Thermocouples are a common type of temperature sensor. In the process of use, the thermocouple is often faulty and inaccurate during use due to various interference factors.
There are 5 points that cause the thermocouple to fail:
1. Connection problem: Accidental thermocouple junction caused by many measurement errors. Remember that any intersection that leads to two different metal junctions. If you need to increase the length of your thermocouple wire, you must use the correct type of thermocouple extension cable (such as the k-type thermocouple k-type). Using any other type of wire, a junction of a thermocouple will be introduced. The correct thermocouple material must be made using any connector that must be followed and the correct polarity.
2. Lead resistance: In order to minimize thermal shunting and improve response time, thermocouples are made of thin metal wires (in the case of platinum type, the cost is also a consideration). This can cause the thermocouple to have a higher resistance, which can be sensitive to noise and can cause errors due to the input impedance of the measuring instrument. A typical thermocouple that exposes the 32awg wire (0.25mm diameter) junction will have a resistance of about 15 ohms/meter. The picotc-082mΩ input impedance will therefore produce an error of less than 0.01% for this cable 12 meters. If a thin wire or long cable thermocouple is needed, it is worth keeping the thermocouple lead short and then using a thermocouple extension wire (which is much thicker and therefore has a lower resistance), between the thermocouple and the measuring instrument run. It is always a good precaution to measure the resistance of your thermocouple before use. The usual cause is that atmospheric particulate matter diffuses into the metal at extreme operating temperatures. Another reason is the diffusion of impurities and chemicals into the thermocouple wire insulation. If operating at high temperatures, check the specifications of the probe insulation.
3. Noise: The output from the thermocouple is a small signal, so it is easy to pick up the electrical noise. Most measuring instruments reject any common-mode noise signal that is the same two wires so that the noise can be twisted together by the cable to help ensure that the two wires pick up the same noise signal minimized. If working in a very noisy environment (such as near a large motor), it is worth considering the use of shielded extension cords. If you suspect that noise pickup first turns off all suspicious devices, see if the readings change.
4. Common mode voltage: Although the thermocouple signal is very small, there is often a larger input of the voltage measuring instrument. These voltages can cause the inductor to pick up (a problem when testing the motor windings and the temperature of the transformer), or "grounding" the intersection. A typical example is the "grounding" junction where the temperature is measured by hot water pipes and non-insulated thermocouples. If there are any bad ground connections, there may be a few volts between the pipe and the geodetic instrument. These signals are then in the normal mode (the same on both thermocouple wires), so it won't cause most instrument problems to be provided they won't be too big. Use the same cable noise overview precautions to reduce the common mode voltage or by using an insulated thermocouple.
5. Heat split: All thermocouples have a certain quality. The quality of the heating, the energy needed, will affect the temperature you are trying to measure. Consider an example of liquid temperature measurement in a test tube: there are two potential problems. The first is that heat will travel to the thermocouple wires and dissipate into the atmosphere, reducing the temperature of the liquid around the wires. A similar problem can occur if the air is not fully immersed in the liquid due to the cold temperature on the wire, the thermocouple, heat conduction, may cause the junction of the thermocouple to a different temperature, the liquid itself. Thermocouples with thinner wires may help because it causes a steep temperature gradient along the thermocouple wires at the junction between the liquid and the surrounding air. If a thin wire thermocouple is used, the lead wire resistance must be considered. The use of thermocouples with thin wires to connect to thicker thermocouple extensions often provides a good compromise.
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