Extension of the measurement range of electrical conductivity by time-domain reflectometry (TDR)

摘要

The electrical conductivity (EC) of a medium invadedby TDR sensors can be estimated from the impedance of a TDR reflectogram.Four categories of sensor were tested in salt solutions and the impedancesof the TDR pulse wave were correlated to the EC of the solution. The relationbetween the impedance and EC over a wide range of conductivities is non-linearbut stable. Second- to fourth-degree polynomials can extend the measurementrange to44 dS m^-1 (equivalent to a NaCl concentrationof 28 g l^-1 or 0.48 N) and result in betterprediction of the conductivities than linear relations. For automatic measurementof EC with a datalogger, the method of Giese and Tiemann (1975, Adv. Mol. Rel. Processes, 7: 45-59) gives accurate measurement of conductivitieslower than 10 dS m^-1. Polynomial relationsbetween EC and the datalogger's record provide an accurate estimate ofthe conductivity over a wide range. However, for both manual and automaticmeasurements, the sensors need to be calibrated individually. In particular,in the non-linear region, the differences between sensors are larger. Fortunately,the relation is sufficiently stable to eliminate significant error.