Among various measurement techniques, temperature measurement may be the most common one, because any application field, grasping the exact numerical value of temperature, understanding the difference between temperature and actual condition, etc., are of extremely important significance. . Take measurement as an example. In the measurement of force, pressure, flow, position and level, etc., in order to improve the measurement accuracy, it is usually required to monitor the temperature, such as the measurement of pressure or force. Stern resistance bridge, but the error caused by the temperature change of the resistance of the constituent bridges will often greatly exceed the resistance value change caused by the force to be measured. If the temperature is not monitored and the measurement result is corrected accordingly, the measurement is completely impossible. Or no effect. Other parameter measurements have similar problems. It can be said that various physical quantities are functions of temperature. To obtain accurate measurement results, accurate corrections must be made for temperature changes. This article is to help readers choose the most appropriate temperature sensor for specific applications and perform accurate temperature measurements.
There are four types of temperature sensors commonly used in the industry: thermocouples, RTDs, thermistors, and integrated circuit temperature sensors; each type of temperature sensor has its own unique temperature measurement range and has its own applicable temperature environment; there is no The temperature sensor can be used for all purposes: The thermocouple has the widest range of measurable temperatures, the thermal resistance has the best linearity, and the thermistor has the highest measurement accuracy. Table 1 shows the unique performance characteristics and comparisons of the four types of sensors. Table 2 is typical application fields of the four types of sensors.
A thermocouple - a universal and economical thermocouple consisting of two different metallic wires, welded together at one end, as shown in Figure 1; the reference junction temperature (also called the cold compensating end) is used to eliminate the iron-copper association and The error contributed by the copper-copper junction; and the two different metal welding ends are placed on the target that needs to measure the temperature.
After the two materials are connected in this way, a voltage is generated at the unwelded end, and the voltage value is a function of the temperature of all the connected terminals. The thermocouple does not require voltage or current excitation. In practical applications, if an attempt is made to provide voltage or current excitations, errors will be introduced into the system.
Since the thermocouple voltage is generated at the open end of two different wires, the interface to the outside seems to be achieved by measuring the voltage directly between the two wires; if the ends of the thermocouple are not connected to another metal, usually copper , then things will really be simple.
However, the fact that thermocouples need to be connected to another type of metal actually creates a new pair of thermocouples, introducing a great deal of error into the system. The only way to eliminate this error is to check the temperature at the reference end (see the figure 1) The error contributed by this connection is subtracted in a hardware or hardware-software combination. The pure hardware elimination technique is more limited than the software-hardware combination technique due to the linearization correction factor. In general, accurate detection of the reference temperature is performed using RTDs, thermistors, or integrated circuit temperature sensors. In principle, a thermocouple can be constructed from any two different metals, but in practice, the two metal combinations that make up the thermocouple have been standardized because of the linearity of the standard combination and the resulting voltage-temperature relationship ideal.
Table 3 and Figure 2 show the characteristics of the commonly used thermocouples E, J, T, K, N, S, and BR.
Thermocouples are highly nonlinear devices that require strong linearization algorithms. The Seebeck coefficient in Table 3 is the average drift of a certain thermocouple at the specified temperature.
When thermocouples are delivered, their performance is guaranteed by the manufacturer according to the NIST 175 standard (this standard has been adopted by ASTM). The standard specifies the thermocouple temperature characteristics and the quality of the raw materials used. Compared with RTDs, thermistors, and integrated circuit silicon sensors, the nonlinearity of thermocouples is extremely severe. Therefore, in the circuit part, complex algorithm processing is required. Table 4 shows an example of a complex algorithm. This is The temperature coefficient of a K-type thermocouple can be linearized from 0 to 1372 degrees. These coefficients are applied to the following equations:
Where: V is the voltage across the thermocouple;
T is the temperature The other application of these complex calculation methods is to make a look-up table in the processing program, so that the coefficient calculation table of the K-type thermocouple listed in Table 4 is a set of 11X14 array decimal numbers. 0.000-13.820;
In addition, the thermocouple can determine the value of the temperature due to a certain function of the reference temperature. (The reference temperature is defined as the temperature of the distal end of the thermocouple wire relative to the soldering end of the thermocouple wire. The RTD is usually used. Thermistor or silicon integrated circuit sensor.
