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PC-Based Instrumentation System for the Detection of Moisture Content of Tea Leaves at Its Final Stage
Abstract
This paper describes a PC-based instrumentationsystem designed for online monitoring of the moisture content(MC) of tea leaves at its final stage. A fringe-field capacitive probeis used as a sensor to detect theMC of a tea leaf sample. The probeis mounted on the shaft of a stepper motor. The stepper motor isused to shake the tea leaf samples to obtain similar compactnessfor all tea leaf samples whose MCs are to be monitored. Theintroduction of shaking facility in the measurement procedureensures consistency of measurement. The three-dimensional finiteelement method is used for the determination of field distributionaround the surface of the sensor and the capacitance of the sensor.
Index Terms—Capacitive sensor, finite element method (FEM)analysis, fringe-field capacitor, moisture measurement, tea leaves.
I. INTRODUCTION
TEA PROCESSING as a whole consists of dryingprocesses and a number of mechanical operations combinedwith or alternated by chemical and enzymatic reactions.The fundamental process in making tea leaves, which is incorrectlycalled fermentation, consists of a series of oxidations andcondensations of certain substances in the withered leaf initiatedby the rolling process. It may be said that a green tea leafis processed qualitatively into the finished product, since duringprocessing, no substances apart from oxygen are added and,not considering the small unavoidable losses by respiration, nosubstances other than water are extracted. The characteristicsof a beverage made from the finished product are determinedby the major components of the leaf, i.e., the polyphenols, thepectic substances, the flavoring constituents, and caffeine: thelatter component particularly for its stimulating effect.From these introductory remarks, it might seem that tealeaves manufacturing is a simple process, but it is emphasizedthat numerous factors affect the final product. From a practicalpoint of view, the chain of operation in tea leaves processingcan be distinguished in
1) withering;
2) rolling;
3) fermentation;
4) drying;
5) sorting and grading.
In general, in tea leaf processing, the withered leaf is rolledfor 2- to 6 half-hour periods and shifted after each rolling. Thefine material and the residual coarse material are subjected tofermentation. The total fermentation time for a roll or batch ofleaf may be 3 to 4 h, after which, it is dried to a keepable productwith the right properties.The percentage of moisture content (MC) of tea leaves at itsfinal stage, i.e., drying phase of processing, plays an importantrole in the following factors:
1) quality of the product;
2) durability of the product (i.e., maintaining its quality fora longer time).It is observed that too much moisture in tea leaves causesdestruction of the product due to action of virus/fungus. Unfortunately,there is yet no quick and accurate device available forthe detection ofMC of tea leaves at its final stage [1]. Therefore,in the factories of North East Region of India (highest producerof quality tea leaves in the world), operators try to produce tealeaves with 3% to 5% MC at the final stage of the product. Themethods used for this purpose are
1) weight method;
2) human taster.In the following, these traditional methods have numerousdisadvantages.
1) The weight method requires weighing samples of samevolume. Tea being a granular substance, the weight ofeach sample depends on the compactness of the sample.Thus, samples with different compactness give differentweight.
2) The human-tasting method is not reliable as it depends onthe experience and efficiency of the taster.These two methods have a huge time lag between the productionspeed and monitoring speed. Therefore, these methods arenot suitable for online monitoring/control of MC of tea leavesat its final stage.Electrical properties of materials such as resistance anddielectric properties are used as a basis for the measurementof MC in different materials. Based on resistance propertiesof wood, probes were developed by Tsongas and Nelson tomeasure the MC in walls [2]. The relationship between thedependence of the electrical resistance with the MC in woodis provided by Caril and TenWolde [3]. Situ sensors based onthe capacitance of a material were developed by researchers todetect MC of different materials. Couvillion et al. [4] madea probe with two parallel rows of 12 pins each that could beinserted into a material to measure its MC.