CALIBRATION AND QUALITY CONTROL OF THE COSHOCTON WEIGHING LYSIMETERS
Robert W. Malone, David Stewardson, J. V. Bonta, Terry A. Nelsen · Transactions of the ASAE · 1999
Many weighing lysimeters, including the Coshocton lysimeters, use load cells to determine evapotranspiration(ET). Because measured ET is sometimes small (less than 0.5 mm for hourly ET), these instruments must be very accuratemass measurement devices. To achieve acceptable accuracy, weighing lysimeters require calibration. Electronic changeswere found to affect the performance of the Coshocton weighing lysimeters, therefore, a quality control plan was needed.The objectives of this article are: to present a weighing lysimeter calibration method; to provide a statistically validquality control plan (using control charts); and to describe the performance of the Coshocton lysimeters operating underthis calibration and quality control plan. Control charts were used to monitor the performance of the weighing mechanismbecause they can: be relatively easy to use; indicate when re-calibration is necessary; identify measurement drift; indicateinstrument problems; indicate human mistakes; provide confidence in measurement results; and be adapted to mostmeasurement processes. A calibration procedure was developed that: could be performed by one person; could beperformed without covering the lysimeter; and could be used in conjunction with the quality control (QC) procedure. Theresults show that the Coshocton lysimeters, operating under the described quality control and calibration procedure, havean error less than 0.025 mm (water equivalent mass) for a single mass measurement. The resulting error in lysimetercomputeddaily ET was about 0.03 mm indicating that the lysimeters are capable of measuring low ET (e.g., hourly ET).The mass measurement process was generally statistically in control which provides confidence in the lysimetercomputedET. Under normal conditions, two of the three weighing lysimeters were found to be significantly affected bywind, likely due to the steep topography (13-26% slopes). However even under somewhat windy conditions (up to4.5 m/s), the lysimeters were very stable and any wind induced error can be considered negligible for most applications.