Compared with thermistors RTD and thermistors, the thermal mass of the thermocouple is small, so its response speed is faster. Due to its wide temperature detection range, this type of temperature sensor is almost unique in some harsh environments.
Thermocouple error analysis Thermocouples are low cost, structurally strong, and small compared to other temperature sensors; however, any stress on the material, such as bending, stretching, and compression, can alter the thermal gradient characteristics; in addition, the corrosive media can penetrate. Its insulating sheath causes changes in its thermodynamic properties. It is advisable to add a protective tube to the thermocouple, such as a ceramic tube for high temperature protection. The metal heat sink can also provide mechanical protection. Thermocouple voltages have voltage drops along the length of two different metals, but this does not mean that a shorter thermocouple will have a different Seebeck coefficient than a larger thermocouple.
The short wire length will certainly make the temperature gradient steep, but from the point of view of the conductive effect, the thermocouple with a longer wire length has its own advantages. At this time, the temperature gradient will be smaller, but the conduction loss will also decrease; but from The negative effect of the long wire is that the output voltage of the long wire thermocouple is small, which increases the burden of the subsequent signal conditioning circuit.
In addition to the small output signal, the linearity of the device needs a large amount of calibration, usually implemented in hardware and software. If it is implemented in hardware, an absolute temperature reference is needed as a cold-side reference. If it is implemented in software, then it is compared. Table or polynomial calculations to reduce thermocouple errors. Finally, electromagnetic interference couples into this two-wire system; small wire gauge wire can be used for high temperature detection and has a longer life, but if sensitivity is the most important factor, large wire gauge wire measurement performance is better.
In general, thermocouples have become a popular choice for temperature measurement due to their large measurable temperature range, high mechanical strength, and low price. The linearity and accuracy required by high-precision systems are not easy to achieve. If the accuracy is more demanding, other temperature sensors should be selected.
Thermal resistance RTD - Absolute substitution of thermocouples The technology of thermistor temperature-sensing elements continues to improve, and the quality of temperature measurements continues to increase, but to truly achieve high-quality, high-accuracy temperature measurement systems, thermal resistance devices The choice is still extremely important. Thermal resistance is a resistive element made of metal, such as platinum, nickel, copper, etc. The selected metal must have a predictable resistance value that varies with temperature, its physical properties must be easy to manufacture, and the temperature coefficient of resistance must be Large enough so that its resistance changes with temperature and is easy to measure accurately. Other temperature-sensing devices, such as thermocouples, do not allow the designer to have a fairly linear resistance change with temperature, and the excellent linearity of the RTDs greatly simplifies the design of signal processing circuits. Figure 5 shows the temperature resistance characteristics of a thermal resistor, in which the platinum resistor has the most accurate and reliable temperature resistance characteristics among the three metals.
Therefore, platinum resistance is most suitable for applications requiring the highest absolute accuracy and reproducibility. Its sensitivity to the environment is extremely low. Compared to this, copper resistance is prone to corrosion, and its long-term stability is poor, while nickel resistance is not tolerated by the environment. Yes, but the applicable temperature range is narrow.
The platinum resistance has good linearity and temperature-responsiveness to the temperature, and it is chemically inert. It can easily be processed into thin wire rods or foils with a small thickness. The platinum's resistivity is higher than that of other thermal resistance materials. When the resistance values ​​are the same, the material is required. Less, suitable for cost considerations, and thermal response.
The thermal response speed of the platinum resistance affects the measurement time. It also depends on the housing of the resistor and its size. The size of the component itself is small, and the size of the housing can also be made smaller. In general, the response speed of the platinum thermal resistance is It is faster than a temperature sensor made of semiconductors.
