[Strong Popular Science] How to achieve thermocouple temperature measurement with one click?
Latest update time：2024-10-15
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Temperature detection is everywhere in modern electronic products and factory equipment.
In wearable devices, human body temperature needs to be detected; in cars, battery temperature needs to be detected; in factories, boiler heating needs to be detected.
Although measuring temperature is a simple task, if you choose the wrong temperature sensor, you will not be able to measure the temperature. Look at what Doudou did.
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「 Several common temperature measurement methods 」
The temperature measurement environment is different, and the temperature sensing products are diverse
, which creates some troubles for engineers. Today, we will sort out several of the most commonly used temperature measurement methods.
01
Thermistor
This circuit symbol is a thermistor and can be seen in many circuit diagrams.
Thermistors are a type of sensitive components whose resistance value changes with temperature. The higher the temperature, the greater the resistance value is a positive temperature coefficient thermistor (PTC). Conversely, the higher the temperature, the lower the resistance value is a negative temperature coefficient thermistor (NTC).
In a circuit, a resistor can be used to measure temperature. Obviously, this temperature measurement method is low-cost, small in size, and simple to use. Although thermistors have poor linearity, they can still be seen in some application scenarios that require fast response and small size.
02
Resistance Temperature Detector (RTD)
This is a temperature sensor with a metal probe built in, whose resistance increases or decreases as the temperature rises. Although the name also contains resistance, it is very different from thermistor. RTD is one of the most accurate and stable temperature sensors. It can be used in some occasions that require high-precision temperature measurement, but RTD has a slow response speed and is expensive.
03
Integrated (IC) Temperature Sensors
The most familiar temperature sensor of this type is DS18B20. An integrated (IC) temperature sensor is a dedicated IC that integrates a temperature sensor on a chip and can perform temperature measurement and signal output functions.
The main features of integrated (IC) temperature sensors are single function (only measuring temperature), small temperature measurement error, low price, fast response speed, long transmission distance, small size, low power consumption, etc. They are suitable for long-distance temperature measurement and control, do not require non-linear calibration, and have simple peripheral circuits.
Integrated (IC) temperature sensors can be divided into two types according to the output signal type: analog integrated temperature sensors (LM35) and digital integrated temperature sensors (DS18B20).
04
Thermal Thermal Sensor
Thermocouple temperature measurement is a closed loop composed of two conductors of different materials. When there is a temperature difference between the two ends, current will flow through the loop. At this time, there is an electromotive force between the two ends - thermoelectric electromotive force. The temperature difference between the two ends can be calculated based on the size of the thermoelectric electromotive force.
One advantage of thermocouples is that they do not require external power supply. In addition, thermocouples have the advantages of wide temperature measurement range and adaptability to various atmospheric environments. However, their disadvantage is that the measurement accuracy is not high, so thermocouples are usually not selected in high-precision measurements and applications.
Here we sort out the advantages and disadvantages of several temperature measurement methods:
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Thermocouple temperature measurement
The four temperature measurement methods introduced above all have their application scenarios. Thermistors, RTDs, and IC temperature sensors are commonly used by hardware engineers. However, thermocouples are often used in industrial applications, so some people don’t know much about them. Next, let’s focus on learning about thermocouple temperature measurement.
As mentioned above, the temperature measurement principle of thermocouples is derived from the "
Seebeck effect
". Two conductors of different materials are welded together to form a closed loop. When one end, which can be called the working end or measuring end, is in a high temperature environment, and the other end, which is the cold end or reference end, is in a lower temperature environment, an electromotive force will be generated between the two ends. By measuring the magnitude of this electromotive force, the temperature of the working end of the thermocouple can be inferred.
The connection between these two materials of different components is standard. According to the two materials used, thermocouples can be divided into K-type thermocouple, S-type thermocouple, E-type thermocouple, N-type thermocouple, J-type thermocouple and so on.
Each thermocouple has a graduation table, which shows the standard value of the thermoelectric potential corresponding to the temperature at the measuring end when the reference end is 0°C. From the graduation table, it can be seen that the voltage signal is at the mV level, which is very small.
In practical applications, since the temperature of the reference end of the thermocouple cannot be kept constant at 0°C or fixed at a certain temperature, and since the thermocouple potential is related to the reference temperature, if the reference end temperature changes, it will inevitably cause measurement errors. In order to eliminate this error, cold end temperature compensation must be performed.
How to perform cold junction compensation? Even those who have never used thermocouples should have thought that they can use compensation wires to extend the thermocouple to a place where the ambient temperature changes more steadily, or place the thermocouple in a container at a constant 0°C. Those who have some basic knowledge of electronics may think of using circuit compensation, which is professionally called bridge compensation.
These methods can indeed allow the thermocouple to measure the correct temperature, but there are some inconveniences in implementation.
Is there a more convenient compensation method? Yes!
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「
EPSH TCM 856 Temperature Measurement Module
」
This is the EPSH TCM 856 temperature measurement module
developed by the former ADI distributor Shijian
. Now, Shijian has been acquired by WT Technology.
In this temperature measurement module, the MAX31856 from ADI is used, a high-precision, linearly compensated thermocouple digital converter that can perform cold-end compensation and digital conversion on the signal of any type of thermocouple, output data in degrees Celsius, and allow readings of temperatures up to +1800°C and as low as 210°C (depending on the type of thermocouple).
As mentioned earlier, the thermoelectric electromotive force of thermocouples is only at the mV level, so data processing is required. The MAX31856 integrates the peripheral circuits required for thermocouple temperature measurement. The built-in input amplifier and ADC amplify and digitally convert the output voltage of the thermocouple. The on-chip temperature sensor measures the cold-end temperature, and then uses the internal lookup table (LUT) to determine the ADC value corresponding to the selected thermocouple type and the cold-end temperature. The thermocouple value and the cold-end value are then summed to generate a value corresponding to the thermocouple temperature after cold-end compensation, and converted into an output value in °C. Finally, the result is sent to the MCU through the SPI interface, and the MCU uploads the data to the PC via RS485.
MAX31856 Internal Structure Diagram
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"
Thermocouple measured temperature
Temperature module
"
With this temperature measurement module, combined with the host computer software developed by WT, you can start temperature measurement with just one click.
1) Hardware Connection
Input 12-24V through the power interface, or input 5V through USB TypeC to power the EPSH TCM 856; connect the thermocouple through the sensor interface; connect the module output to the PC through the RS485 digital interface.
2) Software connection
Open the host computer software and select the corresponding serial port:
3) Configure the main interface
There are five partitions according to the functions: register configuration area, real-time cold-end compensation temperature, real-time linearized thermocouple temperature (i.e. actual temperature), error status, register list and real-time temperature curve.
For some functions of thermocouples, you can configure them through the register configuration area. There are 6 registers here, each representing a different function. By selecting with the mouse, you can write 0 or 1 into the register accordingly.
WT's technical team modularized and visualized the register configuration, making the functions and configuration clear at a glance. Directly through this host computer, you can send instructions to the board to modify the internal registers. This not only reduces the time to learn registers, but also reduces the problems caused by modifying the source code yourself. Such simple operations are the favorite of engineers. In the register list, you can also see the register value changing in real time.
4) Read the temperature
The module has two types of temperature displays: numerical display and linear graph.
Through hands-on testing, we found that although thermocouples are often used in harsh environments such as industrial equipment and electricity, the EPSH TCM 856 temperature measurement module makes thermocouple temperature measurement very convenient, saving engineers a lot of time.
If anyone is interested in EPSH-TCM856, please contact WT for more information.
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