的作用是来延伸热电极即移动热电偶的冷端与显示仪表联接构成的测温系统。 The function of the compensation wire is to extend the temperature measurement system consisting of the hot electrode, that is, the cold end of the mobile thermocouple and the display instrument. Compensation wires are often used in combination with thermocouples.
The material of the cover is not the same for the environment. Sheath materials are polyvinyl chloride, fluoroplastic, low-smoke and low-halogen polyvinyl chloride, and alkali-free glass filament. Imported high-quality fluoroplastics are feasible with a temperature resistance of 260 ° C.
The new process makes the compensation wire have excellent acid resistance, alkali resistance, wear resistance and non-combustibility.
According to the temperature measurement principle of the thermocouple, it can be known that the thermoelectric potential generated by the thermocouple is related to the thermoelectric potential of the hot end (also called the measurement end) and the reference end (also called the cold end). Only the reference end temperature t1 is zero or constant. Change, the thermoelectric potential is the hot end temperature
Single value function graph. If it is not compensated, the larger the temperature difference t1-t2 between the reference temperature of the thermocouple and the temperature t2 of the instrument terminal, the greater the measurement error. Since the relationship between the thermoelectric potential and temperature of most thermocouples is approximately
Measurement, so the measurement error caused is approximately equal to the above temperature difference. Take K-chromium-nickel-nickel-silicon thermocouple as an example, when t1 = 50 ℃ and t2 = 20 ℃, if the hot-end temperature is 1000 ℃, the display temperature is only 969 ℃, the error is up to
31 ° C.
In practical applications, since the junction box of the thermocouple reference end is usually exposed to the atmosphere, the temperature changes greatly. If no measures are taken, the temperature in the junction box can neither be zero nor keep a certain temperature constant. This quote
Measurement error. Because secondary instruments (such as monitors, recorders) and I / O cards connected to thermocouples have ambient temperature compensation, the temperature t2 of the junction point (ie, the instrument terminal) of these devices and the thermocouple can be compensated. thus
It can be seen that the key is how to compensate the reference junction temperature t1 of the thermocouple. At present, there are various reference junction compensation methods, such as constant temperature method, compensated bridge method, compensated thermocouple method, compensated wire method, etc., but the most commonly used method is the compensated wire method.
This article first describes the principle and classification of compensation wires, and then introduces several issues that need to be understood in the application of compensation wires.
的工作原理及分类 Second, the working principle and classification of compensation wires
1.Working principle of compensation wire
In a certain temperature range, the wires whose thermoelectric performance is close to that of thermocouples are called thermocouple compensation wires.
According to the thermocouple intermediate temperature rule, the total potential value of the thermocouple temperature measurement circuit is only related to the temperature of the hot end and the reference end, and is not affected by the change of the intermediate temperature. Therefore, a compensation wire matching the thermocouple material can be used instead. need
For the valuable thermocouple material to be extended, the reference end is extended from the thermocouple junction box to the instrument terminal, and the original reference end temperature is compensated by the compensation wire.
In addition to reducing the measurement error, the compensation wire also has the following advantages: it can improve the physical and mechanical properties of the thermocouple temperature measurement line. For example, the use of multi-core wire or small diameter compensation wire can improve the flexibility of the line and make the connection
Convenient and easy to shield external interference; can reduce the cost of the measurement line.
2. Classification of compensation wires
In principle, it is divided into extended type and compensated type. The nominal chemical composition of the extended type's alloy wire is the same as that of the thermocouple used. Therefore, the thermoelectric potential is also the same. With
Thermocouples are different, but within its operating temperature range, the thermoelectric potential is close to the nominal value of the thermoelectric potential of the thermocouple used, which is represented by "C" in the model.
Compensation accuracy is divided into ordinary and precision grades. The precision after compensation of precision grades is generally only half that of ordinary grades, and is usually used in places where measurement accuracy is required. For example, S, R indexing compensation wire, precision tolerance is
± 2.5 ℃, the tolerance of ordinary grade is ± 5.0 ℃; the compensation lead of K, N graduation number, the tolerance of precision grade is ± 1.5 ℃, the tolerance of ordinary grade is ± 2.5 ℃. In the model, the standard grade is not marked, and the precision grade is marked with "S".
The working temperature is divided into general use and heat resistance. Generally, the working temperature is 0 to 100 ° C (a few are 0 to 70 ° C); the heat resistance is 0 to 200 ° C.
In addition, the core can be divided into single-strand and multi-core (flexible wire) compensation conductors, divided into ordinary and shielded compensation conductors with or without a shield, and there are compensation conductors for intrinsically safe circuits dedicated to explosion-proof applications.
