The C's and D's of dielectric measurements
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2003 2:00:00 AM
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The dielectric properties of materials ranging from electrical insulation to copier toner can indicate a manufacturing process that needs adjustment. While accurate dielectric-constant and dissipation-factor measurements require adherence to standards, you can assess your manufacturing process with some simple measurements that are accurate to approximately ±10%.
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Figure 1. Measure a material's capacitance and resistance (left) compared to that of air (right) to calculate dielectric constanct (k') and dissipation factor (Dx). Make all measurements at the same distance between electrodes. |
You can calculate a material's dielectric constant—its insulating ability—by measuring its capacitance relative to that of a vacuum (1.00). The closer a dielectric constant is to 1, the more the material insulates. Unfortunately, measurements in a vacuum are expensive and cumbersome to make; instead of going to that trouble, you can often substitute the dielectric constant of air (1.0053). To calculate a material's dielectric constant (k'), measure its capacitance (Cx ), then measure the capacitance of air (Cair ), and take the ratio:
Figure 1a shows a specimen between electrodes of an LCR meter or other capacitance-measuring instrument. Figure 1b shows the same setup for measuring air. To make the capacitance measurements, use the same distance, h, between the electrodes for both measurements. To get a repeatable distance between the probes with and without the specimen, you need to build or buy a test fixture that lets you move the electrodes while measuring the distance between them with a micrometer.
In addition to measuring dielectric constant, you can measure dissipation factor (Dx), a material's ability to store energy at a specific frequency. Dx is the ratio of an insulating material's resistance to its capacitive reactance at a specific frequency. You can use the dielectric-constant-measurements to calculate dissipation factor:
Dx = Dxm - Dair
To calculate Dx, measure the dissipation factor of the specimen at electrode distance hm. D xm is the measured dissipation factor with the specimen in the fixture, and D airis the measured dissipation factor of air over distance hm.
To learn more about how to measure dielectric constants, download a paper by Jim Richards of QuadTech.
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You have the dielectric constant wrong for air. It is 1.00059. You left out a zero. See //en.wikipedia.org/wiki/Dielectric_constant
Sherman Barney - 2010-18-1 12:53:35 EST
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