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Cliff Capacitive >sensors measure about 10mm from the active surface (may be a rod or a >flat plate depending on design, TDR about 10mm radius from the antenna >(by the length of the antenna), heat dissapation sensors depend on the >water content measuring up to 30mm from the heat source. You might be interested in the work of Ken Henshall of SAC Soils department, Bush Estate, Penicuik, MIDLOTHIAN. I'll quote relevent extracts >from his Departmental Note 46 "An Assessment of the domain reflectrometry method of water content measurement" March 1992. "The effective volume of soil contributing to the test measurement was examined by making test measurements with the probe at various distances >from the boundary between distinct regions of moist and dry sand. The sample was prepared by evenly packing moist sand into one half of the perspex container, using a wooden block to temporarily fill the remaining half. The wooden block was then removed and air dry sand poured into it's place. The TDR probe with 100 mm pins was then inserted into the moist sand, 20 mm from the boundary, keeping the plane of the pins parallel to the boundary and a trace recorded. Measurements were repeated in 5 mm increments across the boundary up to 20 mm into the dry sand. The tests were carried out as quickly as possible and afterwards, the container as inverted, pouring out the dry sand, to check that water had not moved across the boundary." "The results of the sample volume tests imply that the penetration of the electromagnetic field into the soil is only 15 mm showing that only a small volume of the soil surrounding the centre pin is examined by the test. Although this might be considered a beneficial feature in terms of spatial resolution, in practice, the concentration of the sample volume around the central probe pin, may lead to inaccuracies in the measurement. Such a small volume may not be representative of the intended sample, particularly as the soil close to the pin may be disturbed by the insertion of the pin." Hope you find that useful Best regards Martin Parkes WasteTrim at Technology Transfer Centre, Alrick Building, Mayfield Road, Edinburgh EH9 3JL, SCOTLAND tel (44) 131 472 4708 fax (44) 131 662 4678