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Time Domain Reflectometry Laboratory Calibration in Travel Time, Bulk Electrical Conductivity, and Effective Frequency

Soil and Water Management Research Unit, Conservation & Production Research Laboratory, USDA-ARS, Bushland, TX 79012

View larger version (23K): [in a new window]   Fig. 1. Plot of a waveform and its first derivative from a Tektronix 1502C TDR cable tester set to begin at –0.5 m (inside the cable tester). The voltage step is shown to be injected just before the zero point (BNC connector on instrument front panel). At 3 m from the instrument, a TDR probe is connected to the cable. The relative voltage levels, VI, Vmin, Vo, and VF are used in calculations of the bulk electrical conductivity of the medium in which the probe is inserted and in determination of the probe characteristic impedance. Waveform positions for determining values of these parameters are described numerically in Evett (2000a)(2000c), where Vo here is denoted Vo2.

View larger version (19K): [in a new window]   Fig. 2. Calibration equations for conventional time domain reflectometry in terms of column mean water content vs. column mean travel time for three soils (A, B, and C), disregarding effects of temperature and coaxial cable length. Inset shows horizontal jitter for Soils A and B.

View larger version (9K): [in a new window]   Fig. 3. Water content from TDR measurements at 15-cm depth in all columns vs. soil temperature at the same depth for Soils A, B, and C. Water content data for the second and third column of each soil were adjusted to match the mean of the water content for the first column so that the data would overlap despite small differences in column water contents, allowing any temperature dependency to be apparent.

View larger version (7K): [in a new window]   Fig. 4. Column mean water content from TDR measurements in all columns vs. column mean soil temperature for Soils A, B, and C. Water content data for the second and third column of each soil were adjusted to match the mean of the water content for the first column so the data would overlap despite small differences in column water contents.

View larger version (11K): [in a new window]   Fig. 5. Temperature dependency (regression slope) vs. total coaxial cable length for Soils A and B. Regression slope is for column mean water content vs. column mean temperature.

View larger version (14K): [in a new window]   Fig. 6. Relationship between probe characteristic impedance and cable length for 50- type RG58 coaxial cable (Alpha Wire Co., part no. 9058C).

View larger version (17K): [in a new window]   Fig. 7. Relationship between bulk electrical conductivity, a, and soil temperature for the saturated A, B, and C soils.

View larger version (16K): [in a new window]   Fig. 8. Effective frequency calculated using Eq. [9] vs. cable length for saturated and air-dry soils.