Design of capacitance measurement module for determining critical cold temperature of tea leaves

- A new design thought of the critical cold temperature was provided in this research. Capacitance and temperature of tea leaf were measured by the capacitance and temperature measurement modules and the peak of capacitance was identified to extract the effective peak. Critical cold temperature was indicated by the corresponding temperature where the capacitance appeared the effective peakThe reactance and capacitance of leaves had obvious peak-valley response characteristics at the same low characteristic temperatures. At a certain low temperature cell damage rate increased sharply, when the obvious peak-valley response of reactance and capacitance occurred, where tea leaves suffered serious freezing injury.
- A new detector was preliminarily designed to accurately and convenient measure the critical cold temperature, and capacitance measurement circuit which is the crucial part in the overall design is better enough responding to the change of tea leaf's capacitance in the range of 0 to 100pF only at the room temperature.
- Further study will focus on increasing the measurement range to catch the peak of capacitance through the improvement of capacitance measurement circuit module and the test of the detector in the tea field will be conducted to validate the reliability.

Critical cold temperature is one of the most crucial control factors for crop frost protection. Tea leaf's capacitance has a significant response to cold injury and appears as a peak response to a typical low temperature which is the critical temperature. However, the testing system is complex and inconvenient. In view of these, a tea leaf's critical temperature detector based on capacitance measurement module was designed and developed to measure accurately and conveniently the capacitance. Software was also designed to measure parameters, record data, query data as well as data deletion module. The detector utilized the MSP430F149 MCU as the control core and ILI9320TFT as the display module, and its software was compiled by IAR5.3. Capacitance measurement module was the crucial part in the overall design which was based on the principle of oscillator. Based on hardware debugging and stability analysis of capacitance measurement module, it was found that the output voltage of the capacitance measurement circuit is stable with 0.36% average deviation. The relationship between capacitance and 1/Uc2 was found to be linear distribution with the determination coefficient above 0.99. The result indicated that the output voltage of capacitance measurement module well corresponded to the change in value of the capacitance. The measurement error of the circuit was also within the required range of 0 to 100 pF which meets the requirement of tea leaf's capacitance.