1. Introduction In industries such as metallurgy, petroleum, chemical, machinery manufacturing, and national defense, it is often necessary to measure the temperature of gases and liquids in environments ranging from -200℃ to 1000℃. Previously, glass liquid thermometers, bimetallic thermometers, pressure thermometers, thermocouples, resistance temperature detectors (RTDs), and non-contact thermometers were commonly used for temperature measurement. Among these, thermocouples have a wide temperature measurement range. They can directly generate a voltage (thermoelectric potential) signal without the need for a driving power supply. This signal can be read using DC measuring instruments (such as potentiometers, digital voltmeters, millivoltmeters, etc.) to find the corresponding temperature using a thermocouple temperature characteristic calibration table; alternatively, a small signal voltage can be amplified using a linear correction circuit and displayed on a scale instrument. In some oil and gas pipeline applications, long-term temperature monitoring with rapid and accurate readings is often required. In such cases, the aforementioned thermometers are insufficient. However, if the thermoelectric potential generated by the thermocouple is converted into a digital signal, processed by a microcontroller, and the temperature result displayed on an LCD, this method offers rapid response, high measurement accuracy, low power consumption, and intuitive display. Therefore, a digital low-power, high-precision thermometer composed of a thermocouple, A/D conversion circuit, microcontroller, and LCD module can replace various mechanical thermometers to complete temperature measurement and control work under special circumstances, and is easy to miniaturize. Figure 1 shows the schematic diagram of a portable low-power, high-precision digital thermometer. 2. Hardware Circuit Design During the measurement process, the thermocouple generally generates a thermoelectric electromotive force relative to the cold junction. Industrial standards generally specify the cold junction temperature as 0℃. However, in practical use, placing the cold junction in an ice-water mixture is inconvenient. If the local temperature is not 0℃, the thermoelectric electromotive force may be too large or too small. Therefore, practical circuits usually require temperature compensation for the thermoelectric electromotive force. This system uses an AD7416 to measure the local temperature and calculates the corresponding compensation voltage according to the calibration table. The thermoelectric electromotive force at the true temperature is equal to the difference between the measured electromotive force and the compensation voltage. The portable, low-power, high-precision digital thermometer system consists of four parts: first, a thermocouple; second, a data acquisition circuit composed of AD7705 and AD589, where the A/D conversion circuit converts the thermoelectric electromotive force generated by the thermocouple into a digital signal; third, an AD7416, which measures the cold junction temperature and calculates the compensation voltage; and fourth, a control and display circuit composed of MSP430F413 and a six-digit segment LCD. The specific circuit diagram is shown in Figure 1. To achieve low power consumption and high precision, the chips selected in this design all have low-power modes, allowing them to operate in power-saving mode during measurement intervals. The following is a detailed description of each circuit part. 2.1 Thermocouple This design uses K-type or J-type nickel-chromium-copper-nickel (constantan) thermocouples. These thermocouples are well-suited for temperature measurement systems in oxidizing and weakly reducing environments, with a temperature range of -200℃ to 1000℃ and a thermoelectric potential range of -9.835mV to 76.358mV. Due to their good stability, high sensitivity, and low cost, these thermocouples are ideal for portable temperature measuring instruments. Figure 2 shows the thermoelectric potential-temperature curve of a nickel-chromium-copper-nickel (constantan) thermocouple. Analysis shows that its accuracy can reach ±0.1℃, and its sensitivity can reach 38μV/℃ at -150℃. Figure 2: Thermoelectric potential-temperature curve of a nickel-chromium-copper-nickel (constantan) thermocouple. 