Correct use of thermocouple compensation wires in temperature measurement

Thermocouple compensation wires have been widely used in thermocouple temperature measurement. If you understand the principle, function, method of action, and precautions of thermocouple compensation wires, you can fully utilize the function of thermocouple compensation wires, otherwise it will backfire. In industrial production, although thermocouples have been widely used as temperature sensors for temperature measurement and control, and people are familiar with them, if the correct usage method is not paid attention to, it can cause significant deviation in temperature measurement and control, and in severe cases, directly cause economic losses. Therefore, it should be taken seriously.
1, Introduction to the Temperature Measurement Principle of Thermocouples
A circuit composed of two different homogeneous materials A and B is called a thermocouple. A. The contact points connected at both ends of material B are represented by J1 and J2 respectively. If the temperature T1 and T2 of the contact points of J1 and J2 are different, an electric potential will be generated in the circuit, which is usually referred to as a thermoelectric potential. When A The material of B - timing, the magnitude of thermoelectric potential depends on the temperature difference between T1 and T2, expressed by the formula as
EAB(T1,T2)=eAB(T1)+eBA(T2)=eAB(T1)-eAB(T2)(1)
In the formula: EAB (T1, T2) material is A The thermocouple of B has a thermoelectric potential difference between the junction temperatures T1 and T2.
EAB (T1) - A The potential at point B when the temperature is T1.
EAB (T2), eBA (T1) - A The potential at point B temperature T2 is equal in magnitude and opposite in sign.
In order to unify thermocouple materials and standardize them, relevant national standards have stipulated the composition of thermocouple material A The components of B Purity and provided A The combination form of material B is named with a single letter, such as K-type, S-type, etc. For the convenience of use, the temperature values and potential relationships of various types of thermocouples are unified as the potential values relative to 0C, represented here as T0, and various types of thermocouple calibration tables are made for easy reference and calculation.
Compared to the form in Figure 1, formula (1) is transformed into
EAB(TI,T2)=EAB(T1,T0)-EAB(T2,T0)(2)
Formula (2) is the practical formula we currently use, as long as we know T1 T2, EAB (TI, T0) and EAB (T2, T0) can be found from the graduation table.
Two thermocouple compensation wires
Firstly, let's analyze the connection conductor law and intermediate temperature law of thermocouples.
In practical applications, there is always a distance between measuring and controlling instruments and thermocouples. The materials C and D in the middle are also two homogeneous materials. According to the law of intermediate conductor in thermocouples, the expression for the measured total potential EZ can be derived as follows:
EZ=EAB(T1,T3)+ECD(T3,T2)(3)
Equation (3) is the law of thermocouple connection conductor. If the connection is not in one section, the total potential EZ is also the sum of each section. In the measurement, we hope that the total potential at the measuring end is the thermocouple EAB (T1, T2), which is convenient for controlling the instrument measurement to prevent additional potential from being generated by intermediate connections. The expression is:
EAB (T1, T2)=EZ=EAB (T1, T3)+EAB (T3, T2) (4)
In equation (4), T3 is referred to as the intermediate temperature, hence it is also known as the law of intermediate temperature. This requires us to find a certain material C D, His characteristics are:
ECD(T3,T2)=EAB(T3,T2)(5)
The material that satisfies equation (5) is called the compensating wire of the thermocouple. Because there are many types of thermocouples, there are also many types of thermocouple compensation wires.
3, Common errors in the use of compensating wires and the resulting errors
1. Connect the positive and negative terminals of the thermocouple compensation wire in reverse with the thermocouple
If the positive and negative poles of the thermocouple compensation wire are connected in reverse to the positive and negative poles of the thermocouple, and the positive and negative poles of the thermocouple are connected correctly to the positive and negative poles of the instrument, taking the K-type thermocouple as an example. This error is quite common in applications because after connection, the temperature change trend of the controlled object is consistent with the display instrument. Moreover, many thermocouple compensation wire products currently have non-standard labeling, making it difficult to distinguish; Some manufacturers even label colors incorrectly. Below is an analysis of the errors caused by this situation.
