Electrical-impedance measurements serve as sensitive indications of moisture content.

An apparatus has been developed for measuring the low concentrations of liquid water and ice in relatively dry soil samples. Designed as a prototype of instruments for measuring the liquid-water and ice contents of Lunar and Martian soils, the apparatus could also be applied similarly to terrestrial desert soils and sands. The high sensitivity of this apparatus is best appreciated via a comparison: Whereas soil moisture contents of agricultural interest range between 3 and 30 weight percent, this apparatus is capable of measuring moisture contents from 0.01 to 10 weight percent (at room temperature). Moreover, it has been estimated that optimization of the design of the apparatus could enable measurement of moisture contents as low as 1 part per million by weight.

Figure 1. A Sample Chamber Containing a Four-Electrode Probe is mounted on a printed-circuit board that is plugged into a commercial impedance spectrometer: (a) top view of the soil moisture cup showing the four probes that are spaced 11.18 mm apart; (b) side view of soil moisture chamber inserted into printed wiring board that inserts into the LCR meter, and (c) LCR meter with soil-measuring cup.
The apparatus is a special-purpose impedance spectrometer: Its design is based on the fact that the electrical behavior of a typical soil sample is well approximated by a network of resistors and capacitors in which resistances decrease and capacitances increase (and, hence, the magnitude of impedance decreases) with increasing water content. The apparatus includes a commercial impedance spectrometer and a custom sample chamber. Four stainless-steel screws at the bottom of the jar are used as electrodes of a four-point impedance probe. The leads from the electrodes are routed to a 10-pin connector that is plugged into a printed-circuit board that, in turn, is plugged into the impedance spectrometer (see Figure 1). Special precautions were taken in constructing the printed-circuit board to shield the signal conductors to enable measurement of impedances as high as 3 GΩ, thereby enabling measurement of very low levels of moisture. The lower limit of impedance measurable by this apparatus is 100 Ω.

Figure 2. Three Regions measured by the impedance spectrometer that are explained by the soil moisture model. Measurements were obtained from fine silica sand and two samples of coarse silica sand with a diameter “d”. The soil water was doped with 100 mM KCl and measured at a frequency of 100 Hz. (Note: FSSUCR is fine silica sand from the University of California, Riverside; CSSMAL is coarse silica sand from Mallinckrodt Chemicals; and CSSUCR is coarse silica sand from the University of California, Riverside.)
For a typical measurement run, a sample of soil is placed in the jar and the magnitude and phase angle of impedance are measured at fixed frequencies of 100 Hz, 120 Hz, 1 kHz, 10 kHz, and 100 kHz, using applied AC potentials of 50 mV, 250 mV, and 1 V. The measurement data can then be plotted and analyzed to estimate water content, as illustrated by the example of Figure 2.

This work was done by Martin Buehler of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-41822, volume and number of this NASA Tech Briefs issue, and the page number.

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