Temperature sensor's time constant and hysteresis, temperature sensor temperature measurement factors and temperature sensor error

The Temperature Sensor converts temperature into an available output signal by using various physical properties of the material as a function of temperature. The Temperature sensor is the core part of the temperature measuring instrument and has a wide variety. According to the measurement method, it can be divided into two types: contact type and non-contact type. According to the characteristics of sensor materials and electronic components, it can be divided into two types: thermal resistance and thermocouple. The modern temperature sensor is very small in shape, which makes it widely used in various fields of production practice, and also provides countless conveniences and functions for our lives.

The temperature sensor time constant and hysteresis are related to the heat capacity and thermal resistance of the temperature sensor. In addition to the temperature sensor with time constant and small hysteresis, reasonable insertion depth and correct installation method should be ensured to ensure Temperature Measurement accuracy and temperature control system. Stability and quality of control.

Temperature sensor time constant and hysteresis

Practice has proved that the thermocouple, thermistor, and the Bimetal thermometer will delay the output for a period of time when the measured temperature suddenly changes. This delay time Δτ is generally called pure hysteresis or pure time delay. After the delay Δτ, it will change according to the law of the exponential curve (as shown in the figure below). If △τ is ignored and the timing start is made with the change of the medium temperature, the above curve is consistent with T=△T(1-et/τ In the formula, T is temperature; ΔT is temperature change; t is time; τ is time constant. The time constant corresponds to the time corresponding to the tangent of the starting point of the reaction curve and the equilibrium temperature intersection A, that is, the time required for the output to change by 63.2% ΔT.

Correct understanding and treatment of the time constant and hysteresis of temperature sensors is an important issue. It is related to whether the temperature can be measured correctly and the change of the measured temperature is reflected in time. It has a pivotal effect on the stability and quality of the temperature control system, so it is a problem that cannot be ignored.

How to improve the time constant and hysteresis of the temperature sensor

The temperature sensor time constant and the magnitude of the hysteresis depend on the thermal capacity and thermal resistance of the component. Because the temperature sensor needs to absorb a certain amount of heat, the heat required to change 1 °C is the heat capacity of the temperature sensor, and the smaller the heat capacity, the better. The temperature sensor heat transfer needs to overcome the thermal resistance, which has a direct relationship with the structure and size of the component. Metal is a good conductor of heat, and the size of the thermal resistance is often affected by the air gap of the temperature sensor, the insulation, and the protective sleeve.

Temperature sensors have large time constants and hysteresis, usually ranging from tens of seconds to several minutes, so the influence on measurement and control temperature is very large, especially on the stability of temperature control systems. Therefore, in the field application, in addition to the temperature sensor with a small time constant and low hysteresis, it should also pay attention to the installation method of the temperature sensor. That is to say, there must be a certain insertion depth when installing, especially the thermal resistance. If the insertion depth is not enough, it will cause a large error. When the process pipe is thinner, the pipe must be partially thickened, or the temperature sensor should be bent. On the head, the temperature sensor should be directed to the flow direction of the fluid; when measuring the temperature of the liquid phase medium, it is best to measure the liquidus temperature because the dynamic characteristics and stability of the liquidus temperature are better than the gas phase temperature; if necessary, it can also be taken Fill metal foil or other heat-conducting material between the protective tube and the thermal element (the armored thermocouple or armored platinum resistor is filled with high-purity alumina powder between the protective tube and the component), and the thermocouple can also be used with an open end or Shelled thermocouple.

1. Insert the depth of the measuring medium

The most important thing is the choice of temperature measurement point. For the process of production process, the position of temperature measurement should be typical and representative, otherwise the meaning of measurement will be lost. When the temperature sensor is inserted into the site being measured, some heat flow will occur along the length of the sensor. When the ambient temperature is low, there will be heat loss. This causes the temperature sensor to be inconsistent with the temperature of the medium being measured, which causes a temperature measurement error. Therefore, the measurement error due to heat conduction is related to the depth of insertion. The depth of insertion is related to the material of the protective tube.

Among different materials, due to the good thermal conductivity of the metal protection tube, the insertion depth should be deeper, and the ceramic material has better thermal insulation performance and can be inserted shallower. In some respects, for engineering temperature measurement, the insertion depth is also related to some states such as whether the measuring object is stationary or flowing. For example, the measurement of the flowing liquid or the high-speed airflow temperature is not limited by the above, and the insertion depth may be shallower. The value should be determined by experiment

2. Response time with measuring medium

The basic principle of temperature measurement in contact temperature measurement is that the temperature measuring element should reach a heat balance with the measured object. Therefore, we need to keep them for a certain period of time when measuring temperature, in order to achieve a heat balance between the two. The length or duration of the hold time is related to the thermal response time of the temperature measuring element. The thermal response time depends mainly on the structure of the temperature sensor and the conditions of the measurement, which vary greatly. Therefore, the general temperature sensor not only fails to keep up with the temperature change rate of the object to be measured, but also causes some measurement errors because the heat balance is not achieved.

Therefore, we should choose a sensor that responds faster. For the temperature sensor, in addition to the influence of the protection tube, the diameter of the measuring end of the temperature sensing element is also the main factor, that is, the finer the temperature sensing element, the smaller the diameter of the measuring end, the shorter the thermal response time.