The absolute value of the thermal resistance at zero degrees Celsius is very large and can be specified by the user. For example, the standard resistance of the platinum resistance is 100 ohms, but there are 50, 100, 200, 5001000 or 2000 ohms and other resistances.
General points:
1. Temperature Sensors Overview: Applications, Importance;
2. Horizontal comparison of the four main types of temperature sensors. Thermocouple sensor 4. Thermal resistance sensor 5. Thermistor sensor 6. IC Temperature Sensor and Typical Product Example 7. The correct choice and application of temperature sensor
There are four types of temperature sensors commonly used in the industry: thermocouples, RTDs, thermistors, and integrated circuit temperature sensors; each type of temperature sensor has its own unique temperature measurement range and has its own applicable temperature environment; there is no The temperature sensor can be used for all purposes: The thermocouple has the widest range of measurable temperatures, the thermal resistance has the best linearity, and the thermistor has the highest measurement accuracy. Table 1 shows the unique performance characteristics and comparisons of the four types of sensors. Table 2 is typical application fields of the four types of sensors.
A thermocouple - a universal and economical thermocouple consisting of two different metallic wires, welded together at one end, as shown in Figure 1; the reference junction temperature (also called the cold compensating end) is used to eliminate the iron-copper association and The error contributed by the copper-copper junction; and the two different metal welding ends are placed on the target that needs to measure the temperature.
After the two materials are connected in this way, a voltage is generated at the unwelded end, and the voltage value is a function of the temperature of all the connected terminals. The thermocouple does not require voltage or current excitation. In practical applications, if an attempt is made to provide voltage or current excitations, errors will be introduced into the system.
Since the thermocouple voltage is generated at the open end of two different wires, the interface to the outside seems to be achieved by measuring the voltage directly between the two wires; if the ends of the thermocouple are not connected to another metal, usually copper , then things will really be simple.
However, the fact that thermocouples need to be connected to another type of metal actually creates a new pair of thermocouples, introducing a great deal of error into the system. The only way to eliminate this error is to check the temperature at the reference end (see the figure 1) The error contributed by this connection is subtracted in a hardware or hardware-software combination. The pure hardware elimination technique is more limited than the software-hardware combination technique due to the linearization correction factor. In general, accurate detection of the reference temperature is performed using RTDs, thermistors, or integrated circuit temperature sensors. In principle, a thermocouple can be constructed from any two different metals, but in practice, the two metal combinations that make up the thermocouple have been standardized because of the linearity of the standard combination and the resulting voltage-temperature relationship ideal.
Table 3 and Figure 2 show the characteristics of the commonly used thermocouples E, J, T, K, N, S, and BR.
Thermocouples are highly nonlinear devices that require strong linearization algorithms. The Seebeck coefficient in Table 3 is the average drift of a certain thermocouple at the specified temperature.
When thermocouples are delivered, their performance is guaranteed by the manufacturer according to the NIST 175 standard (this standard has been adopted by ASTM). The standard specifies the thermocouple temperature characteristics and the quality of the raw materials used. Compared with RTDs, thermistors, and integrated circuit silicon sensors, the nonlinearity of thermocouples is extremely severe. Therefore, in the circuit part, complex algorithm processing is required. Table 4 shows an example of a complex algorithm. This is The temperature coefficient of a K-type thermocouple can be linearized from 0 to 1372 degrees. These coefficients are applied to the following equations:
Where: V is the voltage across the thermocouple;
T is the temperature The other application of these complex calculation methods is to make a look-up table in the processing program, so that the coefficient calculation table of the K-type thermocouple listed in Table 4 is a set of 11X14 array decimal numbers. 0.000-13.820;
In addition, the thermocouple can determine the value of the temperature due to a certain function of the reference temperature. (The reference temperature is defined as the temperature of the distal end of the thermocouple wire relative to the soldering end of the thermocouple wire. The RTD is usually used. Thermistor or silicon integrated circuit sensor.
Compared with thermistors RTD and thermistors, the thermal mass of the thermocouple is small, so its response speed is faster. Due to its wide temperature detection range, this type of temperature sensor is almost unique in some harsh environments.
