What is a Thermocouple
A thermocouple is a commonly used temperature-sensing element in temperature measurement instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The various shapes of thermocouples can vary significantly based on different needs, but their basic structures are roughly the same.
The fundamental principle of thermocouple temperature measurement is based on a closed loop composed of two different material conductors. When there is a temperature gradient at both ends, a current will flow in the loop, resulting in an electromotive force – the thermoelectric potential. This phenomenon is known as the Seebeck effect. The two different homogeneous conductors form the thermoelectric junction, with one end being the measurement end and the other end typically maintained at a constant temperature, connected to a temperature controller or a temperature meter. Based on the relationship between thermoelectric potential and temperature, a thermocouple calibration table is created. The calibration table is obtained with the free end temperature set at 0°C and different types of thermocouples have different calibration tables. When using a thermocouple for temperature measurement, the cold-end temperature is significantly affected by the environment and cannot be guaranteed to be 0°C, which introduces errors. Therefore, cold-end compensation is necessary for thermocouples.
Technical Advantages of Thermocouples: Thermocouples have a wide temperature measurement range and exhibit relatively stable performance. They provide high measurement accuracy since they come in direct contact with the measured object and are not influenced by intermediate media. Additionally, thermocouples have fast thermal response times and are flexible in responding to temperature changes. They offer a large measurement range, reliable performance, and excellent mechanical strength. Moreover, they have a long service life and are easy to install (as shown in Figure 1.1 Commonly Used Thermocouples).
Figure 1.1 Commonly Used Thermocouples
Types of Thermocouples
Currently, commonly used thermocouples are standard thermocouples. A standard thermocouple is one that is specified in national standards for its thermoelectric potential-temperature relationship, and allowable error, and has a unified standard calibration table. It comes with compatible display instruments for selection. Seven standardized thermocouples, namely S, B, E, K, R, J, and T, are designated as the unified design thermocouples in China, with K-type thermocouples being the most commonly used.
When using thermocouples for temperature measurement in practical applications, factors such as the temperature range of the operating environment, required accuracy, atmospheric conditions, properties of the measured object, response time, and cost-effectiveness need to be considered. The specific thermocouple model needs to be discussed with the thermocouple manufacturer.
For temperature acquisition using thermocouples, Smacq offers two solutions: one involves using a general data acquisition device (DAQ) along with a thermocouple transmitter, and the other involves using a dedicated thermocouple data acquisition module.
Thermocouples output a relatively small voltage range. For instance, the sensitivity of a K-type thermocouple is approximately 0.04 mV/°C. General Data Acquisition Devices (DAQ) typically do not come with highly amplified amplifiers, and they lack cold-end compensation functionality. Therefore, an external signal conditioning circuit, known as a thermocouple transmitter, is required to address these two issues. In this article, we use the USB-3000 series data acquisition device, which utilizes a temperature transmitter to output standard voltage signals, as shown in Figure 1.2 Temperature Transmitter.
Figure 1.2 Temperature Transmitter
Wiring Connection between Data Acquisition Device and Temperature Transmitter
This section mainly explains the wiring connection between a temperature transmitter capable of outputting standard voltage and a data acquisition device. The temperature transmitter used here has 5 terminals, namely the positive power supply for the temperature transmitter, the negative power supply (power ground), the positive terminal of the thermocouple, the negative terminal of the thermocouple, and the output terminal (interface for standard voltage output). An appropriate power supply should be selected for the temperature sensor (specifically according to the manual of the temperature transmitter), and then the wiring should be done following Figure 1.3. The following diagram shows the wiring diagram using the output voltage form of the temperature transmitter, which is compatible with general data acquisition devices. If the temperature transmitter output is 4-20mA current, the wiring needs to refer to the topic of collecting current data from the Smacq knowledge base.
Figure 1.3 Wiring Connection between Data Acquisition Device and Temperature Transmitter
Regarding the wiring connection between the data acquisition device and the temperature transmitter, it is often ignored whether the power supply of the temperature transmitter is common ground with the data acquisition device. If not common ground, the temperature transmitter is a floating source relative to the data acquisition device, so the wiring should be done as shown in Figure 1.3, short-circuiting AI Sense and AGND. If it can be confirmed that the data acquisition device and the temperature transmitter share a common ground, there is no need to short-circuit AI Sense and AGND, and the negative power supply can be directly connected to AI Sense. In general, temperature transmitters powered by switching power supplies are mostly floating sources. The specific wiring method for the floating source and ground source needs to refer to the USB-3000 series manual.
