Development of digital moisture meter for jute fibre and its products

Transcription

1 Indian Journal of Fibre & Textile Research Vol. 36, June 2011, pp Development of digital moisture meter for jute fibre and its products Gautam Roy a & S C Saha National Institute of Research on Jute and Allied Fibre Technology, 12 Regent Park, Kolkata , India Received 4 February 2010; revised received and accepted 22 July 2010 A new type of digital instrument has been developed using the resistance measurement method with a new concept of built-in lookup table form, which has been proved as more accurate than other available moisture meters. The operations are controlled by a microprocessor unit and provide the result on a digital display. The test is non-destructive and process of sample preparation is free from human errors. The same instrument can also be used for other fibres simply by changing the lookup table for that particular fibre. Keywords: Digital moisture meter, Jute fibre, Microprocessor, Moisture content, Moisture regain 1 Introduction Like most of the other natural fibres, jute fibre absorbs moisture due to the presence of hydrophilic molecules and a large amount of exposed surface in the fibre 1. Since the humidity condition varies so widely, the moisture content value does not remain constant. The quantity of moisture normally retained by jute fibre and its products under equilibrium conditions depends mainly on relative humidity and temperature of the surrounding atmosphere. When jute fibre is kept in an atmosphere of definite humidity, it gradually attains a constant moisture content. The time for attaining equilibrium depends on various factors like the type of packing of the material, air circulation, moisture regain, humidity of the atmosphere, past history of the fibre, quality of the fibre and temperature. It is well known that adsorption of moisture by fibre or its products is accompanied by changes in physical and physico-chemical properties, such as evolution of heat, swelling, and change in strength and extensibility due to variation in crystallinity 2. The moisture content in the fibre plays an important role in mechanical processing and commercial applications. To measure moisture content properly and accurately, it is very much needed to use a very sensitive and accurate instrument, which can show the result digitally and can retain the result until and unless it is reset for next experiment. Moreover, the a To whom all the correspondence should be addressed. gautamroy1234@yahoo.com method should be a non-destructive and with quick response. This kind of technique is not available in the literature and in the market so far. Therefore, in the present study, a moisture measurement meter has been developed to measure the moisture content and moisture regain in the jute and jute products at lower cost with higher accuracy. 2 Materials and Methods Statistically, it has been observed that jute in particular absorbs moisture by weight at 65 ± 5 & RH and the moisture content increases with the increase in RH, following a hysteresis parameter with the change in RH. The two parameters, namely moisture content and moisture regain provide the correct estimation of jute products with respect to the moisture. The values of these parameters were obtained using the following relationship: R = 100[W/D] or M/[1-(M/100)] M = 100[W/(D+W)] or R/[1+(R/100)] where D is the oven dry weight; W, the weight of water content; R, the moisture regain; and M, the moisture content. Using a CSIRO make direct moisture regain tester (HB43- Halogen moisture analyser), the moisture regain () and content () were computed using the following equations:

2 ROY & SAHA: DEVELOPMENT OF DIGITAL MOISTURE METER 179 Regain () = 100 (Weight of moisture present/ weight of oven dried sample) = 100 (W 1 -W 2 )/ W 2 Moisture content () = 100 [(W 1 -W 2 )/ W 1 ], where W 1 is the actual weight of the sample; and W 2, the oven dried weight of sample. The absolute humidity is expressed as the weight of water present in a unit volume of moist air, i.e. gram per cubic foot or gram per cubic meter. The relative humidity is expressed as the ratio of the actual vapour pressure to the saturated vapour pressure at the same temperature as given below: Actual vapour pressure Re lative humidity () = 100 Saturated vapour pressure At the time of testing using dry and weight measurement method, the humidity was maintained as 65 ± 2 at a room temperature of 20 ± 2 C to get the best result. It has been found that the resistance of fibre changes considerably with the variation in moisture content. So, to measure the moisture content in the jute fibre and its products, we employed resistance measurement method. In conventional resistance measurement system, the resistance changes in non-linear form with the change in moisture content of the fibre 3-6. This creates lots of problem 7. Therefore, a novel technique of resistance moisture lookup table has been used. Accurate sets of data of resistance have been collected with respect to moisture content using the gravimetric method. These sets of data have been stored in the built-in-memory block of the instrument. At the time of measurement using this instrument the leakage current is measured through the test sample to obtain the resistance of the fibre under test. The control unit of this instrument searches the corresponding moisture content from the stored lookup table of the built-in-memory. 2.1 Description of Proposed Instrument Figure 1 shows the sketch of newly developed digital moisture measurement meter. To start the measurement, the power cord is connected to the power supply and the power switch (1) is turned on. A LED indicator (3) indicates the power line connection with the instrument. Depending on the types of sample used, the probe is connected to the sensor port (2). The probe is brought into the close contact with the jute sample under test with certain amount of pressure. The types of probe to be used depends upon the types of sample used, e.g. for jute bale, the spike type of probe (5); for jute fabric, the flat type of probe (6); and for jute yarns, the small round type with small spikes (7) are used (Fig. 2). Applied pressure on the sample by the probe is kept constant by using a clutch type of arrangement present in the probe. In the tip of the probes there are terminals through which current is allowed to flow through the sample under test. To make sure of the proper contact, a clutch type notch is provided along with the pressing arrangement of the probes. When the probe is properly in contact with the sample, a leakage current is passed through the test sample. This leakage current can be expressed as a function of moisture content of the test sample. This small leakage current is amplified and digitized to a suitable range. This value is then compared with the Fig. 1 Schematic diagram of digital moisture meter [1 power switch, 2 sensor port, 3 LED indicator, and 4 to 7 segment digital display] Fig. 2 Sketch of probes [5 spike type probe, 6 flat type probe, and 7 small multi spike probe]

