## 2 Materials and Methods
### Design of the Device
#### 2.1.1 Hardware  
The main parts of the system are the microcontroller (Adafruit feather 32u4 proto board [23]), which is the processing part that coordinates all the others. The second most important parts are the Amphenol NTC thermistor MC65 series [24] and the Amphenol Industrial Temperature Sensor JS8746A [25]. The former sensor is a very fine point medical temperature sensor (1.65 mm diameter maximum), and the latter is a harsh-environment sensor for air temperature with a cylindrical enclosure. The size of the MC65 series makes it the best available solution for the experiment, which requires the least intrusive way to measure the temperature of the fruit's core. Furthermore, we did not choose a smaller diameter for the sensing part although available, because, as the thermistor is inserted in the core of the fruit it might be damaged due to the friction between the flesh of the fruit and the thermistor during its insertion. The vital parts that contribute to the precision and accuracy of the system are the ADS1115 16-bit ADC--4 channel with Programmable Gain Amplifier [26]--which increases the ADC resolution from 10 to 15 bits and amplifies the analog signal of the sensor and the LM4040 Voltage Reference Breakout [27], which stabilizes the reference voltage to achieve more accurate readings. Table 1 displays the specification of the thermistors used.  
The battery used for the system to be portable is a 1200 Mah Li-po battery, which can be recharged by means of a micro-USB connector while in operation. The use of headers in a Printed Circuit Board (PCB) allows the researchers to easily replace parts that might  
Figure 1: Initially the device was designed according to the specifications based on the nature of the experiment. Then, the laboratory experiment was designed and data collection followed. Data analysis involved statistical methods to assess the Agreement. Our findings confirmed the initial hypothesis which allowed us to design the field experiment.  
Figure 2: The device consists of two precise medical sensors inserted in the fruit and an environment temperature probe. There is also an On/Off waterproof switch. The box is waterproof (IP 54) and flanged.  
be damaged or not working properly. The detailed list of the components can be found in Appendix A.  
The design is simple but efficient and is based on open-source technology. We have used a 32u4, 8-bit microcontroller, clocked in 8 Mhz. This choice was based on low consumption of the microcontroller and the vast number of guides and tutorials available that can provide guidance even for amateur users [23]. Since the 32u4 is equipped with a 10-bit analog to digital converter and the input range to the ADC is 0 to 2.048 Volts, a 16-bit analog to digital converter (ADS1115) equipped with an amplifier was added to the device. The readings from the thermistor are sent directly to the ADS1115, which uses an I2C connection with the microcontroller and the signal is amplified and digitized using 15 bits, instead of the microcontroller's 10-bit ADC. The amplification gain according to the datasheet and the input voltage from the thermistor is 2, which means that the Vcc used in order to calculate the value of the thermistor in volts (Table A2, Appendix B, ADC value formula) is 2.048 Volts. Moreover, since the microcontroller reference voltage is not stable, an LM4040 voltage reference breakout is used (0.1% output voltage tolerance) by defining the reference as 2.048 volts. Last, all the reference resistors used to create a voltage divider to read the resistance of each thermistor are of high precision and are described in details in Appendix A. A logger shield [28] was used to write all the data to a micro-SD card integrated with a timestamp. The current time was acquired by the Real time clock PCF8523. The logger shield uses a 3 V CR1220 battery to keep track of the time even if the power is cut from the microcontroller to the logger shield. The schematic is available on Figure A1 (Appendix B).