COSINE RESPONSE OF QUANTUM SENSORS

Apogee sensors are fully cosine corrected.  Some radiation sensors under correct in low angle light (70 - 90 degrees); other sensors over correct.  The exact placement of the sensor in the head allows the correct amount of low angle light to reach the sensor, which provides the cosine correction. The solid black line in the graph at the right indicates a perfect cosine response.

Apogee Quantum Sensors (Model QSO) have a nearly perfect cosine response, as do LI-COR Quantum Sensors (Model LI-191SA).

CLICK ON IMAGE FOR LARGER VIEW.

EFFECTS OF RAINFALL ON RADIATION SENSORS

Water accumulates on the top of many types of radiation sensors after it rains.  The standard "castle" design (shown in the photo)  provides good cosine correction, but is prone to trapping water and subsequently dust deposits on the sensor surface.  Adding a drain hole on the side of these sensors helps to remove the water but the hole diameter must be large to compensate for the surface tension of the rainwater.  The hole size in most "castle" type sensors is inadequate to provide this drainage.  Our measurements in long-term studies indicate that sensors with the "castle" design read about 15% low until the water evaporates.   More detailed analysis has indicated that small amounts of water can also accumulate on the top of even our "flat top" design.  We tested different amounts of taper on the sides of our sensor in an effort to design a water-repellant, self-cleaning sensor.  The dome shape sensor achieves this objective.   In addition to minimizing water accumulation on the sensor surface, this new shape allows rainfall to rinse and clean the sensor.   

This design retains the cosine correction, height, and diameter of the original sensor.

LONG-TERM ROOF TOP STUDY

This is a study started August 1997 to measure long term stability and sensor drift quantum sensors, pyranometers, and UV sensors.  Below is a list of sensors we are currently testing:

Quantum Sensors (400 - 700 nm)  Test Data (updated 11-01-00)
        6 Apogee
        8 LI-COR

UV Sensors (250 - 400 nm)
        3 Apogee sensors

Pyranometer Sensors (280 - 3000 nm)
        2 LI-COR silicon pyranometers
        3 Apogee silicon pyranometers
        1 Kipp & Zonen silicon pyranometer
        1 Kipp & Zonen thermopile pyranometer
        3 Eppley precision PSP pyranometers