Thermocouples are a common passive temperature measuring element
Measuring principle
It is also an active sensor that does not require an external power supply and has a wide measuring temperature range (up to 2000°C). It responds quickly, so there is virtually no significant delay in the operation of the system.
Thermocouple structure diagram: The thermocouple structure is simple, mainly composed of two metal wires to form a loop.
In general, the output voltage generated by thermocouples is very small (about 40 μV/°C for K-type), so accurate op amp support is required. Otherwise, external noise, especially when using long wires between the thermocouple and the measurement circuit, can distort the signal. The table below shows some common thermocouple types and characteristics.
Another problem is the "cold end," which is where the thermocouple wire connects to the wire of the signal circuit, like a second thermocouple in the line.
To compensate for the effects of the cold junction, try measuring the temperature of the cold junction and add the resulting voltage to the thermocouple voltage (Vout) so that the voltage sensed by the thermocouple measuring end (Vcj) is officially displayed
Vtc = Vout + Vcj
where Vtc = voltage obtained by the thermoelectric inductance
Vcj = voltage at the "cold junction".
The following is a typical thermocouple compensation circuit. The temperature sensor is located at the cold end for monitoring, and the ADC provides the output data at the required resolution.
(来源:Digi-Key)
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Thermocouple measuring principle
Thermocouple is a common temperature measuring element, the principle of thermocouple is relatively simple, it directly converts the temperature signal into a thermal electromotive force signal, and converts it into the temperature of the measured medium through the electrical instrument, although the principle is simple, but the measurement is not simple.
Thermocouple temperature measurement principle
The thermoelectric potential generated by a thermocouple consists of two parts: the contact potential and the temperature difference potential.
Contact potential: Conductors of two different materials have different electron densities. When the two ends of a conductor of two different materials are joined together, at the junction, electron diffusion takes place, and the rate of electron diffusion is proportional to the density of the free electrons as well as the temperature of the conductor. A potential difference is then formed at the junction, i.e. the contact potential.
Thermoelectric potential: When the temperature of the two ends of a conductor is different, the rate of diffusion of free electrons at the two ends of the conductor is different, which is an electrostatic field between the high and low ends. At this point, a corresponding potential difference is generated on the conductor, which is called the temperature difference potential. This potential is only related to the nature of the conductor and the temperature at both ends of the conductor, and not to the length of the conductor, the size of the cross-section, the temperature distribution along the length of the conductor.
Schematic diagram of a thermocouple
Figure 1 Schematic diagram of a thermocouple
Measurement circuit diagram of a thermocouple
Fig.2. Measurement circuit diagram of a thermocouple
One end that is directly used to measure the temperature of the medium is called the working end (also known as the measuring end) and the other end is called the cold end (also known as the compensation end), and the cold end is connected to a display or companion instrument, which indicates the thermoelectric potential generated by the thermocouple.
In the measurement of actual thermocouples, the measurement circuit of thermocouples is generally composed of thermocouples (conductors A and B), connecting wires C and measuring instruments. When the thermocouple is placed in the measured environment, as shown in Figure 2, three contact potentials of J1, J2, and J3 and two temperature difference potentials are formed, and the potential of the entire thermocouple is composed of these parts.
In order to better understand the electric potential of these parts, we need to understand a law:
The law of intermediate conductors: When an intermediate conductor (third conductor) is connected to a thermocouple loop, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple loop, as long as the temperature at both ends of the intermediate conductor is the same.
First, the 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.
01 Measuring principle
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 principle of temperature measurement is to use the thermoelectric effect (thermoelectric effect refers to a phenomenon in which the electrons in the heated object accumulate 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.
02 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).
03 Signal detection: Thermocouple signal is detected using a 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.
04 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.
05 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.
06 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.
07 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.
08 size
Thermal resistance sensors are comparatively larger in size as compared to thermocouples.
2. 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.
The principle of temperature measurement of RTD
From physics, we know that the resistance value of a conductor (or semiconductor) changes with temperature, and generally speaking, there is a relationship between them as follows, namely
The resistance of a metal conductor generally increases with increasing temperature, and α is a positive value, which is called a positive temperature coefficient of resistance. The semiconductor material used for temperature measurement has a negative α, i.e., it has a negative temperature coefficient of resistance. The α value of various materials is not the same, and for pure metals, it is generally about 0.38%~0.68%. Its size is related to the purity of the conductor itself, the larger the α, the higher the purity of the conductor material.
From the above, it can be seen that the measuring principle of RTD thermometer and thermocouple thermometer is different. The thermocouple thermometer measures the temperature change by converting the thermocouple of the thermometer element into the change of the thermoelectric potential, while the RTD thermometer measures the temperature by converting the temperature change into the change of the resistance value of the thermoresistance of the thermometer.
RTD thermometer is suitable for measuring the surface temperature of liquids, gases, vapors and solids in the low temperature range of -200~+850°C. In addition, its output signal is large and the measurement is accurate, so in 1990 the international temperature scale (ITS-90) stipulates: 13.8033K~961.78 °C temperature range with platinum resistance thermometer as the reference. Materials and requirements for RTD
The mechanism of RTD temperature measurement is to use the property that the resistance value of a conductor or semiconductor changes with temperature, but not all conductors or semiconductor materials can be used as measurement sensors, and they must be considered and selected from other aspects of performance.
1. Stable physical and chemical properties, high measurement accuracy, corrosion resistance and long service life.
2. The temperature coefficient of resistance should be large, that is, the sensitivity should be high.
3. The resistivity should be high, so that the volume of the RTD is smaller, and the time constant of temperature measurement is reduced.
4. The heat capacity should be small, so that the resistive body is less thermally inert and the response is more sensitive.
5. Good linearity, that is, the relationship between resistance and temperature is linear or smooth.
6. Easy to process, cheap price, reduce manufacturing cost.
7. Good reproducibility, easy to batch production and parts exchange.