Wide range and high precision temperature measurement device using platinum resistance

　　The temperature measurement system designed according to the method studied in this paper has the characteristics of small size and high precision. It can not only be used for high-precision temperature measurement in industrial production and scientific research processes, but also can be used as a transferable measurement standard.

　　0 Preface

　　-30-300℃ is the most commonly used temperature range for scientific research and industrial production. With the increasing requirements for temperature measurement accuracy and resolution in scientific research and industrial production, the research on high-resolution and high-precision temperature measurement methods and the development of related high-resolution and high-precision temperature measuring instruments have received widespread attention. Standard platinum resistance thermometer, as an interpolation temperature measuring instrument specified by the 1990 International Temperature Scale (ITS-90), is the temperature measuring instrument with the highest temperature measurement accuracy in the temperature range of -30-300 degrees. Platinum resistance, as a temperature sensitive element, is the core component of platinum resistance thermometer. Pt100, as a commonly used sensor for precision temperature measurement, has the advantages of stable performance, good repeatability and small error.

　　There are usually two methods to convert the resistance change of platinum resistors with temperature changes into voltage signals that can be further processed: the bridge method and the constant current source method. The inherent nonlinearity of the bridge method will introduce systematic errors in the temperature measurement system; the constant current source method has good linearity and a simple structure, but due to the stability of the constant current source, it will introduce random errors. This paper studies a high-precision, high-resolution temperature measurement method based on the constant current source method with Pt100 as the temperature sensor. The proportional measurement principle is used to eliminate the random errors caused by the stability of the constant current source; the segmented measurement method is used to perform high-precision measurements in the range of -30-300℃.

　　1 Measurement principle

　　The instability of the constant current source output current is the main source of error in the temperature measurement of platinum resistance based on the constant current source. The proportional measurement method is used to eliminate the fluctuation of the constant current source current intensity. The principle is shown in Figure 1:

　　Connect Pt100 in series with a high-precision standard resistor. Assume that at a certain moment, the current intensity output by the constant current source is Ia, then the voltage drop formed on Pt100 is Up=Ia*Rp, and the voltage drop formed on the standard resistor is Us=Ia*Rs. At the same time, the voltage drops of the platinum resistor and the standard resistor are collected, and the voltage drop ratio is divided, then the ratio λ=Up/Us=Rp/Rs, so that the random error caused by the instability of the current source can be eliminated.

　　Assume that the number of bits of the AD converter that collects the temperature voltage signal is, if the temperature range of -30-300℃ is considered as a whole and its output voltage range is the same as the range of the AD converter, then the change in LSB indicates that the temperature changes by ℃.

　　In fact, since the range of temperature voltage is slightly smaller than the range of AD converter, the temperature represented by the change of LSB is greater than ℃. This paper adopts a segmented measurement method to divide the measured temperature range into three sections: -30-80℃, 80-190℃, and 190-300℃. Under the control of the intelligent microprocessor, the constant current source outputs test currents of different intensities for three different temperature sections. The voltage drop formed by Pt100 in each temperature section is amplified and conditioned to a voltage range close to the range of AD converter. Through this segmented measurement method, the resolution of temperature measurement can be effectively improved, making technical preparations for high-precision temperature measurement.

　　2 Temperature measurement system design

　　According to the principle of proportional measurement and segmented measurement, a high-resolution and high-precision platinum resistance temperature measurement device is designed. Its structure and main component models are shown in Figure 2.

　　The platinum resistor is a four-wire A-class Pt100. LM134 is used as the core device of the constant current source. Three feedback resistors with different resistance values ​​are set to enable it to output three constant currents of 0.5mA, 0.7mA, and 1mA. The switching of the resistor is completed by the signal relay. The signal relay is controlled by the MCU  through the relay driver. The test current is 1mA in the range of -30-80℃, 0.7mA in the range of 80-190℃, and 0.5mA in the range of 190-300℃. The standard resistor is a wire-wound resistor with an accuracy of 0.01% and a temperature stability of 5ppm/℃. INA114 is used as the core device of the signal amplification circuit of the platinum resistor and the standard resistor. The voltage drop of the platinum resistor is amplified 100 times, and the voltage drop of the standard resistor is amplified 40 times. Taking -30-80℃ as an example, when using 1mA test current, the voltage drop range across the platinum resistor is 88.22-130.9mV. After amplification 100 times, the voltage range is 8.822-13.09V. At the REF end of INA114, a voltage of -8.192V is input, and the output voltage range of INA114 is 0.63-4.898V. The output voltage of INA114 is converted to digital using TI's 24-bit AD converter ADS1247. The input range of ADS1247 is 0-5V, so theoretically, the temperature resolution in the range of -30-80℃ can reach ℃. However, due to the stability and error problems of ADS1247, the actual resolution will be lower than the theoretical resolution. The working process of the temperature measuring device is: MCU initiates a temperature measurement based on the input information of the keyboard. MCU first controls the signal relay to make LM134 output 1mA current, then collects the output signal voltage of the platinum resistor. If it reaches 5V, it drives the relay to switch the feedback resistor of LM134 until the output voltage of the platinum resistor is lower than 5V. Then, it collects the voltage signals output by the platinum resistor and the standard resistor at the same time, performs division operation and temperature conversion, and sends the conversion result to the display.

　　Through experiments in low temperature and constant temperature oil tanks with a resolution of 0.001°C and an accuracy of 0.01°C, the measurement accuracy of the device can reach 0.03°C, and the maximum temperature resolution is 0.003°C.

　　3 Conclusion

　　This paper discusses the design principle and implementation method of a wide-range, high-precision temperature measurement device based on platinum resistance. The proportional measurement method can effectively eliminate the random error caused by the fluctuation of the constant current source. The segmented measurement method can partially enlarge the measured temperature range, thereby improving the resolution and accuracy of the measurement.