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What are the reasons for the failure of the thermocouple

by:JVTIA     2022-01-23
Let's take a look at the failure factors of thermocouples: Thermocouples are a common temperature sensor. In the process of use, they are often affected by a variety of interference factors, resulting in failures and errors in the use of thermocouples. There are 5 points: 1. Connection problems: accidental thermocouple junctions caused by many measurement errors. Remember, any intersection that leads to the junction of two different metals. If you need to increase the length of your thermocouple wire, you must use the correct type of thermocouple extension wire (such as type k thermocouple type k). Using any other type of wire will introduce a thermocouple junction. Any connector used must be made of the correct thermocouple material, and the correct polarity must be observed. 2. Lead resistance: In order to minimize heat shunting and improve response time, the thermocouple is made of thin metal wires (in the case of platinum type, cost is also a consideration). This may cause the thermocouple to have a higher resistance, which can make noise sensitive, or it may cause errors due to the input impedance of the measuring instrument. A typical thermocouple exposed at the junction of a 32awg wire (0.25 mm diameter) will have a resistance of about 15 ohms/meter. The picotc-082mΩ input impedance, so the error of 12 meters of this cable will be less than 0.01%. If you need to use a thin wire or a long cable thermocouple, it is worth keeping the thermocouple lead short, and then use the 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 that impurities and chemicals diffuse 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 electrical noise. Most measuring instruments reject any common-mode noise signal that is the same as the two wires, so that the noise can be twisted together through the cable to help ensure that the two wires pick up the same noise signal and are minimized. If you are working in a very noisy environment (such as near a large motor), it is worth considering using a shielded extension cord. If you suspect noise pickup, turn off all suspicious equipment first and see if the reading changes. 4. Common mode voltage: Although the thermocouple signal is very small, there is often a larger voltage measuring instrument input. These voltages may cause the inductance to pick up (a problem when testing the temperature of the motor windings and transformers), or 'ground' junctions. A typical example is the 'grounded' junction of hot water pipes and non-insulated thermocouples that measure temperature. If there is any bad ground connection a few volts may exist between the pipeline and the geodetic measuring instrument. These signals are in normal mode (the same in both thermocouple wires), so they will not cause problems for most instruments provided they are not too big. Using the same precautions outlined for cable noise can reduce the common-mode voltage, or by using insulated thermocouples. 5. Heat shunt: all thermocouples have a certain quality. The quality of heating, the energy required, will thus affect the temperature you are trying to measure. Consider the example of liquid temperature measurement in a test tube: There are two potential problems. It is the heat energy that will travel to the thermocouple wire and dissipate into the atmosphere, thereby reducing the temperature of the liquid surrounding the wire. A similar problem may occur if the thermocouple is not sufficiently immersed in the liquid, due to the air temperature of the thermocouple in the colder environment, and heat conduction may cause the junction of the thermocouple to a different temperature in the liquid itself. Thermocouples with thinner wires may be helpful because it causes a steep temperature gradient along the thermocouple wires at the junction between the liquid and the surrounding air. If a thermocouple with a thin wire is used, the lead resistance must be considered. The use of thermocouples with thin wires connected to thicker thermocouple extension wires often provides a good compromise.
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