The Trellis Tension Measurement System
Figure 1. Schematic diagram depicting components of the continuous monitoring and yield estimation system in a trellised crop. A single cordon wire is shown for simplicity. Yellow arrows depict forces or loads on the trellis wire that influence wire tension
We insert an electronic device, a load cell, into the trellis wire to measure tension. Many forces affect tension in the wire including vine growth, temperature, and wind. Our goal is to develop a predictive model and computer program that account for these variables to improve yield estimates.
The load cell is connected to a data logger or computer that records increases in tension during the season. Tension can be used to follow crop development and to make estimates of final yield. The physical concept is straightforward but there are many environmental and biological factors to be considered for successful application.
For example:
• daily temperature cycles
• wind
• management practices
• variation in support among posts and vine trunks
• variation in a biological system from year-to-year
After four seasons of working with our system, we have learned how to account for most of these factors to obtain more accurate yield estimates. 
To improve our ability to estimate yield from wire tension it is useful at any given time to know how much of the mass hanging on the wire is due to fruit or vegetative growth. Before budbreak, a standard weight is hung on the cordon wire. The weight is correlated with the change in tension that we measure.
During the growing season, samples of shoots and fruit are collected and weighed so that we can develop a system that will automatically account for the weight of the shoots upon the trellis wire.
Details of the method can be found in Transactions of the American Society for Agricultural Engineering (2004, volume 47, pages 647-657; or by request ).
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