changed and similar measurements are made at the same timings.
The direct irradiance is also measured simultaneously using a standard pyrheliometer.
Thus, global, diffuse and direct irradiance measurements are made simultaneously.

8. Principle:
For any instant on first day, the relation among global, diffuse and direct irradiance yields-
v1g/k1 –v2d/k2= S1. sinθ1 … (1)
Similarly, for any instant on second day, we have-
v2g/k2 –v1d/k1= S2. sinθ2 … (2)

9. TABLE-I Calibration using shading-unshading method
Date of calibration
Pyranometer No.
Calibration constant obtained
No. of sets
Mean value
Dispersion (/k)
14-Jan-2005
CM11-77 63
4.78
0.027
15-Jan-2005 81
4.76
0.018
17-Jan-2005 83
4.83
0.012
21-Jan-2005
2299 76
11.32
0.011
22-Jan-2005 39
11.38
0.024
24-Jan-2005 79
11.41
0.016
4-Feb-2005
1154 69
10.66
0.021
5-Feb-2005 77
10.57
0.014
7-Feb-2005 85
10.64
0.022

10. Table II Calibration using new technique of calibration
Date of calibration
Pyranometer No
Calibration constant obtained
No. of sets
Mean value
Dispersion (/k)
21st and 22nd Jan 2005
1154 46
10.66
0.009
CM11-77
4.74
0.012
21st and 24th Jan 2005 83
10.69
0.015
4th and 5th Feb 2005
1039 90
12.04
0.005
2299
11.44
0.008
5th and 7th Feb 2005
12.00

11. TABLE-III Comparision of results
Pyranometer No
New method
Shade-Unshade
Mean value
Dispersion (/k)
Dispersion
(/k)
CM11-77
4.74
0.012
4.78
0.027
0.009
4.76
0.018 -
4.83
2299
11.44
0.008
11.32
0.011
0/008
11.38
0.024
11.41
0.016
1154
10.66
0.021
10.69
0.015
10.57
0.014
10.64
0.022

12. Pyranometer No. CM11-76
Shading-Unshading
New Method

13. Pyranometer No. 2299
4th and 5th Feb 2005
5th and 7th Feb 2005

14. Advantage of new method
This is free from errors which the shading-unshading method suffers.
The results have better repeatability.
On the basis of theoretical and practical considerations, this provides another fundamental method for pyranometer calibration.
This probably better method for simultaneous calibration of two pyranometers with reference to primary standard pyrheliometers.
Angular dependence of the pyranometer response can also be studied in details.

15. Over and all
Thank You