A Measurement System for Time Constant of Thermocouple Sensor Based on High Temperature Furnace

Abstract

In dynamic temperature measurement, thermocouple sensors are widely used, and their dynamic characteristics directly affect the accuracy of the test results. So before applying thermocouple sensors to dynamic temperature measurement, their dynamic characteristics should be obtained, and their dynamic performance parameters should be analyzed. The time constant is the most important dynamic parameter, which reflects the response speed of a thermocouple sensor. Therefore, it is necessary to measure the time constant. The time constant is closely related to the heat transfer mode, so the heat transfer environment of the time constant measurement system should be similar to the application environment of thermocouple sensor. When using the thermocouple to measure the temperature in various kilns, the heat transfer is mainly through the radiative mode, and existing equipment, such as constant temperature water/oil tank, shock wave tube and other devices, cannot be used to measure the time constant to reflect scenarios in actual measurement applications. Therefore, this paper proposed a new method to measure the time constant of the thermocouple by improved high temperature furnace. In the system, the high temperature furnace was used to generate the stable temperature field, and the fast feed device was used to insert the thermocouple into the high temperature furnace and generates the ramp temperature excitation. The temperature can reach 1500 • C in the furnace, and the temperature error in uniform temperature field is ±1 • C. Finally, the time constant of a K-type thermocouple was measured, and the uncertainty of the measurement result was analyzed.2 of 12 transfer mode. At present, there are many kinds of measurement methods and devices of time constant, such as constant temperature water/oil tank, hot wind tunnel, shock tube, laser, and so on.Stable temperature fields in water flow and airflow environments are generated by constant temperature water/oil tank and hot wind tunnel respectively. The thermocouple is inserted into the stable flow field rapidly, and the thermocouple is excited by step temperature. The time constant is obtained by the thermocouple response. Xiong J. and Wang X.N. [1,2] measured the response time and delay time of the thermocouple by constant temperature water/oil tank, and the dynamic error was corrected. Zhao S.A. [3] developed a hot wind tunnel with the temperature range from 1000 • C to 1700 • C, and the step response of the thermocouple was obtained. Shock tube is used to generate step pressure, which is usually used for dynamic calibration of pressure sensors. After breaking the film, the gas in the high pressure chamber compresses the gas in the low pressure chamber to produce the shock wave. The step pressure is generated on the pressure sensor by the shock wave and used to excite the pressure sensor. At the same time, due to the gas compression, there is also a temperature step behind the shock wave, which can be used to excite the thermocouple. Meng X.F. et al. [4,5] modified ...

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“…However, step functions of temperature and humidity are not possible in real-world conditions. Instead, Li et al (2018) used a ramp function to model the thermodynamic shock and found that the error of assuming an ideal step function is less than 10% if the transition time from the initial to the final state is less than the time response of the sensor. According to the iMet-XF bead thermistor datasheet, the sensor response time can be less than 1sec.…”

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“…However, under normal operational conditions, the sensor will be responding to temperature changes in the surrounding medium. Creating well-known and reproducible dynamic temperature changes in the medium is challenging to perform experimentally: the plunge method may be used to produce a temperature step [ 17 , 18 , 35 , 36 , 50 ], and a ramp input may be produced by rapid insertion into an oven [ 37 , 68 , 70 ]; however, there are limitations to these methods as described in Section 1 . Therefore, a FE transient heat-transfer model was used to study the sensors’ response to changes in the temperature of the surrounding medium, and the experimental results described above were used to validate the model.…”

“…Characterising the dynamic response can be challenging: a common method is to rapidly immerse the sensor into water at a different temperature to the ambient room temperature, providing temperature step input [ 17 , 18 , 35 , 36 , 50 ]. However, limitations to this technique include errors caused by unknown temperature gradients in the air and water, uncontrolled speed and depth of immersion [ 68 ], and temperature gradients in the fibre that would not be present in normal use. The speed of response is also strongly dependent on the thermal properties of the medium surrounding the sensor, and it is challenging to study this dependence using the immersion method.…”

“…Furthermore, in polymer-based sensors, water absorption into the polymer elements can be significant [ 32 , 62 , 69 ] and therefore a sudden change of surrounding medium (e.g., air to water, or vice versa) could add an unwanted contribution to the sensor response. A ramped temperature input can also be used for measuring the dynamic temperature response [ 37 , 68 , 70 ]; however, the minimum measurable time constant is limited by the gradient of the ramp [ 68 ]. Other approaches include heat-transfer modelling [ 35 ], and optical heating methods: several investigators have studied the dynamic responses of thermocouples and resistance thermometers using pulsed or modulated light to induce rapid temperature cycles [ 71 , 72 , 73 , 74 , 75 ].…”

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