In the Laboratory
Digital Capacitance Meters in the Physical Chemistry Lab Roy W. Clark* and Michael J. Smith Department of Chemistry and Physics, Middle Tennessee State University, Murfreesboro, TN 37132
The measurement of the dielectric constant of liquids has been an experiment in our physical chemistry labs for many years. However, the Dipolmeter1 we use is an old tubetype instrument whose tubes will eventually expire. We investigated the use of modern digital capacitance meters to measure the capacitance of our cells. Digital capacitance meters have been developed for the electronics trade and are much less expensive than the older laboratory-quality capacitance instruments. We purchased five digital capacitance meters2 and experimented with them in conjunction with cells containing aluminum variable capacitors. This is a popular type of cell for dielectric constant measurements (1). Our results can be summarized briefly as follows: There are two distinct designs of digital capacitance meters. One is a classic bridge model (B&K) with selfbalancing circuits and digital readout. The other type places the cell in an RC oscillator circuit so that C determines the frequency of oscillation. A counter circuit then determines the frequency and reads out the result as a capacitance. This latter type can be subdivided into two groups, one in which there is no dc component to the voltages applied to the cell *Corresponding author.
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(Kelvin, Metex) and another in which there is a dc component to the cell voltages. We found that only the bridge type gave reliable and consistent readings with a variety of solvents. The second type with zero offset voltage gave reasonable readings with high Q solvents, but with some drift, and those in this second category (Elenco, Protek) with a dc component to the applied voltage gave erratic and unreproducible readings. We therefore recommend that instructors wishing to buy a modern digital capacitance meter for their laboratory consider only the bridge type self-balancing digital meters. Notes 1. Dipolmeter, Model DM01, Kahl Scientific Instrument Corp., El Cajon, CA. 2. B & K Precision 878 Capacitance Bridge, B. & K. Precision, 6470 West Cortland St., Chicago, IL 66035; Elenco CM-1555, Elenco, 150 West Carpenter, Wheeling, IL 66090; Kelvin 400LE, 10 Hub Dr, Melville, NY 11747; Metex M-4650CR, 903 S. Lake St., Suite 102, Burbank, CA 91502; Protek CM-108, Protek, 154 Veterans Dr., Northvale, NJ 07647.
Literature Cited 1. Shoemaker, D. P.; Garland, C. W. Experiments in Physical Chemistry, 2 ed.; McGraw-Hill: New York, 1967.
Journal of Chemical Education • Vol. 75 No. 6 June 1998 • JChemEd.chem.wisc.edu
