3. The difference between thermocouple and thermal resistance
1. Basic differences
In temperature measurement, RTDs and thermocouples are both contact temperature measurements. Although they have the same function, they are both used to measure the temperature of an object, but their working principle and usage characteristics are different.
The principle of temperature measurement by RTD thermometers is based on the property that the resistance value of a conductor or semiconductor changes with temperature (the resistance value of a metal conductor increases with increasing temperature). It is a metallic conductor that is usually made of platinum, nickel, or copper. While measuring temperature, a thermal resistance is connected to a circuit, and when an electric current passes through it, its resistance changes with the temperature. Based on the resistance value of the RTD, the temperature of the substance can be calculated.
The thermocouple thermometer uses the thermoelectric effect (thermoelectric effect refers to a phenomenon in which the electrons in the heated object accumulate current or charge when they move from the high temperature region to the low temperature region with the temperature gradient).
A thermocouple is a sensor made up of two different metal wires that create a voltage difference at their contact. When these wires come into contact with substances of different temperatures, the magnitude of the voltage difference changes. The magnitude of this change can be measured to deduce the temperature of the substance.
2. Measuring range
The measurement range of RTD thermometer is relatively low, generally used to measure medium and low temperatures, generally between -200-600 °C, he is characterized by high accuracy, when measuring medium and low temperatures, the output signal is much larger than that of thermocouples, high sensitivity, can realize remote transmission, automatic recording and multi-point measurement. RTD thermometers are not accurate in high temperature (>850°C) measurements, are susceptible to oxidation and are not resistant to corrosion.
The thermocouple thermometer has a relatively high measurement range, generally between -200-2000°C, but when measuring low temperatures, temperature compensation is required, and the measurement accuracy of the low temperature section is low. Some special thermocouples can be used to measure as low as -269 degrees (such as gold, iron, nickel and chromium) and up to 2800 degrees (such as tungsten and rhenium).
3. The thermocouple signal is detected by millivolt meter.
Cold junction temperature and linearity compensation are added when needed. Resistance signals are detected using a resistance meter, with added linearity compensation if needed.
4. Accuracy
RTD provides high accuracy and may be the preferred solution when temperature measurement accuracy is required to be around ±0.05 to ±0.1°C. In contrast, thermocouples have a lower accuracy of about ±0.2 to ±0.5°C.
5. Sensitivity
Although a thermocouple sensor system typically has a faster response time due to temperature changes at its contact points, it also typically takes longer to reach thermal equilibrium. This is mainly due to the presence of cold junction compensation, which does not respond as quickly to temperature changes as the hot junction located at the tip of the sensor. In contrast, RTD sensors are designed to be more durable and react more quickly to temperature changes.
6. Reading drift
Due to the design of RTD sensors, there is little drift, which allows them to produce stable readings for longer periods of time than thermocouples. Unlike RTD sensors, thermocouples have relatively high drift times, which are often due to wire inhomogeneity due to thermal and chemical exposure or mechanical damage. Therefore, thermocouples need to be calibrated frequently.
7. Cost-of-use thermocouples are usually cheaper than RTD sensors.
Because most thermocouples cost between half and a third of the resistance of a thermal resistance. However, as mentioned earlier, thermocouples need to be adjusted and calibrated regularly, which adds to the long-term cost of the product in addition to longer installation and setup times.
8. Size
Thermal resistance sensors are comparatively larger in size as compared to thermocouples.
9. Advantages and disadvantages
RTDs and thermocouples have their own advantages and disadvantages, and when choosing between the two temperature sensors, you should take into account the differences between them and choose the right thermometer according to your measurement needs.
Measuring 600~1300 °C temperature range, thermocouple is ideal, but for the measurement of medium and low temperatures, thermocouples have certain limitations, this is because the thermocouple in the low temperature area output thermoelectric potential is very small, the quality of the instrument is higher, such as platinum rhodium-platinum thermocouple at 10O °C temperature at the thermoelectric potential is only 0.64mV, such a small thermoelectric potential to the amplifier and anti-interference requirements of the electronic potentiometer are very high, the maintenance of the instrument is also difficult, in addition, the thermocouple cold end temperature compensation problem, in the range of medium and low temperature influence is more prominent, on the one hand, to take temperature compensation will inevitably increase the inconvenience of work, on the other handIf the cold junction temperature can not be fully compensated, its influence will be greater, and at low temperatures, the linearity of thermoelectric characteristics is poor, and certain measures must be taken when adjusting the temperature, which are the shortcomings of thermocouples in temperature measurement. For this reason, another type of measuring element is often used in the industry to measure low temperatures, i.e. thermal resistance. The measurement range of RTD thermometer is -20O°C~+850°C.
