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In <199904111908.TAA41308@aqua.ccwr.ac.za>, on 04/11/99 at 07:08 PM, owner-sowacs@aqua.ccwr.ac.za said: >With regard to the measured radius of capacitance sensors, the EnviroSCAN >sensor must be differentiated from most others. Concerning the >EnviroSCAN, Paltineau and Starr, 1997: Real Time Soil Water Dynamics >Using Capacitance Probes, Soil Sci. Aoc. Am. J., Vol 61, report that "99% >of the [EnviroSCAN] sensor's response was obtained within a radial >distance of 10 cm from the wall of the PVC access pipe. The remainder of >the sensor readings extended to a soil thickness of 18 cm from the access >pipe wall." This greater radial distance, and consequently greater >measured volume, is one of the characteristics that differentiate the >EnviroSCAN from other capacitance probes. The 99% value in the paper by Paltineau and Starr is misleading. One could also say that 99% of the response was found within a radial distance of 10 meters. That also would be true, but irrelevant. A more useful idea of the sensor's response is obtained by examining their figure showing response versus radial distance. As stated, the 99% response is shown to occur at 10 cm. But, 95% response occurs at 4 cm, 92% response occurs at 3 cm, and 82% to 84% response occurs at 2.5 cm. So, most (82%) of the response of this sensor is to the moisture content of the soil within a 2.5 cm radius outside the access tube. The figure does not give response values for radii smaller than 2.5 cm, but it does show that the proportion of full response drops off quickly at smaller radii. The sensor uses a PVC tube that is about 5 cm in diameter. Using a radius of 2.5 cm for non-soil volume inside the tube, a radius of 5 cm for the 82% response, and a radius of 12.5 cm for the 99% response, I find that the soil volume sampled at 82% response is about 12% of that sampled at 99% response (assuming a reponse volume shape that is approximately a right circular cylinder). The extra 88% of the total volume sampled at 99% response accounts for only 17% of the response. To be sure, there is a real lack of good scientific literature on actual response vs. volume for the many different kinds of probes. There are some nice theoretical attempts, but not that much in the way of actual measurements. -------- 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 ---------------------------------------------------