Reports

This report describes our homemade Rotating Shadow Band (RSB) diffuse PAR sensor.  The sensors uses a Kipp & Zonen PAR sensor, a 0.5rpm DC motor and some magnetic proximity sensors. 

The main parts are:

motor - https://www.servocity.com/html/0_5_rpm_gear_motor.html
hub - 545576 https://www.servocity.com/html/0_770__set_screw_hubs.html#.VWYlAGdFCHt
box - http://www.digikey.com/product-detail/en/NBF-32002/377-1761-ND/2328533
proximity sensor - http://www.digikey.com/product-detail/en/59140-010/59140-010-ND/43977
proximity magnet - http://www.digikey.com/product-detail/en/57140-000/57140-000-ND/43978
Aluminum - 1" wide x 1/16" thick x 24" long

The concept is to measure PAR quickly (2-10Hz) while a shadow band rotates slowly (0.5rpm) around the PAR sensor periodically casting a shadow on the PAR sensor.  The magnetic proximity sensors detect when the shadow band is horizontal to delimit the time periods where one shadow passing has occurred and the minimum during that period will be the diffuse PAR.

Advantages:

• Single high quality PAR sensor can be used and easily swapped
• Proximity sensor automatically corrects for any change in motor speed due to varying power (solar) or wear.
• Can be used to measure both diffuse PAR (minimum during 1/2 revolution) and total PAR (when RSB is horizontal)
• No tricky alignment with the sun - align it generally towards the equator and the rotation solves the problem of time of day or season.

Disadvantages:

• Might need a dedicated data logger to record/process high frequency measurements
• Blocks different amounts of sky (4%-13%) depending on sun angle.
• Moving parts eventually wear out
• Relatively slow sample rate of about once per minute

Diagram 1. Front View of the RSB diffuse PAR sensor.  This diagrams shows the K&Z PAR sensor and the shadow band at a 45deg angle in black.  A dotted blue rectangle shows the position of the shadow band when vertical.  The black arc and light blue wedges show a projection of the shadow band to illustrate that different amounts of sky are blocked when the shadow band is in different positions. This ranges from about 4% at dawn and dusk to about 13% at midday.

Figure 1. A portion of the raw 10Hz data from the RSB during midday on a partially cloudy day.  Each dip the the green line is when the RSB casted a shadow over the PAR sensor and the blue line shows the minima of these dips - the diffuse PAR.

Figure 2. A zoomed in portion of the data in Figure 1 displaying the individual data points.  Here you can see that there are 10-12 points during a shadow's passing and that the sampling rate could be reduced to about 2Hz as it is in later systems and still catch the minima.

Figure 3. After the minima of each shadow passings have been found, these minima are averaged into 30min averages to be saved with the other 30min micromet data.