3 180 INDIAN J. FIBRE TEXT. RES., JUNE 2011 built-in lookup table to get the corresponding moisture content. This moisture content is displayed on the 7 segment digital display (4). Figure 3 shows the schematic circuit diagram of the instrument. Here, the moisture sensor senses the moisture content of the test sample and in the next stage moisture to electrical signal converter unit converts the moisture content into its equivalent electrical signal of varying amplitude. This signal is fed to the next unit where it is manipulated and amplified to its suitable form and value. After that, the analog data is fed to the A/D converter unit to convert it to its equivalent digital form. In the final stage, the processed digital data comes to the control and computer interface unit, which, in turn, sends the output to the digital display unit and computer. This unit also controls the inflow of data using sample and hold mechanism. The occurrence of steady state value indicates the moisture content of the jute sample under test. The moisture measurement algorithm is shown in the flow chart (Fig. 4) and the flow of data is shown in Fig Results and Discussion Jute fibre, yarn and fabric have been tested with this newly developed moisture meter and the results are given in Tables 1-3. The results are compared with those obtained from a standard type of HB 43 halogen moisture analyzer (dry and wet measurement system) using the same batch of samples. It is found that average value of moisture determined by the newly developed moisture meter is very close to the value of halogen moisture analyzer. Minimum error is found to be 1.13 and maximum error is Moisture regain value of jute fibre in different relative humidity 8 is given in the Table 4. It is found that the moisture regain values increase with the increase in relative humidity. Figure 6 shows the calibration curve for the experiments using the instrument. From the calibration curve it appears that two values obtained from the developed instrument and halogen moisture analyzer are more or less identical. The instrument has been used for a number of repetitive tests for jute fibres, yarns and fabrics. The consistency in the results has been observed 9. The repeatability was calculated using the following relationship: Fig. 3 Schematic diagram of circuit design of instrument Fig. 4 Flow chart of measurement algorithm r p = 2 2/3 σ, where σ is the standard deviation of the readings from the mean. If the difference between the successive moisture meter readings is less than r p, then the Fig. 5 Flow chart of data for digital moisture measurement meter

4 ROY & SAHA: DEVELOPMENT OF DIGITAL MOISTURE METER Table 1 Moisture content in jute fibre A HB43 halogen moisture analyser Error Table 2 Moisture content in jute fabrics Moisture content by HB43 halogen moisture analyser, Error A B C Table 3 Moisture content in jute yarn Moisture content by HB43 Halogen moisture analyser, Error Table 4 Jute moisture regain with relative humidity Relative humidity, Moisture regain, B C Fig. 6 Calibration curve of drying and weight measurement system using HB 43 halogen moisture analyzer (-- --), and newly developed digital moisture meter (-- --)

5 182 INDIAN J. FIBRE TEXT. RES., JUNE 2011 successive readings are said to be repeatable. All the results given in Tables 1-3 are repeatable. Some of the advantages of the newly developed instruments are: Online indication of moisture content value and number of stored records. Data storage/retrieval on demand. Stored data transfer to computer via RS232C port Accuracy level is within ± 2.5 Resolution of the reading is 0.1. Overall accuracy is ± 1 digit 4 Conclusion It is a unique instrument suitable for the moisture measurement for the jute and jute products. It is not only a low cost and microprocessor based precision device, but also have the advantage of nondestructive, template free operation as well. It can accommodate different types of jute and jute product samples for precise measurement of moisture content. Another unique feature of this instrument is by changing the built-in lookup table, it can also be used for moisture content measurement in non-jute products simply by changing the lookup table of the instrument with a suitable one for the required non jute material. References 1 Jute in India A Monograph (Indian Central Jute Committee, Calcutta), Ray P K, J Appl Polym Sci, 15 (1971) Booth J E, Principles of Textile Testing (Butterworth Scientific), Hearle J W S, J Text Inst, 44 (1953) T Cusick G E, Hearle J W S, J Text Inst, 46 (1955) T Cusick G E & Hearle J W S, J Text Inst, 46 (1955) T J W S Hearle & R H Peters, Moisture in Textiles (Butterworth Publication Ltd. and the Textile Institute), Indian Standards Specifications IS: 7032 (Bureau of Indian Standards, New Delhi), Precision of test methods Determination of repeatability and reproducibility by inter laboratory test, ISO Standard 5725, 1981 (E).