## 3 Measurement System for Time Constant Based on High Temperature Furnace
### Generation of Ramp Temperature Excitation  
In the system, the inner diameter of the high temperature furnace is 20 mm. At the distance of 120 mm from the furnace mouth, there is a uniform temperature zone with the thickness of 20 mm, temperature error of \(\pm\)1 \({}^{\circ}\)C and temperature fluctuation of \(\leq\)0.2 \({}^{\circ}\)C/min. The temperature of the uniform temperature zone can be set by the temperature controller, and the temperature range is from 300 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C. The high temperature furnace is heated by heating wire, and the heat insulation layer is made up of ceramic fiber blocks with low thermal conductivity. The temperature of the uniform temperature zone is measured by a standard thermocouple which is a second-class standard B-type thermocouple and has been verified by Shanghai Institute of Metrology. The output signal of the standard thermocouple is also fed back to the temperature controller which makes a high temperature furnace generate different temperatures through the PID control method.  
In order to study the temperature distribution from the furnace mouth to the uniform temperature zone in the high temperature furnace, the standard thermocouple was used to measure the temperature at the distance of 10, 20, 40, 60, 80, 100, and 120 mm respectively, as shown in Figure 4. The thermocouple was moved by the fast feed device and accurately stays at different positions in the furnace. The temperature of the uniform temperature zone was 400 \({}^{\circ}\)C, and the temperatures at different locations are shown in Figure 5. It can be seen from the figure that the temperature from the furnace mouth to the uniform temperature zone increases linearly. The temperature distributions from the furnace mouth to the uniform temperature zone were measured at different temperatures of uniform temperature zone. The experimental results show that when the temperature of uniform temperature zone is less than 700 \({}^{\circ}\)C, the temperature profile from the furnace mouth to uniform temperature zone is linear or approximately linear. When the temperature is greater than 700 \({}^{\circ}\)C, the temperature profile is not linear. However, the nonlinear distribution makes the measurement error of thermocouple time constant smaller. The research in this paper was carried out at the temperature of uniform temperature zone of between 300 \({}^{\circ}\)C and 700 \({}^{\circ}\)C, and at this moment, the temperature distribution from the furnace mouth to the uniform temperature zone is linear.  
In order to realize the rapid movement of the thermocouple, AC servo motor, screw, and slid block are used to build the fast feed device. The thermocouple is fixed on the slide block, and the screw drives the slide block to realize the movement of the thermocouple. AC servo motor has the advantages of high control precision, low frequency stability, resonance suppression, fast response speed and  
Figure 4: Schematic diagram of temperature measurement positions in high temperature furnace.  
Figure 5: Temperatures at different positions.  
so on, and cooperating with the lead screw, it can realize the fast start-stop and fast movement of the thermocouple.  
The thermocouple is quickly inserted into the uniform temperature zone from the furnace mouth by the fast feed device, and the temperature distribution in the furnace is linear, so the thermocouple is excited by the ramp temperature during the insertion process. In this system, the speed of the fast feed device can reach 300 mm/s, and the distance from the furnace mouth to the uniform temperature zone is 120 mm. Therefore, the rise time of the ramp temperature excitation is 0.4 s. According to Table 1, when the measurement error of time constant is less than 5%, this time constant measurement system can be used for time constant of the thermocouple whose time constant is larger than 4 s.