The biggest advantages of RTD thermometers are:
The measurement accuracy is high, there is no cold end compensation problem, and it is especially suitable for low temperature measurement, so it is widely used in industry. The platinum resistance thermometer can measure -200°C, and the indium resistance temperature can measure the low temperature of 3.4 K. The disadvantages are that it cannot measure too high temperatures, that it requires an external power supply, so its use is limited, and that the resistance of the connected wires is easily affected by the ambient temperature, which can lead to measurement errors.
Fourth, on-site judgment in the work
1. Thermocouple
The thermocouple has positive and negative poles, and the compensation wire also has positive and negative points. In operation, the common ones are short circuit, open circuit, poor contact (there is a multimeter to judge) and deterioration (according to the surface color to identify). If it is normal, please check the process).
2. Thermal resistance
Thermal resistance short circuit and open circuit can be judged with a multimeter, in operation, suspect short circuit, as long as the resistor end is removed from a wire head to see the display instrument, such as to the maximum, the thermal resistance short circuit back to zero, wire short circuit, to ensure normal connection and configuration, the table value shows low or unstable, the possibility of water ingress into the protective tube shows the maximum, and the thermal resistance open circuit shows the minimum short circuit.
5. Summary
1. What is the measurement principle of thermocouples?
The working principle of thermocouples is based on the Seeback effect, which is a physical phenomenon in which two ends of conductors of different compositions are connected to form a loop, and if the two connections have different temperatures, a thermal current is generated in the loop.
A thermocouple consists of two different wires (thermoelectrodes) that are welded to each other at one end to form the measuring end (also known as the working end) of the thermocouple. It is inserted into the medium at which the temperature is to be measured, while the other end of the thermocouple (reference or free) is connected to the display instrument. If there is a temperature difference between the measuring and reference ends of the thermocouple, the display gauge will indicate the thermal electromotive force generated by the thermocouple.
Thermocouples are currently generally available in K, B, and S grades, representing different materials for different temperature ranges. For example, type K is nickel-chromium-nickel-silicon material, which is generally measured at 0-800 degrees, and type B is platinum rhodium 30-platinum rhodium 6, which is generally measured at 800-1600 degrees.
2. What is the measurement principle of RTD?
Thermal resistance measures the temperature by using the resistance of a metal conductor or semiconductor when the temperature changes, and the heating part of the resistance (temperature sensing element) is formed on the substrate by a thin metal wire evenly wound around the frame made of insulating material or by laser sputtering. When there is a temperature gradient in the measured medium, the measured temperature is the average temperature of the dielectric layer in the range where the temperature sensing element is located.
As the temperature changes, the resistance of the resistance changes, and the thermoelectric potential of the thermocouple changes.
At present, the thermal resistance adopts copper thermal resistance and platinum thermal resistance, which are divided into different graduation numbers according to the different thermal resistance values at 0 degrees, such as PT100, PT1000, CU50, etc., taking PT100 as an example, PT represents platinum, and the resistance value of thermal resistance is 100 ohms when 100 degrees represents 0 degrees.
3. What is an armored thermocouple and what are its advantages?
In the IEC1515 standard, the name is "mineral insulated thermocouple cable", that is, inorganic mineral insulated thermocouple cable. The thermoelectrode, insulator and sheath are formed by drawing them as a whole, and the outer surface seems to be covered with a layer of "armor", so it is called an armored thermocouple. Compared with general prefabricated thermocouples, it has the advantages of high pressure resistance, good bendability, good oxidation resistance and long service life.
