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Hello colleages With regards to the discussion on measurement of matric potential, has anyone had any experience with Campbell Scientific's 229 thermal diffusivity sensor? They are similar to gypsum block sensors expcept they measure thermal conductivity rather than electrical conductivity and hence the conductivity of the water is not an inherent problem. I am in the process of analyzing my initial field data and making sense of my calibration curves (which relate sensor mV output to matric suction). I have calibration points at .1, 20, 100, 200, and 400 kPa. What I have found so far is that the sensor predicts suction very well above 10 kPa using my calibration curve (a log-linear relationship between sensor output and suction using the 20-400 kPa calibration points) when compared to suctions obtained with tensiometers which have been paired up with the 229 sensors. But below 10 kPa, the calibration data is useless and the suctions obtained with the sensor do not have a good correlation with the tensiometer data. I have concluded that the air entry pressure of the 229 sensor must be around 10 kPa and hence suctions below 10 kPa cannot be properly measured with this sensor without more detailed calibration in this range. The manual which is extremely limited agrees roughly with this last statement. For many this lack of accuracy in the very low suction range may be acceptable, however, several sensors rarely get above 20 kPa making the data unreliable or useless. I also have CS615 (FDR?) sensors installed adjacent to the matric suction sensors/tensiometers which have not been working very well. I would be interested in corresponding with anyone who has attempted to calibrate CS615 sensors in the lab. I had a very little luck in getting any sort of agreement between sensors under lab conditions and less luck in the field (unless vol. water contents of 190% sound realistic). I don't think this is what the manual refers to as small errors associated with the default calibration equation :) Any thoughts on this? David Thomas -- David Thomas Geology and Geophysics, University of Calgary ph (403) 220-6596 fax (403) 284-0074