If connected correctly, the total thermoelectric potential received by the instrument is
EZ=EK(TI,T3)+EKX(T3,T2)=EK(T1,T3)+EK(T3,T2)=EK(T1,T2)(6)
Due to a connection error, according to the law of intermediate conductors, the total thermoelectric potential received by the instrument is
EZ=EK(T1,T3)+EKX(T3,T2)(7)
For KX extended compensation wires, there are
E'KX(T3,T2)=-EKX(T3,T2)=--EK(T3,T2)(8)
Calculate the error generated by the instrument measurement value
EZ'-EZ=EK(T1,T3)-EK(T3,T2)-EK(T1,T3)-EK(T3,T2)=-2EK(T3,T2)(9)
The temperature near the general industrial furnace is at least 8 ℃ higher than the temperature in the control room. So the resulting error is exactly twice the compensation value of the compensating wire. For K-type thermocouples, the differential potential value is generally around 40C/(μ V), and the measured temperature is approximately 16 ℃ lower than the actual temperature. If the control temperature is set at 600 ℃, the actual temperature should be around 616 ℃.
From the above analysis, it can be seen that when the positive and negative poles of the thermocouple compensation wire are reversed, it not only fails to compensate, but also doubles the error compared to not connecting the compensation wire. Therefore, the polarity of the compensation wire must be taken into account when connecting it.
2. The compensation wire model used is incorrect
If the same type of compensating wire is paired with the same type of thermocouple, errors will still occur if the selected type of compensating wire is incorrect. Assuming an S-type thermocouple is used, the compensation wire KX of the K-type thermocouple is selected.
According to the law of intermediate conductor, the total thermoelectric potential received by the instrument is
E'Z(T1,T2)=ES(T1,T3)+EKX(T3,T2)(10)
If the S-type compensating wire SC is used correctly without considering the self error of the compensating wire, the total potential measured by the instrument is EZ (T1, T2)=ES (T1, T3)+ES (T3, T2) (11)
Due to selecting the wrong compensation wire, the measurement value of the instrument resulted in errors as shown in equations (10) - (11)
EZ'-EZ=EKX(T3,T2)-ES(T3,T2)(12)
If the working temperature of the S-type thermocouple is 900 ℃ and the ambient temperature in the control room is 25 ℃, still according to T3-- T2==8 ℃, check the S-pair and K-pair calibration tables separately, and obtain the potential difference as
EKX(T3,T2)-ES(T3,T2)=0.278mV
The temperature measured by the instrument is higher than the actual temperature. If the instrument is controlled at 900 ℃, the actual value is only 875.1 ℃, with an error of 24.9 ℃.
If the polarity is reversed again in the above situation, the instrument measurement value will be higher. When the instrument displays 900 ℃, the actual temperature is 933.2 ℃, with an error of 33.2 ℃.
3. Mixing compensation wires with ordinary wires
In practical applications, it is often found that ordinary wires are used to connect compensating wires due to their insufficient length, or a section of ordinary wire is connected after the compensating wire is broken.
4, Precautions for using compensating wires
1. Selection of compensation wires
The compensation wire must be selected correctly according to the type of thermocouple used and the occasion in which it is used.
2. Contact connection
Try to keep the temperature of the two contact points as close as possible to the thermocouple terminal. The temperature at the connection point with the instrument terminal should be as close as possible. In areas where there is a fan in the instrument cabinet, the contact points should be protected to prevent the fan from blowing directly to the contact points.
3. Use length
Because the signal of the thermocouple is very low, at the microvolt level, if used over a long distance, signal attenuation and strong electrical interference in the environment can cause distortion of the thermocouple signal, resulting in inaccurate temperature measurement and control. In severe cases, temperature fluctuations may occur during control. Based on experience, it is usually better to control the length of the thermocouple compensation wire within 15 meters. If it exceeds 15 meters, it is recommended to use a temperature transmitter to transmit the signal. The temperature transmitter converts the potential value corresponding to the temperature into direct current for transmission, and has strong anti-interference ability.