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Rick Allen said: >Has anyone else observed a temperature bias in the NP? I wouldn't go to >the bank on our findings here at USU, but they do suggest some caution. >This effect would impact the use of Trevor's VSW = >(Count/WaterDrumCount) * Slope + Intercept approach somewhat, as the >user must be careful to not bring a "hot" or "cold" gauge into the lab >for the "water drum count". >Rick Allen >Utah State University Rick and all, I've put a PDF document on neutron probe calibration at http://www.cprl.ars.usda.gov/programs that describes what I measured in 1985 during an extensive field calibration. There was a negative effect of temperature on shield counts for a CPN 503DR gauge. The relationship was significant but not necessarily important. Over a 30 C temperature range the change in water content would be a little less than 0.01 m3/m3 (assuming that the shield count changed but the access tube reading did not because the soil buffered the temperature change). This might be important in our climate because we often experience wide diurnal temperature swings. But, because we don't know what the probe temperature is in the access tube we can't really say what the effect is. This would make a very good master's degree research project for someone. Place a thermocouple or other temperature sensor inside the probe and log temperatures for standard counts over a wide range of temperatures. Then log temperatures over the course of normal operations and see if there would be an appreciable effect, for instance as the probe warmed or cooled while taking multiple readings in an access tube. -------- Steve Evett srevett@ag.gov http://www.cprl.ars.usda.gov/ USDA-ARS, P.O. Drawer 10, 2300 Experiment Station Road Bushland, Texas 79012. Tel:806.356.5775 FAX:806.356.5750 ---------------------------------------------------