2.2 Data Acquisition Circuit In this part of the circuit, the AD7705 is the front-end device for the low-frequency measurement system. It has high resolution and a power-saving mode, meeting the requirements of high accuracy and low power consumption. Furthermore, the AD7705 also has on-chip digital filtering, calibration, and compensation circuits, thus better ensuring high-precision temperature measurement. The AD7705 operates on a single 2.7V to 3.3V supply and has two analog differential input channels. With a 3V supply and a 1.235V reference voltage, the maximum amplitude range of the bipolar input signal is 0 to ±10mV (Gain = 128) to 0 to ±1.235V (Gain = 1). Additionally, the AD7705 can directly receive small signals from sensors for A/D conversion and output a serial digital signal. It uses Σ-Δ technology to achieve 16-bit A/D conversion. The sampling rate is determined by the master clock at the MCLKIN pin and the variable gain of the amplifier. In practice, the AD7705 can simultaneously perform on-chip amplification, modulation conversion, and digital filtering of the input signal. Its digital filter has programmable stopband control to adjust the filter cutoff frequency and output data update rate. The filter's response is similar to that of a median filter, but with a steeper falling edge. Since the output rate of the digital filter coincides with the first concave frequency of the filter's amplitude-frequency response, when the output rate is 25Hz, the first concave frequency of the filter is also 25Hz. Additionally, the (sinx/x)3 filter can suppress harmonic components at the first concave frequency, with a suppression amount greater than 40dB. When FS0 and FS1 are 0 and 1 respectively, the output rate and the first concave frequency are 25Hz, and 6.55Hz at the -3dB point. If the ambient temperature changes slowly, this circuit can effectively suppress interference signals greater than 6.55Hz, including 50Hz interference signals, during analog-to-digital conversion. When the AD7705 operates at 3V and the on-chip programmable amplifier gain is set to 1, the A/D accuracy is 16 bits, and the minimum resolution voltage is 37.69μV (1.235V×2/65536). The thermocouple's output thermoelectric potential changes by 38μV to 81μV/℃ for every 1℃ change (-150℃ to 1000℃), which is greater than the AD7705's minimum resolution voltage. Therefore, the system resolution can reach 1℃, meeting the requirements of most industrial measurements. Because the AD7705 can directly perform analog-to-digital conversion on voltages from -0.6175V to 0.6175V, it can operate normally without additional circuitry when the thermocouple is measuring temperatures below 0°C and the thermoelectric potential is below 0V. The AD589 is the voltage reference source for the AD7705. The AD589 is an inexpensive two-terminal device that provides a 1.235V bandgap reference voltage output with temperature compensation. Its on-chip component matching and thermal tracking characteristics give the AD589 high stability. Furthermore, the AD589's output impedance is 10 times lower than that of a typical low-temperature-coefficient Zener diode; therefore, even with varying loads, the circuit can maintain high accuracy without external components. 2.3 Measuring Cold Junction Temperature with the AD7416 The AD7416 is a complete monolithic temperature monitoring system with a temperature measurement range of -55°C to 125°C. This device incorporates a bandgap temperature sensor and a 10-bit A/D converter. The A/D converter monitors the temperature and digitizes the value, achieving a resolution of 0.25°C. The digital thermometer described in this paper uses the AD7416 to measure the local temperature and can output the required compensation voltage when the thermocouple reference junction temperature is not 0°C. 2.4 Control and Display Circuit The MSP430F413 is an ultra-low-power microcontroller manufactured by Texas Instruments, with a voltage range of 1.8–3.6V. Because the MSP430F413 includes various functional modules (such as frequency-locked loop, timer, watchdog, comparator, LCD driver circuit, and input/output ports), it is suitable for various applications. Its low power consumption and low voltage characteristics are particularly suitable for battery-powered portable instruments. The MSP430F413 is connected to the AD7705's SCLK, DIN, and DOUT pins via P1.4, P1.5, and P1.6 respectively to form a three-wire interface. When P1.3 is low, the AD7705 is selected to initiate A/D conversion, calibration, and data reading. Once the A/D conversion is complete, the level change of the DRDY pin can be read from P1.7, allowing the system to respond accordingly. Local temperature data can be acquired via the I2C bus formed by P6.5, P6.4, and the AD7416. The P3, P4, and P5 ports of the MSP430F413 have secondary functions; in addition to being ordinary I/O ports, they can drive a 24-segment LCD module with four COM ports. In this design, a six-digit segment LCD is used for the display. The three 1MΩ resistor divider between pins R33, R23, R13, and R03 provides a reference bias voltage for the LCD