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Sensors 2017, 17(1), 77; doi:10.3390/s17010077

A 3D Faraday Shield for Interdigitated Dielectrometry Sensors and Its Effect on Capacitance

1
School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6012, New Zealand
2
Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5010, New Zealand
3
School of Engineering and Computer Sciences, Victoria University of Wellington, Wellington 6012, New Zealand
*
Author to whom correspondence should be addressed.
Academic Editor: Vittorio M. N. Passaro
Received: 31 October 2016 / Revised: 25 November 2016 / Accepted: 27 December 2016 / Published: 31 December 2016
(This article belongs to the Section Physical Sensors)
View Full-Text   |   Download PDF [5858 KB, uploaded 10 January 2017]   |  

Abstract

Interdigitated dielectrometry sensors (IDS) are capacitive sensors investigated to precisely measure the relative permittivity ( ϵ r ) of insulating liquids. Such liquids used in the power industry exhibit a change in ϵ r as they degrade. The IDS ability to measure ϵ r in-situ can potentially reduce maintenance, increase grid stability and improve safety. Noise from external electric field sources is a prominent issue with IDS. This paper investigates the novelty of applying a Faraday cage onto an IDS as a 3D shield to reduce this noise. This alters the spatially distributed electric field of an IDS affecting its sensing properties. Therefore, dependency of the sensor’s signal with the distance to a shield above the IDS electrodes has been investigated experimentally and theoretically via a Green’s function calculation and FEM. A criteria of the shield’s distance s = s 0 has been defined as the distance which gives a capacitance for the IDS equal to 1 e 2 = 86.5 % of its unshielded value. Theoretical calculations using a simplified geometry gave a constant value for s 0 / λ = 1.65, where λ is the IDS wavelength. In the experiment, values for s 0 were found to be lower than predicted as from theory and the ratio s 0 / λ variable. This was analyzed in detail and it was found to be resulting from the specific spatial structure of the IDS. A subsequent measurement of a common insulating liquid with a nearby noise source demonstrates a considerable reduction in the standard deviation of the relative permittivity from σ unshielded = ± 9.5% to σ shielded = ± 0.6%. The presented findings enhance our understanding of IDS in respect to the influence of a Faraday shield on the capacitance, parasitic capacitances of the IDS and external noise impact on the measurement of ϵ r . View Full-Text
Keywords: shielding; guard; noise reduction; electric field distribution; Green’s function; FEM; interdigitated dielectrometry sensor; co-planar electrodes; capacitance measurement shielding; guard; noise reduction; electric field distribution; Green’s function; FEM; interdigitated dielectrometry sensor; co-planar electrodes; capacitance measurement
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Risos, A.; Long, N.; Hunze, A.; Gouws, G. A 3D Faraday Shield for Interdigitated Dielectrometry Sensors and Its Effect on Capacitance. Sensors 2017, 17, 77.

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