Traceable Measurements of the Static Permittivity of Dielectric Reference Liquids at Temperatures from 5 to 50 deg C using a Micrometer Driven Admittance Cell

摘要

Traceability of RF and Microwave dielectric measurements is much facilitated by the use of reference dielectrics and NPL has been pursuing a programme of measurements to make more of these materials available [1]. Both solid and liquid reference materials have their value, but the former suffer from variability of electrical properties from one manufactured batch to another. Reference liquids have the advantage that samples of a high-purity grade taken straight from a newly-opened bottle are very consistent and so their properties can be published for use in the calibration and checking of instruments for complex permittivity measurements. To suit this purpose liquids having a range of complex permittivities are required. Both non-polar and polar reference liquids are useful, as they have quite different properties. Non-polar liquids (e.g. cyclohexane and silicone oil) generally have low permittivity, low loss, low temperature coefficient and low frequency dependence of permittivity, whereas polar liquids (e.g. alcohols and water) have none of these characteristics. Polar liquids with short-chain molecules (e.g. water, methanol and ethanol) are of value for calibrating and checking high loss, aqueous and particularly biological tissue measurements. Their dielectric behaviour throughout much of the RF and microwave range is conveniently described by dielectric relaxation models, such as the single and multiple Debye models [2, 3]. They allow the complex permittivity to be calculated easily from the relaxation parameters at a specified frequency and temperature. One of the most important of these parameters is the 'static' (i.e. low frequency) permittivity [2, 3]. A limited programme of RF and Microwave measurements on reference liquids was carried out at NPL during the years 1990-1992 [2]. This covered measurements of static permittivity in a micrometer-driven admittance cell and measurements of high-frequency permittivity (up to 3 GHz) in coaxial transmission line cells. The earlier project was limited, however, to a temperature range of 20 - 30 deg C, which does not encompass the full industrial range of interest, and the number of measurements undertaken was limited by the slowness of the temperature control in the admittance cell. Full experimental details of this earlier work have not previously been published and some of the details are given here for the first time. The current programme extends the earlier work in three ways: (i) the temperature control of the liquid admittance cell has been improved, allowing for faster, more reliable control, (ii) a wider range of temperatures is possible: 5 - 50 deg C, (iii) a wider range of liquids is being measured. The static permittivity measurements reported here will be followed by RF & Microwave measurements next year. When the work has been completed, results will be published in NPL reports.