Feasibility Study of a Capacitance Method for Measuring Concentrations of Suspended Sediment in Water

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Many determinations have been made of the dielectric constant of water, and of its variations caused by temperature and frequency. The dielectric constant of water has a relatively high value as compared with those of many other substances with which water is commonly associated. Consequently, measurements of the dielectric constant (or of the electrical capacitance) of materials such as soil, cereal and grain have been used as indications of their moisture contents.

This study was undertaken to determine whether or not a capacitance method can be used to measure sediment concentrations in water. Laboratory measurements were made of carrying concentrations of several individual artificial sediments, natural sediments, and pure salts in distilled water. A flow-through-type conductivity cell was used in which the capacitor plates are fixed in relation to each other. The equation for capacitance of the flow through cell can be written as C = kD where C= capacitance in farads, k = a constant determined by the physical configuration of the cell, D = dielectric constant, and it is seen that C varies directly as D.

Typical values of salt and sediment concentrations existing in natural streams in the United States were determined from the literature.

Conclusions of the study are summarized as follows:

  1. The capacitance method of measuring concentrations of sediment in natural streams is not practical for the following reasons.

a) The presence of even small quantities of conducting and/or magnetic sediments in the measured samples invalidates the measurements.

b) Concentrations of sediments in most streams in the United States are too low during most of the year to be sensed by the capacitance method.

c) Concentrations of dissolved solids in many streams in the United States are high enough during at least part of the year to render capacitance measurements inaccurate.

d) The surface chemistry of clay-size particles prevents their being reliably measured with the capacitance method.

2. The capacitance method of measurement may be applicable for particular or specialized uses, such as the determination in the laboratory of concentrations of mixtures of known nonconducting and nonmagnetic particulate matter. Such measurements are not noticeably affected by the salts used in this study until their concentrations exceed about 10 meq/1.

It is recommended that further efforts to adapt the capacitance method to the measurement of stream sediments to discontinued.

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