INTRODUCTION

The first part of this manual provides a brief summary of the factors you need to consider when installing the Net Radiometer type NR2 and connecting it to Delta-T Loggers. Reference is made to the appendix pages (called A1, A2 etc) which contain more detailed information on the performance and specifications and maintenance of the instrument.

Net Radiation

Net radiation is an important quantity governing evapotranspiration from surfaces and vegetation. When combined with measurements of windspeed, temperature and humidity, estimates of potential evapotranspiration can be derived from the Penman-Monteith equation. This, together with rainfall data can be used to calculate soil moisture deficit.

The NR2 may also be used as a reference instrument in experiments requiring the use of Tube Net Radiometers.

Instruments used for measuring net radiation are called net pyrradiometers by the WMO (World Meteorological Organisation), but the term net radiometer will be used in this manual for brevity. Full details of WMO procedures and guidance for using net radiometers can be found in ref.1.

DESCRIPTION

The NR2 net radiometer has a thermopile sensor head which is exposed to both the downward and the upward fluxes of radiation. The thermopile produces a millivolt output proportional to the net flux of radiant energy. If the net flux is downwards, a positive signal is produced, whereas if the net flux is upwards (which may occur at night) the signal becomes negative.

The thermopile is shielded from wind and rain by thin polythene domes. In order to prevent condensation occurring inside the domes, a desiccant tube is incorporated in the support arm of the radiometer. The desiccant material must be refreshed from time to time, and the polythene domes must be replaced if they become degraded by sunlight, or physically damaged.