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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs I conducted an experiment seven years ago to measure volumetric water content for potting soil from 0 - 0.50 m3/m3 moisture range. I found that TDR can be used to accurately measure volumetric water content in potting soil. I prefer to express the time delay as T/Tair, the ratio of propagating time in medium to that in air with the same travel distance science the relationship between T/Tair vs. volumetric water content is linear. The experimental results show: 1. Volumetric water content = (T/Tair - Ts/Tair)/[sqrt(dielectric constant of water)-1] 2. The slope of the regression line (T/Tair vs. volumetric water content) for potting soil is almost the same as that for fine sand (0.1256 vs 0.1263 with R2 = 0.995). 3. The X intercept (Ts/Tair, the ratio of propagating time in dry soil to that in air with the same travel distance) needs to be adjusted. For loamy soil Ts/Tair value is approximately 1.55 and for potting soil it should be adjusted to approximately 1.14. This adjustment is required because the dielectric constant of dry loamy soil is different to that of dry potting soil, not related to so-called "isolated water". 4. Part of the results has been published in Soil Science of American Journal volume 60, 1996. More details can be found in my Ph.D thesis. Z. J. Sun Ph.D Research Scientist ESI Environmental Sensors Inc. 100-4243 Glanford Avenue Victoria, BC, Canada V8Z 4B9 Tel: 250-479-6588 Fax: 250-479-1412 e-mail: jason@esica.com