Anderson, R. Y., G. S. Campbell, and J. Greene. 1994. 
A Transmission Delay Time Probe for Measurement of Water Content in Porous Substances such as Soil or Grain. 
Proceedings of the Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, Illinois, Sept 7-9, U.S. Bureau of Mines, Special Publication SP 19-94, NTIS PB95-105789, p. 66
(posted July 12, 1996)

The influence of the dielectric constant of a medium on the propagation velocity of a pulse traveling through that medium allows Time Domain Reflectometry (TDR) to be a useful tool for measuring dielectrics. The high dielectric of water relative to other naturally occurring substances makes TDR useful for measuring water content in soils and other porous media. The main disadvantage of TDR is the complexity of the circuitry required for the measurement, and therefore the expense of the equipment. Circuitry that measures the time required for the transmission of a pulse over a fixed length waveguide is much simpler. There is no need to detect a reflected pulse. We call the device a Transmission Delay Time (TDT) probe. Testing of TDT probes has shown that the response to water content in soil and grain is essentially the same as that measured with TDR.