4. What are the indexing numbers of thermocouples, and what are their characteristics?
热电偶的分度号有主要有S、R、B、N、K、E、J、T等几种。 其中S、R、B属于贵金属热电偶，N、K、E、J、T属于廉金属热电偶。
The S index number is characterized by strong oxidation resistance, and it is suitable for continuous use in oxidizing and inert atmospheres, with a long-term use temperature of 1400 °C and a short-term temperature of 1600 °C. Of all thermocouples, the S-graduation number has the highest accuracy class and is often used as a standard thermocouple;
Compared with the S index, the R index number is about 15% larger except for the thermal electromotive force, and the other properties are almost identical;}
The thermal electromotive force (EMF) of the B index is extremely small at room temperature, so the compensation wire is generally not required for measurement. It has a long-term use temperature of 1600°C and a short-term temperature of 1800°C. It can be used in oxidizing or neutral atmospheres, and can also be used for short periods of time under vacuum conditions.
The characteristics of the N index number are strong oxidation resistance at high temperature at 1300 °C, good long-term stability of thermal electromotive force and reproducibility of short-term thermal cycle, good resistance to nuclear radiation and low temperature resistance, and can partially replace the S index thermocouple;
The K graduation number is characterized by strong oxidation resistance, and it is suitable for continuous use in oxidizing and inert atmospheres, with a long-term use temperature of 1000 °C and a short-term temperature of 1200 °C. The most widely used of all thermocouples;
The E-index number is characterized by the highest thermal electromotive force, i.e., the highest sensitivity, among commonly used thermocouples. It is suitable for continuous use in oxidizing and inert atmosphere, and the operating temperature is 0-800°C;
The characteristics of the J index number are that it can be used in both oxidizing atmosphere (upper limit of 750 °C) and reducing atmosphere (upper limit of 950 °C), and is resistant to H2 and CO gas corrosion, and is mostly used in oil refining and chemical industry;
The T-graduation number is characterized by the highest accuracy class of all inexpensive metal thermocouples and is typically used to measure temperatures below 300°C.
5. How many ways are there for the lead wire of the thermal resistance, and what are the effects?
There are three types of RTD lead outs: 2-wire system, 3-wire system, and 4-wire system.
The wiring of the 2-wire RTD is simple, but it has to bring in the additional error of the lead resistor. Therefore, it is not suitable for manufacturing A-level precision RTD, and the lead wire and wire should not be too long when used.
The 3-wire system eliminates the influence of lead resistance, and the measurement accuracy is higher than that of the 2-wire system. It is the most widely used as a process sensing component.
The 4-wire system not only eliminates the effect of the lead resistor, but also eliminates the effect of the resistance of the connected wire when the resistance of the wire is the same. For high-precision measurements, a 4-wire system is used.
6. What are the advantages and disadvantages of N-type thermocouples compared with K-type thermocouples?
Advantages of Type N Thermocouples:
-High temperature oxidation resistance and long-term stability. The selective oxidation of Cr and Si elements in the positive electrode of K-type thermocouple nickel-chromium causes the uneven alloy composition and thermoelectromotive force drift, etc., and the increase of Cr and Si content in N-type thermocouple makes the oxidation mode of nickel-chromium alloy change from internal oxidation to external oxidation, resulting in the oxidation reaction only on the surface.
-Good stability of low-temperature short-term thermal cycling, and inhibition of magnetic transition;
- High resistance to nuclear radiation. The N-type thermocouple cancels the metamorphosis elements Mn and Co in the K-type, which further strengthens the anti-neutron irradiation ability.
-In the range of 400~1300°C, the linearity of the thermoelectric characteristics of N-type thermocouples is better than that of K-type.
Disadvantages of Type N Thermocouples:
-The material of N-type thermocouples is harder than that of K-type thermocouples, which is more difficult to machine;
- Relatively expensive price. The coefficient of thermal expansion of N-type thermocouple is 15% lower than that of stainless steel, so the outer sleeve of N-type armored thermocouple should be made of NiCrSi/NiSi alloy, and the nonlinearity error is large in the range of -200~400°C.
7. How to choose thermocouples and RTDs?
According to the temperature measurement range: thermocouples are generally selected above 500°C, and thermal resistance is generally selected below 500°C;
Selection according to measurement accuracy: thermal resistance is selected for high accuracy requirements, and thermocouples are selected for low accuracy requirements;
According to the measurement range, the temperature measured by thermocouples generally refers to the "point" temperature, and the temperature measured by thermal resistance generally refers to the spatial average temperature.