Data Acquisition Device Software Operation
Before using the temperature transmitter, it is necessary to understand some important parameters of the temperature transmitter being used, such as whether the selected temperature transmitter is suitable for the thermocouple type, the power supply voltage of the temperature transmitter, the temperature range for measurement, and the output voltage. In the case of Smacq, the actual test uses a temperature transmitter with a power supply voltage of 24V, a temperature range of -50°C to 400°C, and an output voltage of 0V to 10V.
With the correct external device wiring, enter the setting interface of Smacq DAQ Software, and set the software to use single-ended mode. Choose AI channel 0 for data acquisition. Select a range of ±10.24V and a sampling rate of 10Sa/s/ch (the actual sampling rate can be set based on the actual situation and the sampling rate available on the data acquisition device (DAQ)). Then, set the range and output voltage of the temperature transmitter. Therefore, some important parameters of the temperature transmitter need to be understood based on the product manual, as shown in Figure 1.4 Temperature Transmitter Parameters.
Figure 1.4 Temperature Transmitter Parameters
If you want to convert the voltage signal collected by the data acquisition device (DAQ) into temperature, check the unit conversion. Figure 1.5 shows the voltage curve collected by the data acquisition device (DAQ), and Figure 1.6 shows the temperature curve collected by the data acquisition device (DAQ) after unit conversion.
Figure 1.5 Voltage Curve
Figure 1.6 Temperature Curve
This way, the data acquisition device (DAQ) can obtain both the voltage data and the temperature data after conversion. It is also possible to use the software provided by Smacq to export the collected data and generate Excel-formatted data in the form of voltage or temperature. For details on data acquisition device (DAQ) software operation, please refer to Smacq DAQ Software Quick Start Guide.
Remote IO Module for Thermocouple Data Acquisition
Another solution for thermocouple temperature acquisition involves using Smacq’s M2101 remote IO module, which is specially designed for thermocouple data acquisition within the M2100 series. The M2100 series temperature acquisition remote IO module is a set of computer interface modules based on the Modbus RTU standard protocol. Being a standard MODBUS RTU device, it can be easily connected and used with PLCs, industrial control screens, and other devices. It supports 8 types of thermocouples and has 8 thermocouple acquisition channels with an RS-485 (2-wire) data interface. For specific details, refer to the product manual on Smacq’s official website. Since modern PCs rarely have RS-485 interfaces, Smacq provides the SDS1011 serial port converter, which uses the USB standard protocol to support USB-to-RS-485 interface conversion. Additionally, the SDS1011 also features a 24V 100mA power output, solving the power supply issue for the M2101 module.
Wiring Connection between Remote IO Module M2101 and Thermocouples
Normally, a direct current power supply is required to power the M2101 remote IO module. However, the SDS1011 has upgraded power supply capabilities compared to the SDS1001 serial port converter, allowing it to directly power the M2101, simplifying the circuit connection. Refer to Figure 1.7 for the schematic diagram of the connection between the M2101 remote IO module and thermocouples.
Figure 1.7 Remote IO Module M2101 and Thermocouple Connection Schematic
Software Operation for Remote IO Module M2101
The corresponding operating software for the remote IO module M2101 is M Console. The following example shows the temperature acquisition of a K-type thermocouple during product testing by Smacq, as shown in Figure 18 Temperature Display Interface.
Figure 18 Temperature Display Interface
Alternatively, by clicking on the Data logger on the main page, users can choose the channel to record temperature data, then set the file generation address. This way, real-time temperature data can be displayed and recorded in .CSV format files. Additionally, the software provides real-time displays of the current data as well as the maximum, minimum, and average values of the collected data, facilitating data analysis, as shown in Figure 1.9 Data Waveform.
Figure 1.9 Data Waveform
The above information presents two solutions provided by Smacq for temperature acquisition using thermocouple sensors. Users can choose the most suitable solution based on their specific requirements and needs.