Solarimeter for field use

Using simple materials a thermopile solarimeter has been constructed with output 5.5 mV cal -1 cm -2 min -1 ; resistance 13 Ω; response time 20 s; weight 35 g; dimensions 4 × 4 × 4 cm. It can therefore be used with a portable millivoltmeter for measuring reflexion coefficients, radiation within crops, etc. General equations for solarimeter performance and a simplified theory for the present design give a non-linearity of -0.8% cal -1 cm -2 min -1 and a temperature coefficient of -0.12% per deg C. Outdoor comparison with a Kipp solarimeter gave ±0.03 cal cm -2 min -1 as the maximum deviation from the mean response with radiation up to 1.1 cal cm -2 min -1 . The standard deviation for thirty observations in bright sunshine and with overcast skies was ±0.002 cal cm -2 min -1 . Cosine and azimuth response were improved using a larger instrument of the same design and comparisons of this with a Kipp solarimeter gave maximum deviation of ±0.02 cal cm -2 min -1 .