Thermocouple error analysis Thermocouples are low cost, structurally strong, and small compared to other temperature sensors; however, any stress on the material, such as bending, stretching, and compression, can alter the thermal gradient characteristics; in addition, the corrosive media can penetrate. Its insulating sheath causes changes in its thermodynamic properties. It is advisable to add a protective tube to the thermocouple, such as a ceramic tube for high temperature protection. The metal heat sink can also provide mechanical protection. Thermocouple voltages have voltage drops along the length of two different metals, but this does not mean that a shorter thermocouple will have a different Seebeck coefficient than a larger thermocouple.
The short wire length will certainly make the temperature gradient steep, but from the point of view of the conductive effect, the thermocouple with a longer wire length has its own advantages. At this time, the temperature gradient will be smaller, but the conduction loss will also decrease; but from The negative effect of the long wire is that the output voltage of the long wire thermocouple is small, which increases the burden of the subsequent signal conditioning circuit.
In addition to the small output signal, the linearity of the device needs a large amount of calibration, usually implemented in hardware and software. If it is implemented in hardware, an absolute temperature reference is needed as a cold-side reference. If it is implemented in software, then it is compared. Table or polynomial calculations to reduce thermocouple errors. Finally, electromagnetic interference couples into this two-wire system; small wire gauge wire can be used for high temperature detection and has a longer life, but if sensitivity is the most important factor, large wire gauge wire measurement performance is better.
In general, thermocouples have become a popular choice for temperature measurement due to their large measurable temperature range, high mechanical strength, and low price. The linearity and accuracy required by high-precision systems are not easy to achieve. If the accuracy is more demanding, other temperature sensors should be selected.
Thermal resistance RTD - Absolute substitution of thermocouples The technology of thermistor temperature-sensing elements continues to improve, and the quality of temperature measurements continues to increase, but to truly achieve high-quality, high-accuracy temperature measurement systems, thermal resistance devices The choice is still extremely important. Thermal resistance is a resistive element made of metal, such as platinum, nickel, copper, etc. The selected metal must have a predictable resistance value that varies with temperature, its physical properties must be easy to manufacture, and the temperature coefficient of resistance must be Large enough so that its resistance changes with temperature and is easy to measure accurately. Other temperature-sensing devices, such as thermocouples, do not allow the designer to have a fairly linear resistance change with temperature, and the excellent linearity of the RTDs greatly simplifies the design of signal processing circuits. Figure 5 shows the temperature resistance characteristics of a thermal resistor, in which the platinum resistor has the most accurate and reliable temperature resistance characteristics among the three metals.
Therefore, platinum resistance is most suitable for applications requiring the highest absolute accuracy and reproducibility. Its sensitivity to the environment is extremely low. Compared to this, copper resistance is prone to corrosion, and its long-term stability is poor, while nickel resistance is not tolerated by the environment. Yes, but the applicable temperature range is narrow.
The platinum resistance has good linearity and temperature-responsiveness to the temperature, and it is chemically inert. It can easily be processed into thin wire rods or foils with a small thickness. The platinum's resistivity is higher than that of other thermal resistance materials. When the resistance values ​​are the same, the material is required. Less, suitable for cost considerations, and thermal response.
The thermal response speed of the platinum resistance affects the measurement time. It also depends on the housing of the resistor and its size. The size of the component itself is small, and the size of the housing can also be made smaller. In general, the response speed of the platinum thermal resistance is It is faster than a temperature sensor made of semiconductors.
The absolute value of the thermal resistance at zero degrees Celsius is very large and can be specified by the user. For example, the standard resistance of the platinum resistance is 100 ohms, but there are 50, 100, 200, 5001000 or 2000 ohms and other resistances.
General points:
1. Temperature Sensors Overview: Applications, Importance;
2. Horizontal comparison of the four main types of temperature sensors. Thermocouple sensor 4. Thermal resistance sensor 5. Thermistor sensor 6. IC Temperature Sensor and Typical Product Example 7. The correct choice and application of temperature sensor
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