PRACTICAL CONSIDERATIONS FOR USING A TDR CABLE TESTER

Time domain reflectometry (TDR) technology can be used to measure soil water content, but due to an incomplete understanding of this technol ogy, some scientists are still reluctant to adopt it. The objectives o f this paper are to discuss the basic principles of TDR to measure soi l water content, to present the equations to convert TDR screen readin gs to soil water content values, and to describe a custom built TDR sy stem. Commercial cable testers operate under the assumption the dielec tric constant of the medium is known. For soils applications the diele ctric constant varies with soil water content and is unknown. The equa tions which govern the TDR technology presented in logical order allow the user to understand the relationships between wave velocity, diele ctric constant, and soil water content. The custom-built TDR system de scribed herr uses a commercially available cable tester; a balun trans former, shielded, two-wire transmission cable; and wave guides constru cted from stainless steel welding rod. Several of these systems have b een successfully used for six years.