3. The measured medium will increase the thermal impedance

In some cases, if the temperature sensor used at high temperature is in a gaseous state, some dust and the like deposited on the surface of the protective tube will be melted on the surface of the protective tube, so that the protective tube is thickened and its heat is The impedance increases. If the measured medium is in the form of a melt, there will be slag deposited on it during use, which not only increases the response time of the temperature sensor, but also causes the indication temperature to be low, which causes deviations. Therefore, we have to periodically In addition to the verification, in order to reduce the error, it is also necessary to check frequently, which is very helpful for the use of the temperature sensor.

In order to avoid the temperature sensor error, we must understand the cause of the error in the temperature sensor, in order to better try the temperature sensor to make it play its maximum value. Next, Xiaobian teaches you how to avoid errors.

1: Temperature sensor installation position problem: The installation position should not be close to or heated, at least the insertion depth should be 8 to 10 times the diameter of the protection tube. The temperature sensor and power cable should not be installed in the same conduit to avoid introducing interference and causing errors.

2: Insulation problem of temperature sensor: If the thermocouple is insulated, the dirt or salt slag of the protection tube and the cable plate will cause poor insulation between the thermocouple and the furnace wall, which is more serious at high temperature, which will not only cause thermoelectric potential. The loss is also introduced into the interference, and the resulting error can sometimes reach up to 100 degrees.

3. Thermal resistance error: When there is a layer of coal ash on the protection tube at high temperature, the dust is attached to the upper part, the thermal resistance increases, which hinders 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 the error.

4. Error introduced by thermal inertia: Due to the thermal inertia of the thermocouple, the indication value of the meter lags behind the measured temperature, which is especially noticeable when performing rapid measurement. Therefore, thermocouples with thinner hot electrodes and smaller diameters of protective tubes should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The larger the measurement hysteresis, the smaller the amplitude of the thermocouple fluctuation and the greater the difference from the actual furnace temperature.

After the above problems occur, the user may have doubts about the product. If you are not familiar with the operation error caused by improper installation, do not doubt the temperature sensor problem. Check whether the temperature sensor is insulated and installed correctly according to the above points.

When the temperature sensor is installed and used, in fact, as long as more attention can ensure the best measurement, the following small series will be introduced specifically.

1. Error introduced by improper installation

If the position and insertion depth of the thermocouple are not reflected, the true temperature of the furnace, etc., in other words, the thermocouple should not be placed too close to the door and heated, the depth of insertion should be at least 8 to 10 times the diameter of the protection tube; The gap between the protective sleeve and the wall of the thermocouple is not filled with thermal substances, so that the heat in the furnace overflows or the cold air invades. Therefore, the gap between the thermocouple protection tube and the hole of the furnace wall is blocked by heat insulating material such as refractory mud or asbestos rope to avoid hot and cold. Air convection affects the accuracy of temperature measurement; the cold junction of the thermocouple is too close to the furnace body to make the temperature exceed 100 °C; the installation of the thermocouple should avoid strong magnetic field and strong electric field as much as possible, so the thermocouple and power cable should not be installed. In the same duct, the error may be caused by introducing interference; the thermocouple cannot be installed in the area where the measured medium rarely flows. When the temperature of the gas in the tube is measured by the thermocouple, the thermocouple must be installed against the flow direction and fully in contact with the gas. .

2. The error introduced by the insulation is poor

If the thermocouple is insulated, the dirt or salt slag of the protection tube and the cable plate may cause poor insulation between the thermocouple and the furnace wall, and it is more serious at high temperature, which not only causes the loss of the thermoelectric potential but also introduces interference. The resulting error can sometimes reach Baidu.

3. Error introduced by thermal inertia

Since the thermal inertia of the thermocouple causes the meter's indicated value to lag behind the measured temperature, this effect is particularly pronounced when making rapid measurements. Therefore, thermocouples with thinner hot electrodes and smaller diameters of protective tubes should be used as much as possible. When the temperature measurement environment permits, the protection tube can even be removed. Due to the measurement lag, the amplitude of the temperature fluctuation detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuation. The larger the measurement hysteresis, the smaller the amplitude of the thermocouple fluctuation and the greater the difference from the actual furnace temperature. When the temperature is measured or controlled by a thermocouple with a large time constant, although the temperature displayed by the meter fluctuates little, the fluctuation of the actual furnace temperature may be large. In order to accurately measure the temperature, a hot-selective galvanic couple with a small time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, and is proportional to the diameter of the hot end of the thermocouple, the density of the material, and the specific heat. To reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end. . In use, it is usually a material with good thermal conductivity, a protective tube with a thin tube wall and a small inner diameter. In the more precise temperature measurement, a bare wire thermocouple without a protective sleeve is used, but the thermocouple is easily damaged and should be corrected and replaced in time.

4, thermal resistance error

At high temperatures, if there is a layer of coal ash on the protective tube, the dust is attached to it, the thermal resistance increases, hindering the conduction of heat, and the temperature indication is lower than the true value of the measured temperature. Therefore, the outside of the thermocouple protection tube should be kept clean to reduce the error.