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Sally Logsdon writes to the sowacs list: ===8<============== I wanted to contribute to the discussion of soil moisture probes such as theta probe, old Tecktronics unit combined with old Campbell equipment, and newer CS615, 616. Concerns were expressed about longer rise times with CS616. Although the Tektronics cable tester has short rise time, after adding old Campbell hardware (transient suppressor, multiplexers, low loss cables, etc.), the rise time increases up to 1 or 2 ns (Logsdon, 2000). The longer rise times associated with all these attachments knocks off the higher frequencies and causes the response to be more sensitive to salinity, clay, organic matter, etc. The same would be true for CS616 and for theta probe (measurement frequency of 100 MHz). The other concern was linearity. The voltage from the theta probe is not linear with water content; however, the voltage can be converted to "square root of apparent dielectric" which is linear with water content. All three systems must be specifically calibrated when encountering high salinity or high charged clay or organic matter. Reference Logsdon, S.D. 2000. SSSAJ 64:54-61. ===8<============== NOTE: To get off this list, send an email to list@sowacs.com with the subject line: unsubscribe_sowacs For full instructions see this page: <http://www.sowacs.com/subscribe/index.html>