2. Judgment of the index number and polarity of the compensation wire
Sometimes it can be judged according to the material, insulation layer and sheath color of the compensation wires listed in the data. However, due to the differences between the old and new standards and IEC standards, it is often difficult to accurately determine the graduation number and polarity of the compensation wires with this method.
的两端剥去绝缘层，把两根导线绞合在一起制成热电偶的热端，放到沸腾的水中，两根导线的另一端与直流电位 The most reliable and commonly used method is the test method, which is to peel off the insulation layer at both ends of the compensation wire , twist the two wires together to make the hot end of the thermocouple, and place it in boiling water. The other end of the two wires With DC potential
The differential meter is connected (it should not be connected with a moving coil direct reading mV meter, because the current reading is low when measuring). Compare the measured thermoelectric potential with Table 1, and the closest is the graduation of the compensation wire. No. according to the sign of the potentiometer
The pole determines the polarity of the compensation lead. Because the reference junction temperature of the thermocouple formed by the compensating wire is not necessarily 0 ° C, for example, 20 ° C, the measured thermoelectric potential is lower than the thermoelectric value of the reference junction at 0 ° C. In some unknown
For example, if the reference lead temperature is about 20 ° C and the measured value is in the range of 3.928 ± 0.150mV, the graduation number of this compensation lead is K. 3.928 is a K-scale thermocouple at 100 ℃ and 20 ℃
The difference of thermoelectric potential, 0.150 is the tolerance of ordinary grade compensation wire of K division number.
3. Position of compensation wire instrument panel connection point
We know that the compensation wire only extends the reference end of the thermocouple to move the position of the reference end. The extended reference end temperature should be constant or equipped with a device that automatically compensates for the reference end temperature, otherwise can
Can cause measurement errors due to new reference junction temperature changes.
For example, when wiring in the instrument panel, the temperature at the terminal of the instrument panel is higher than the temperature at the terminal of the instrument panel, because the panel-mounted display and the recorder itself generate heat due to power on. When the thermocouple's compensation wire is introduced into the instrument panel,
If it is connected to the terminals of the instrument panel, and the terminals of the instrument panel and the instrument terminals are connected by copper wire, the measurement error will be caused due to the above temperature difference. So it ’s best to run the compensation leads across the terminals of the dashboard.
The sub is directly connected to the terminal of the meter.
4.Line resistance of compensation wire
For early dynamic coil instruments equipped with thermocouples, there were two line resistance requirements of 5Ω and 15Ω. When the installation location of the thermocouple is far from the moving coil meter, or the index numbers K, N, E, J, T and other compensation guides containing copper-nickel materials
When wiring, its line resistance is relatively large. When selecting, pay attention to select a compensation wire with a larger cross section. For example, when a dynamic coil meter with an external 15 Ω line resistance E graduation number is selected, the compensating conductor cross section is 1.0 mm2, 2.5 mm2,
While the corresponding unit length line resistance is 1.25Ω / m and 0.5Ω / m, respectively, the maximum allowable length of the compensation wire is only 12 m and 30 m. If you do not pay attention to the design, this length can easily be exceeded, causing measurement errors.
5, R, S indexing thermocouple compensation wire
The thermocouples also known as platinum-rhodium-platinum have two indexing numbers, R and S, which respectively represent platinum-rhodium 13-platinum and platinum-rhodium 10-platinum thermocouples. The former is less widely used in China, but its thermoelectric potential is larger ( Thermoelectric potential analysis of R and S indexed thermocouples at 1600 ° C
(The difference is 18.849mV and 16.777 mV), and the two are basically the same at low temperature (100 ° C) (the thermoelectric potentials of R and S graduation numbers are 0.647 mV and 0.646 mV, respectively), and slightly different at 200 ° C (R, S minutes Thermoelectric potential in degrees
(Respectively 1.467 mV and 1.441 mV), so at present, the compensation wires of the R and S graduation numbers on the domestic market are universal. For example, the S-indexed compensation wire usually purchased on the market is used for the R-indexed thermocouple at 100 ° C.
的极限温度200℃，当热电偶的热端温度分别为600℃、1000℃、1300℃时，所引起的误差仅为2.5℃、2.2℃、2.0℃。 There is no error below, even when the limit temperature of the heat-resistant compensation wire is 200 ° C, when the hot-end temperature of the thermocouple is 600 ° C, 1000 ° C, and 1300 ° C, the errors caused are only 2.5 ° C, 2.2 ° C, 2.0 ° C. .
This is a special case of Section 1.
Among the commonly used thermocouples, the accuracy of the R and S graduated number compensation leads is the lowest, but from the temperature range of use, the error is small in the range of 0 to 60 ° C, and the error of 100 to 150 ° C is relatively large. When measurement error requirements are high,
The temperature of the reference junction must be kept below 100 ° C.