How Thermocouple Sensors Work
When there are two different conductors and semiconductors A and B to form a loop, and the two ends are connected to each other, as long as the temperatures at the two junctions are different, the temperature of one end is T, which is called the working end or the hot end, and the temperature of the other end is TO , called the free end or the cold end, there is a current in the loop, that is, the electromotive force existing in the loop is called the thermoelectromotive force. This phenomenon of generating electromotive force due to differences in temperature is called the Seebeck effect. There are two effects related to Seebeck: first, when a current flows through the junction of two different conductors, heat is absorbed or released here (depending on the direction of the current), which is called the Peltier effect; Second, when a current flows through a conductor with a temperature gradient, the conductor absorbs or releases heat (depending on the direction of the current relative to the temperature gradient), known as the Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple.
How Resistive Sensors Work
The resistance value of the conductor changes with the temperature, and the temperature of the object to be measured is calculated by measuring the resistance value. The sensor formed by this principle is the resistance temperature sensor, which is mainly used for the temperature in the temperature range of -200-500 °C. Measurement. Pure metal is the main manufacturing material of thermal resistance, and the material of thermal resistance should have the following characteristics:
(1) The temperature coefficient of resistance should be large and stable, and there should be a good linear relationship between the resistance value and the temperature.
(2) High resistivity, small heat capacity and fast reaction speed.
(3) The material has good reproducibility and craftsmanship, and the price is low.
(4) The chemical and physical properties are stable within the temperature measurement range.
At present, platinum and copper are the most widely used in the industry, and have been made into standard temperature measuring thermal resistance.
Considerations when choosing a temperature sensor
1. Whether the environmental conditions of the measured object have any damage to the temperature measuring element.
2. Whether the temperature of the measured object needs to be recorded, alarmed and automatically controlled, and whether it needs to be measured and transmitted remotely. 3800 100
3. In the case where the temperature of the measured object changes with time, whether the lag of the temperature measuring element can meet the temperature measuring requirements.
4. The size and accuracy of the temperature measurement range.
5. Whether the size of the temperature measuring element is appropriate.
6. The price is guaranteed and whether it is convenient to use.
How to avoid errors
When installing and using the temperature sensor, the following errors should be avoided to ensure the best measurement effect.
1. Errors caused by improper installation
For example, the installation position and insertion depth of the thermocouple cannot reflect the real temperature of the furnace. In other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least 8 to 10 times the diameter of the protection tube.
2. Thermal resistance error
When the temperature is high, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance will increase and hinder the conduction of heat. At this time, the temperature indication value is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce errors.
3. Errors caused by poor insulation
If the thermocouple is insulated, too much dirt or salt slag on the protection tube and the wire drawing board will lead to poor insulation between the thermocouple and the furnace wall, which is more serious at high temperature, which will not only cause the loss of thermoelectric potential but also introduce interference. The error caused by this can sometimes reach Baidu.
4. Errors introduced by thermal inertia
This effect is especially pronounced when making fast measurements because the thermal inertia of the thermocouple causes the meter’s indicated value to lag behind the change in the temperature being measured. Therefore, a thermocouple with a thinner thermal electrode and a smaller diameter of the protection tube should be used as much as possible. When the temperature measurement environment permits, the protective tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than that of the furnace temperature fluctuation. The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuations and the larger the difference from the actual furnace temperature.
Post time: Nov-24-2022