High-precision temperature measurement system built with multimeter

This article will use RIGOL's benchtop multimeter DM3068 to provide a convenient and accurate solution. Temperature measurement is common in various industrial sites, but it is not easy to achieve accurate and fast temperature measurement. The main reason for the difficulty is that the temperature signal itself is not as easy to detect directly as a general physical signal, and the temperature measurement data usually needs to be saved or recorded in digital form, which will involve sensor technology, precise measurement and data processing.

System structure
The temperature measurement system mainly consists of three parts: temperature sensor (used to convert temperature changes into voltage/resistance changes), measurement part (measurement of voltage/resistance signals), and calculation and conversion part (converting the measured voltage/resistance value into temperature value). Among them, temperature sensors have become industrial standardized products, and only the products suitable for the application need to be selected. The remaining two parts can be directly undertaken by DM3068. In addition, a PC is used to record and analyze the measurement results. The schematic diagram of the system structure is shown in Figure 1.

Figure 1 System structure diagram

Selecting a temperature sensor
Currently, the most common temperature sensors in industrial applications are thermocouples (TC), thermal resistors (RTD), and thermistors. DM3068 has built-in measurement support for each of these three types of sensors, making temperature measurement easier.

DM3068 temperature measurement function
DM3068 is a benchtop multimeter with an accuracy of 61/2. In addition to being able to measure various electrical signals, it has a built-in temperature measurement function and can be directly connected to a variety of temperature sensors for temperature measurement. It supports front panel operation and remote communication to control and obtain measurement results.

Since the DM3068 front end has a built-in cold-end reference temperature measurement, you only need to simply connect the TC temperature sensor directly to the DM3068 in voltage measurement mode, and select the matching TC sensor type in the DM3068 setting interface to start temperature measurement. DM3068 also supports 2-wire RTD, 4-wire RTD temperature sensors and Thermistor temperature sensors, and you only need to select the parameters that match the sensor to directly measure the temperature.

DM3068 directly supports TC sensors of type B, E, J, K, N, R, S, and T. It supports 2-wire or 4-wire RTD sensors with an R0 parameter range of 49 to 2100Ω and a sensor coefficient α of 0.00385, 0.00389, 0.00391, and 0.00392. It supports five types of Thermistor sensors: 2.2kΩ, 3kΩ, 5kΩ, 10kΩ, and 30kΩ, which are sufficient to meet common applications.

Since DM3068 can provide high-precision voltage and resistance measurement, the minimum resolvable accuracy can reach 0.1μV voltage or 0.1mΩ resistance. Therefore, when measuring temperature, the reliability and stability of temperature measurement can be fully guaranteed. The internal calculation method adopts the calculation method that conforms to the international temperature standard ITS-90, and the temperature calculation accuracy can reach 0.001℃. In addition, it also has a variety of statistical and control functions to facilitate the integration of temperature test systems with other equipment.

Measurement application example
Application scenario
Use RTD sensor, model Pt100, α parameter is 0.00385, use the statistical function of DM3068 to record the current temperature change and save the measurement data for later reference.

Setting method
Refer to the system structure in Figure 1, connect the two terminals of the sensor to the resistance measurement input of DM3068, and then set the temperature measurement function of DM3068. In order to ensure the reliable connection of the measured signal, use the rubber head connector shown in Figure 2 (a). The lead end can be directly connected to the lead of the temperature sensor, and the metal head can be directly connected to the voltage measurement input probe jack of DM3068. The effect after connection is shown in Figure 2 (b).

(a) Rubber head for connection (b) Effect after connection

Figure 2 Connection between DM3068 and sensor

On the DM3068 front panel, press the "Sensor" button to enter the sensor configuration interface, select "New" → "Temperature" → "Type" → "RTD" → "R0", set the resistance to 100Ω, and select the α parameter to 0.00385. After the settings are completed, return to the previous level and click the "OK" → "Apply" button to start temperature measurement.

Use the 9-hole serial communication cable that comes with DM3068 to connect the computer and DM3068. Press the "Utility" button on the front panel of DM3068 to enter the setting interface, then select "Interface" → "RS232" → "Settings" → "Baud Rate" in sequence, where the baud rate is 9600, and then through "Interface" → "RS232" → "Print" → "Open" to turn on the function of DM3068 to automatically print data to the serial port in character mode through the serial port. Open the "HyperTerminal" serial communication software in the "Start → Programs → Accessories → Communication" directory of the computer, set the baud rate to 9600, hardware handshake is not available, and other parameters are the default, and set the data saving path and data file name in the "Transfer → Capture Text" menu of the software. After startup, the hyperterminal will automatically save the data displayed on the screen in the corresponding file, and you can directly open the saved file in Notepad or Excel in the future.

At the same time, you can also turn on the trend drawing function of DM3068, and you can directly save the drawing results to the USB flash drive after the measurement is completed. Press the "Math" key on the front panel of DM3068 to enter the setting interface, and press "Trend Drawing" to directly enter the trend drawing function. Press the first blue soft key at the bottom of the screen to call up the menu. After the measurement is completed, press the "Stop" key to stop drawing. At this time, a "PrtScn" menu will pop up. Insert the USB flash drive from the USB interface on the front panel and press the "PrtScn" menu key to directly save the drawing. The result is shown in Figure 3.

Figure 3 Trend drawing

Conclusion
It is relatively complicated to realize a high-precision and fast temperature measurement system, but by making full use of the rich temperature measurement functions of DM3068, convenient and high-precision temperature measurement can be achieved. In addition, DM3068 also has rich mathematical statistics functions and pass/fail detection output functions, as well as rich communication interfaces, which can quickly build a high-precision temperature measurement system as desired.