SOLAR RADIATION SENSORS

Sensor Type

Sensor

Description

Solar Radiation

The aim of solar radiation sensors is to measure the  amount of sunlight being received at a given location. Global Radiation sensor measures across the full light spectrum, whereas a PAR (photo-synthetically active) sensor measures only the wavelengths taken up by plants.
Solar radiation sensors are typically broken down in to two types: cheaper sensors, which use a photo-detector and more expensive units which use a thermopile. The latter are typically referred to as pyranometers and may be first or second class, depending on their level of accuracy. 
The simpler sensors are often referred to as global radiation sensor. The cheaper sensors built using a photocell, do not measure equally across the light spectrum and while they  return reasonable results in clear sky conditions,  the accuracy will fall off in cloudy conditions. A pyranometer will have a spectral response from 300 to  3200 nm, whereas lesser quality sensors will only read from 400  to 11000nm
Constructing a proper pyranometer is a complex and expensive process. The measurement element comprises a large number of thermocouple junctions placed end to end. The thermocouples re made using two dis-similar metals and a voltage is generated across the junction which is proportional to the temperature difference. One side of the junction (the cold side) is fixed to the body of the sensor. The other side, the hot side, is mounted on a surface coated with black energy absorbent material. The voltage generated on any one junction is small, but by stacking multiple thermocouples together, a higher voltage is produced. A separate temperature sensor measures the body temperature and is used to apply temperature compensation to the output. The result is a sensor with very high sensitivity (i.e. starts reading in very low light levels) and high linearity, which translates to high accuracy.
 Global Radiation sensors  can usually be identified by the presence of a Teflon diffuser on the top of the sensor element. Higher quality sensors use a glass dome which ensures that regardless of the angle of the sun, the same amount of line reaches the sensor element.  If you picture the path of the sun through the sky, it starts in the east, rises to be directly above at midday then sets in the east. Without a glass lens, the amount of light captured at the different solar angles will change. Similarly the angle of the midday sun changes from summer to winter: with the sun directly overhead in summer but to the north in winter.. The error produced by the changes in solar angle is referred to as Cosine Error. This is because the Cosine function produces its smallest output at 0 and 180 degrees and its maximum at 90 degrees, which matches the error from the angle of the sun. The smaller the cosine error, the better the sensor quality. 
 

Solar Radiation

RK200-04

The RK200-04 is a silicon cell solar radiation sensor r with an SDI-12 output. Unlike many competing units, it has a glass dome to reduce cosine errors. The RK200-04 is ideal for use in agricultural applications, for instance in the calculation of evapo-transpiration. 
 

Pyranometer

RK200-03

The RK200-03 is a thermopile type pyranometer with an SDI-12 interface. It is fitted with  a glass dome, a bubble level and  and leveling feet. After installing the sensor the feet are adjusted until the bubble level shows the sensor is exactly horizontal. Use the RK200-03 in any application which requires high accuracy and consistent results. The price enables you to use the sensor in areas where previously you would have chosen a silicon cell sensor.