It is important to measure the rapid temperature change of high-speed gas flow using a rapid response temperature sensor in a few special cases. Foreign products can meet the measurement requirements, but they are costly and inconvenient to acquire; domestic products are rare to obtain. For the accurate measurement of rapid temperature changes in high-speed gas flow, there are higher requirements for rapid response characteristics of thermocouples. Rapid response thermocouples differ from regular thermocouples as they must have a fast response speed, good gas-tightness, sufficient physical strength to cope with the impact of high-speed gas, and minimal effect on the original gas flow state. In temperature sensors with rapid response characteristics, the lumped parameter method has been used to analyze the measures to comprehensively decrease the time constant of the thermocouples by analyzing the heat transfer characteristics of their welding joints. Gases with no radiation ability and high temperature flue gas with radiation ability are selected for analysis. The results of these analyses show the diameter of the sensing point and gas flow velocity to be the controllable factors. However, under the condition of a high gas flow velocity, the heat radiation from the flue gas or the inner wall of the flow passage to the temperature sensing point has little influence on the temperature measurement results. Based on the aforementioned analyses, 30 mu m copper wire and constantan wire are used to make a T-type rapid response thermocouple. The sensing point is welded by the controllable pulse discharge device. The welding joints (temperature sensing points) are all within 100 mu m. The wires and the armor structure may influence the state of gas flow near the sensing point. Therefore, rapid response thermocouples use a new armor structure with an exposed temperature sensing point, which provides them with high reliability and superior dynamic response characteristics. To calibrate the thermocouples, the temperature rise time of the welding joints of the thermocouples must be considerably smaller than the time constant of the thermocouple. Thus, the method of applying heat to the welding joints becomes important in ensuring that the measurement results are highly accurate and reliable. Through comparison and comprehensive consideration, this study uses a laser to heat the thermocouple and to achieve ideal positive and negative temperature steps. The time constant of the thermocouples measured by the experiments is approximately 40 ms. To be cost-effective, thermocouple wires should be selected for different applications based on appropriate time constant requirements. Therefore, the dynamic calibration of thermocouples with three diameters has been performed as a basis for the selection. The purpose of the dynamic calibration experiment is to measure the time constants of the thermocouples with three different wires of 30, 40, and 50 mu m diameters. It is found that the time constant increases with the wire diameter. In addition, the negative step time constant of the thermocouple is always lower than the positive step time constant. This is because laser light travels along a straight line. In the positive step, laser light directly heats the front part of the sensing point, and there is no heating in the back part of the sensing point from the laser light, and some heat is lost via radiation to the colder environment, resulting in a decrease of the strength heating the sensing point from the laser light. In the negative step, the entire surface of the sensing point is involved in radiation heat transfer with the external environment, so the time constant measured is closer to the true value.