display. Each character is 15mm × 10mm in size, allowing for convenient data reading from a certain distance. In addition, a button is connected to each of the three pins P1.0, P1.1, and P1.2, which can be set to interrupt mode. These three buttons can also be used to set the system's sampling interval, threshold value, and control the system to enter low-power or active mode. The microcontroller's clock signal is generated by a 32.768kHz crystal oscillator and an on-chip oscillation circuit, thus reducing power consumption. The watchdog circuit ensures the program's long-term normal operation. If the system collects data every 10 seconds, the average current of the entire system in one cycle is 103.2μA. If the entire system is powered by a 3V/1Ah battery, it can operate continuously for 13 months. Reducing the data acquisition frequency can further extend battery life. 3. Software and System Experiments The software of this digital thermometer system consists of a data acquisition program, a timer interrupt service routine (entry address 0FFE0h, priority 0), a watchdog interrupt service routine (entry address 0FFF4h, priority 10), a button interrupt service routine (entry address 0FFE8h, priority 4), an LCD display program, and thermocouple calibration table data. The microcontroller in the system operates in active mode, and during working intervals, it can be set to low-power mode 2 to reduce power consumption and extend battery life. Since the MSP430F413's on-chip ROM is only 8KB, it cannot completely store the data in the calibration table. Therefore, within a certain error range, for the approximately linear portion, it can be approximated using piecewise linear segments. For curves with large curvature, Chebyshev approximation expressions can be used for programming calculations. Because the ambient temperature and local temperature do not change abruptly in industrial environments, temperature can be measured and displayed at regular intervals. The data acquisition program's algorithm has adaptive characteristics; therefore, when a temperature changes beyond a set threshold within a unit time interval, the MSP430F413 will shorten the sampling interval to increase the sampling frequency and issue an audible and visual alarm signal via buzzer U1 and LED D1. Experiments show that the AD7416 has high sensitivity and can use smoothing filtering methods to reduce errors in local temperature data. The MSP430F413 microcontroller can be programmed in C, offering strong readability and portability. The program can be compiled into machine code using IAR's IAREmbedded Workbench and IARC-SPYdebugger. The IAREmbedded Workbench system software includes the MSP430F413 microcontroller header files msp430x41x.h and in430.h, which define the on-chip special function register names, operating modes, input/output registers, timers, system clock, power management, comparators, LCD display registers, watchdog timer, interrupt vectors, and library functions. The program code can be written to the on-chip Flash ROM via the JTAG interface of the MSP430 Flash Emulation Tool using a computer. Since the MSP430F413 microcontroller's JTAG interface supports in-circuit programming, programming is very convenient, and upgrading existing programs is also easy. The system's main flowchart and the timer interrupt service routine data acquisition flowchart are shown in Figure 3. Figure 3. System Main Flow and Timer Interrupt Service Routine Data Acquisition Flowchart 4. Conclusion The accuracy and resolution of this system mainly depend on the accuracy and resolution of the sensor and A/D chip. Since digitization cannot completely eliminate errors in applications, it is necessary to eliminate errors caused by cold junction temperature or cold junction compensation, as well as errors in connecting compensation wires, when measuring temperature. Attention should also be paid to circuit errors and errors caused by noise, insulation resistance, thermal resistance, etc. Depending on actual needs, highly stable thermocouples and local temperature sensors with an accuracy of 1℃ can be used, thus improving accuracy within a certain temperature range. This system uses MSP430F413 and AD7705 as the core to implement the design of a low-power, high-precision portable thermometer. For portable instruments, this design achieves low power consumption and high accuracy requirements under low-cost, wide temperature measurement range conditions, and has certain practical value. Currently, this circuit has been put into application. Practice shows that the entire portable low-power, high-precision digital thermometer is easy to use, operates stably, has a long standby time, and has broad application prospects.