Moisture content measurement in paddy

Moisture content is an important quantity for agriculture product, especially in paddy. In principle, the moisture content can be measured by a gravimetric method which is a direct method. However, the gravimetric method is time-consuming. There are indirect methods such as resistance and capacitance methods. In this work, we developed an indirect method based on a 555 integrated circuit timer. The moisture content sensor was capacitive parallel plates using the dielectric constant property of the moisture. The instrument generated the output frequency that depended on the capacitance of the sensor. We fitted a linear relation between periods and moisture contents. The measurement results have a standard uncertainty of 1.23 % of the moisture content in the range of 14 % to 20 %.


Introduction
Rice is an important agriculture product. In general, it is recommended that rice for food purposes be stored in paddy form rather than milled rice as the husk helps prevent quality deterioration and provides some protection against insects. To control the quality of paddy, moisture content plays a vital role. Safe storage for a longer period is possible if paddy is maintained at moisture levels of 14 % or less [1][2][3].
Moisture in paddy is defined as the water in liquid state. To measure moisture content, there are direct methods and indirect methods [4]. Loss on drying of moisture is a direct method that can be used as a reference method [5]. The direct method provides high accuracy and high precision of the value of moisture content. However, this method is time-consuming. Hence, some indirect methods such as resistance type [6] and capacitance type [7][8] instruments were developed.
The resistance type instruments have low accuracy because of pure water does not conduct electrical current. The moisture can conduct by some contaminated ions. The 555 integrated circuit timer was configured for a soil moisture meter based on the resistance method [9].
The capacitance type instruments have higher accuracy because the dielectric constant that is an intrinsic property of the moisture is linearly proportional to the capacitance. In this work, we used the 555 timers to develop a capacitance type instrument for moisture content measurement in paddy.

Experiment
To develop a capacitance type instrument, we need a direct method to be the reference method. We employed the air-oven method. The paddy sample was sampling about 5 g before putting into an oven to heat at 130 °C for 2 h. After drying, the paddy was weighted again. The moisture content was calculated in % given by Where is the mass of paddy before drying and is the mass of paddy after drying. We designed an instrument circuit based on the IC555 timer. The circuit was controlled by Arduino Uno R3 and displayed the result of moisture content measurement on LCD display. The circuit diagram is shown in figure 1. In figure 1, we configured the IC555 timer to be astable multivibrator mode for frequency generating output. We replaced the capacitor at pin 2 with a container for the paddy. The container was designed as a parallel capacitor. The paddy sample was put between the two conductive parallel plates. The capacitor C2 was selected as . The resistors were selected as , and for R1, R2 and R3 respectively. The output frequency was related to the value of resistances R1, R2 and the capacitance of the paddy container C as given by (2) In equation 2, the frequency is inverse proportional to C. The linear relation is preferred for the capacitance sensor. Using we obtained where T is the period of the output signal.

Results and discussion
The paddy samples were measured by the capacitance instrument before measured again by the reference method. The results of frequency, period and moisture content are tabulated in

Conclusion
A capacitive instrument based on IC555 timer circuit can be used to measure the moisture content in paddy. The standard uncertainty of the measurement was 1.23 % of the moisture content in the range of 14 % to 20 %.