Detailed studies of light in forest canopies typically require a large number of light sensors. However, the high cost of commercially available sensors can make such studies very expensive. This report describes the construction and testing of a practical, rugged, and inexpensive sensor for measuring photosynthetic photon flux density. Detailed instructions are provided for assembly and calibration. The sensor was made from a gallium arsenide phosphide (GaAsP) photodiode held within a protective casing of acrylic and aluminum. The LI-190 Quantum Sensor (LI-cor Inc.) was used as a standard for comparison. The stability of the GaAsP sensor compared favourably with the LI-190 during a 24-month open-sky test. Results from nine GaAsP sensors exhibited signal drift of 3-23%, and from four LI-190 quantum sensors of 5-18.5%. The GaAsP sensors generally drifted by <6% over a single season except for one of the Type 1 sensors (#42) that displayed intermittent signal drift. Open-sky daily calibration coefficients of GaAsP Types 2, 3, and 4 showed that about 95% of the data points were within 2-4% of the mean for June to September 1998, and within 3-10% of the mean for January to November 1999. Data variability almost doubled during the wetter months and Type 1 (#41 and #42) sensors were particularly affected. GaAsP (Types 2 and 3) and LI-190 sensors exhibited similar signal drift beneath a canopy as in the open-sky test. Daily calibrations made beneath the canopy were variable over the season, suggesting that care should be taken in making in situ calibrations. Field testing beneath a birch canopy indicated that, compared to a quantum sensor (LI-190), one GaAsP sensor had a spectral response error of <7% in the densest canopy. Recommended sensor care includes minimizing contact with humidity, frequent calibration (pre- and post-season), frequent field checks, and regular maintenance, including diffuser cleaning. Low cost may make these sensors desirable for studies that require a large number of sensors.
Fielder, P., Comeau, P.G.. 2000. Construction and Testing of an Inexpensive PAR Sensor. British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development. Working Paper (FLNRORD). WP53
Topic: FLNRORD Research Program
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