1 Introduction

 

The Sunshine Duration sensor measures status

and sunshine duration. The WMO (World Meteorological Organization) defines the sunshine duration as the time

during which the direct solar radiation exceeds the level of 120 W/m2.

 

The LP SD18 performs the measure of radiation with an array of photodiodes arranged in a particular geometry which allows to obtain an accurate measurement in any weather conditions. This solution avoids the use of mechanical moving parts and ensures high  reliability over time.

 

The instrument, besides indicating the presence of sun as required by the WMO, measures also direct radiation (SRD), therefore it can be used as a low cost alternative to a pyrheliometer, which use is bound to a solar tracker.

 

 The instrument is available in three versions, which differ in the type of output:

 1    RS485 MODBUS-RTU output and volt-free contact output

(contact closed = SRD ≥ 120 W/m2, contact open = SRD < 120 W/m2)

 2    RS485 MODBUS-RTU output, analog voltage output 0…1 Vdc, which corresponds to 0…2000 W/m2 of direct radiation, and digital output voltage

(digital voltage output: 1V = SRD ≥ 120 W/m2, 0V = SRD < 120 W/m2)

 3    SDI-12 output and volt-free contact output

(contact closed = SRD ≥ 120 W/m2, contact open = SRD < 120 W/m2)

The SD is equipped with a heating element separately powered and galvanically isolated,  which prevents the formation of condensation on the glass surface onto which the sensitive elements are placed. For harsh climates, the above-mentioned versions are available with a second heating element (option R, LP SD18.xR), which prevents the formation of ice and prevents snow from settling.

The instrument does not need any positioning adjustment during the year and it can be installed on a mast or on a proper fixing base (optional).

The application fields are multiple: from the agronomy (agricultural science) to the study the growth  of  crops,  to photovoltaic  systems  for  verifying  their  performance,  to  buildingautomations for automatic opening/closing of blinds, shutters and, in general, to all those areas where it is necessary to monitor the presence of sunlight.

 2 Operating Principle

 The Sunshine Duration is based on the use of 16 sensors arranged in such a way that,

in the presence of sun, at least one of the photo-detectors is exposed to sun light directly from the sun (besides the diffusion component).

Those sensors which are not directly illuminated by the sun are used for the measurement of the diffused light that is subtracted from the measurement of the sensor which sees the sun directly to get direct radiation.