Definition - TDR
MoisturePoint uses TDR to provide accurate moisture data, however, before the introduction of Moisture.Point® , TDR was not considered an appropriate technology for gathering this type of data due to it's limitations in certain soil types. In order to understand how ESI have overcome this hurdle, it is useful to consider the technical basis for TDR.
With TDR, a step pulse with a very fast rise time is transmitted into a transmission line. The step pulse travels down the transmission line and reflects off any impedance discontinuities (shorts or open circuits) in the line and then propagates back to the pulse source. The shape of the reflected waveform shows a record of the transmission line that the pulse traveled through. Wherever the reflected pulse has an upward or positive step in it, there is a higher impedance in the transmission line. Wherever the reflected pulse has a downward or negative step, there is a lower impedance in the transmission line. As a result, these pulse characteristics can be used to locate the position and attributes of different parts of the transmission line. Of particular importance to soil moisture applications, is the fact that the return time of the reflection is related to the characteristics of what ever surrounds the transmission line. One of the properties of TDR is that the round trip time of the pulse through a transmission line embedded in soil is directly related to the moisture content of the soil, e.g. the more moisture in the soil, the longer the round trip time. This strength, however, can also be a weakness. In particular, the presence of clay and salinity will attenuate the signals to the point that even a visual interpretation of the waveforms is impossible.
Figure 1 A typical TDR pulse reflection observed in a probe emulator with the following features: 1. probe cable leading into the probe emulator (nominal impedance: 50Ω), 2. impedance 'bump' encountered at the higher impedance of the probe connector; 3. impedance drop to a lower impedance level (approximately half, or 25Ω) inside the probe emulator.
E.S.I. MoisturePoint Technology and TDR
The MoisturePoint MP917 measures the round trip propagation time of a signal across a segment of a probe, and it is this propagation time that is directly related to the moisture content of the soil. The MP-917 uses a transmission line embedded in the soil as a probe, and transmits a step pulse into the probe. By measuring the time it takes for the pulse to travel across a small portion, or segment of the probe, the moisture content of that segment can be determined.
Unlike traditional TDR instruments, MoisturePoint probes use switching diodes and a patented technique to create difference functions. A diode is an electrical device that conducts current in only one direction. By using this property, diodes can be used to introduce short circuits (impedance discontinuities) across a transmission line. MoisturePoint probes use shorting diodes across the rails of a probe to separate the probe into discrete segments. By controlling which shorting diodes are conducting, specific segments of the probe can be examined.
Figure 2 The same probe emulator, with a short circuit introduced into the probe emulator transmission line at 2.
A particular shorting diode in a probe can be either off (not conducting) or on (conducting). For each condition, a TDR pulse will return a different reflected pulse, but they will be different only past the point where the diode is located. The reflected pulse with the diode on (transmission line shorted) will exhibit a downward or negative step at the diode location. The reflected pulse with the diode off will not exhibit this.
If we subtract the "diode off" waveform from the "diode on" waveform, we get what is called a diode difference function. Common features of both waveforms cancel out, and only the differences between the waveforms are seen. This emphasizes the effect in the waveform of shorting the diode. It ignores features that may be present in both of the reflected waveforms, but are not caused by the diode short (e.g. an impedance bump caused at the probe connector). A typical difference function will have the following characteristics: a flat, smooth baseline, a sharply rising edge, and a relatively flat trailing upper plateau. Because of multiple reflections, which are generally only present for the diode off condition, the plateau will often be irregular, and not as flat as the baseline. The most important parts of the difference function are the baseline and rising edge of the step, because they allow the MP-917 to locate the diode's position in the time domain.
Difference functions are used to pinpoint the position of a diode. The MP-917 uses special sampling circuitry to measure and acquire diode difference functions (and raw TDR waveforms as well). During probe scans, examining the rising edge of the difference function allows the MP-917 to accurately determine the position of a diode in the time domain.
This is done by fitting a straight line to the rising edge of the difference function, and extrapolating the line down until it intersects the baseline (see Figure 4). The intercept point on the baseline is the diode's position in time. By doing this for the diodes at each end of a segment, the propagation time of a pulse across the overall segment can be found. The MP-917 uses this propagation time, or time delay, to calculate the moisture content of the soil around the probe.
In field use, the MP-917 performs all of the above steps automatically, and displays only the resulting moisture content for the probe. ViewPoint, a graphical analysis program available as an optional accessory for the MP-917, provides a means for the user to examine in detail the waveforms that are used by the MP-917 to calculate moisture readings. Raw TDR waveforms and diode difference functions can be examined, as well as complete probe scans.
MoisturePoint technology allows the reliable automated measurements of soil moisture content under very difficult conditions: all soil types including clay and bentonite, organic and saline soils, high electrical conductivity, wide moisture range.
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