Wheat straw moisture meter based on alterative current impedance method

Abstract: Wheat straw is a main kind of straws in China. Since it contains a large amount of fiber, wheat straw is used to make paper and produce particleboards in different densities. Moreover, high energy content in wheat straw provides an opportunity to convert straw's heat energy to electric power. Since the unit price of wheat straw is based on weight and straw decays when kept in a high moisture state, moisture content is listed as a main criterion in wheat straw consuming market. It is also an important factor in straw's application. Therefore, moisture content detection is very important during wheat straw marketing, storing and processing. Since the traditional oven drying method has the major disadvantages for long time measurement and unsuitable for on-site detection in collection fields, developing the new kinds of wheat straw moisture meters with advantages of convenience, rapid measurement and high precision is needed in wheat straw industry. In this study, based on alterative current (AC) impedance method, a straw moisture meter, which can detect impedance, temperature and pressure, was designed with an 8-bit single-chip microcomputer as controller. Experiment was conducted at five moisture content levels from 10.4% to 19.7% at wet basis, temperature range from 5 to 40oC at 5oC interval, and three bulk density levels (75.3~101.3 kg/m3) at each moisture content to study the effect of moisture content, temperature and bulk density on impedance. A cubic equation on impedance as functions of moisture content, temperature and bulk density was established. The significance effect of each variable on the regressed equation was analyzed. The results indicate that bulk density has no significant effect on the equation at the significance level of 0.05. Therefore, a cubic equation of impedance with two variables of moisture content and temperature was regressed. The test results showed that the regressed cubic model with two variables can describe the relationship between impedance and moisture content and temperature exactly. Based on the obtained impedance and temperature, Newton iteration algorithm was used to calculate the moisture content of wheat straw. The experiment indicated that the absolute error between calculated moisture content with the developed straw moisture meter and actual value obtained by drying method was within -2.0%-0.9%, and the response time for predicting moisture content was within 3 s. The study can provide a reference for developing convenient and rapid moisture meters of wheat straw.