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United States Department of Agriculture

Agricultural Research Service

Title: Coaxial-probe contact-force monitoring for dielectric properties measurements

item Nelson, Stuart

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2011
Publication Date: 3/8/2012
Citation: Nelson, S.O. 2012. Coaxial-probe contact-force monitoring for dielectric properties measurements. Transactions of the ASABE. 28(1):149-152.

Interpretive Summary: Dielectric properties of materials are electrical properties that determine how materials interact with electric fields such as those of high-frequency and microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens and lower radio-frequency dielectric heating equipment. Dielectric properties are also important in low power applications, such as their use in the rapid measurement of moisture content in grain and other commodities. Dielectric properties of materials have to be measured for particular applications. One technique which is being used extensively in the food and agricultural industries, because commercial equipment has been marketed for such measurements, is the use of an open-ended coaxial-line probe. This probe contacts the sample material for which dielectric properties are being measured and is connected to a network analyzer that takes the measurements from the reflected waves and calculates the dielectric properties of the material being measured. Experience has shown that the values of such measured properties vary depending on the force applied to hold the probe in contact with semisolid materials such as fruit and vegetables and meat product tissues. For such measurements, the variation can be limited by using a constant force to provide uniform measurements for comparison. This paper describes a contact-force monitoring system developed for use with these kinds of measurements. A commercial load cell and strain meter were adapted to the particular measurement system. An alignment sleeve was designed and built to permit the force on the sample exerted by the probe to be properly transferred to the load cell so that the magnitude of that force can be displayed on the strain meter. The alignment sleeve for a stainless-steel sample cup and load cell is supported on a platform, which permits the force acting between the sample and the probe to be easily adjusted and monitored to maintain a constant force for the measurements. The force-monitoring system will reduce variation among repeated measurements of dielectric properties of products and permit improvements in research and development projects involving the measurement of radio-frequency and microwave dielectric properties of materials.

Technical Abstract: A means is described for measuring and monitoring the contact force applied to a material sample with an open-ended coaxial-line probe for purposes of measuring the dielectric properties of semisolid material samples such as fruit, vegetable and animal tissues. The equipment consists of a stainless steel sample cup, load cell and associated strain meter and an aluminum alignment sleeve to insure that the force acting on the sample is properly transferred to the load cell. The assembly (sample, sample cup, load cell and spacer, in the alignment sleeve) rests on a platform with position adjustable vertically to raise the assembly and provide contact of the sample with a rigidly mounted coaxial probe above the sample holder. The strain meter to which the load cell is connected displays the force between the probe and the sample to facilitate monitoring this contact force. Use of a consistent contact force helps to reduce the variation in results of dielectric properties measurements by this technique.

Last Modified: 06/21/2017
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