Transient High Surface Temperature Measurement And Dynamic Calibration

Temperature is a significant physical quantity in the fields of scientific research anddevelopment and one of the foundational SI quantities. The concept and measurement oftemperature are all based on thermal balance. At present, thermocouple, thermo resistive andradiance thermometer are already technologically mature and can be adapted to measure thetransient high surface temperature in many research projects. But new technology is needed inweapon test, where the common feature of transient surface temperature measurement is thatthe temperature is quite high and changes quickly, and usually non-reproductive singularprocess probably accompanied with high pressure or high speed gas flow, and there is toomuch disposable processing. Therefore, the measurement condition is harsh and thetechnological difficulty is very high. In order to obtain real-time, online, rapid and correctmeasurement, the dynamic calibration is much important.In transient temperature measurement, temperature changes very quickly, due to thethermal inertia and finite heat conduction of sensing element of temperature sensor, errorexists between the measured result and the true value, it was so called dynamic response error.The purpose of dynamic calibration of transient surface high-temperature sensors is that: Usea traceable, fast changing temperature signal to obtain the dynamic response of certaintemperature sensor, and modify the measured result so as to make it more close to the truevalue, consequently reduce the dynamic response error. To trace a dynamic parameter to theNational fundamental standard is the base of correcting any dynamic measurement.In the past, transient high-temperature measurement technique mainly focused on the hightransducer function of temperature, and seldom involves the research of the transientcharacteristic. The author has made a transient surface high-temperature sensor with sapphirefiber blackbody cavity, on which ceramics film is splashed. It can be suitable for workingunder harsh environment. The working principle of this sensor is based upon Planck radiancelaw. With a time response on the order of 10-2s, the sensor can be used to measure thetransient high temperature from 900 to 1900℃ under harsh environment. The relationbetween the thermo physical property of the black body's ceramic film and the responsivespeed has been analyzed. Both the theoretical and experimental results show that: The smallerthe thickness of black body's ceramic coating is, the faster its heating responsive speed will be.The armor structure of the sensor has been designed to fit the harsh environment. A hightemperature metal pool equalizer has been used to realize the static calibration of the sensor.High power CO2 laser pulse was used as the step-function excitation source of thetemperature sensor to dynamically calibrate it. Thus the problem of calibration of this sensorin the engineering application is solved and the foundation of the design of this kind ofsensors is established. Using this transient high-temperature sensor we have succeeded inmeasuring the temperature of oxyacetylene torch and rocket engine plume as well as thetransient high surface temperature at the forward frame of a missile launching box.In this foundation, a new principle is proposed, that is, a high frequency modulated highpower CO2 laser is used as the driving source to heat up the sensor. On the surface a fastelevating temperature signal appears. Using the temperature signal obtained by high speed IRdetector traceable high-temperature dynamic calibration can be carried through. Thefrequency-response characteristics of the former surpass that of the latter, therefore it can beused as the true value to calibrate the latter and let system error be corrected.Tellurium-cadmium-mercury infrared detector which has good response to the CO2 laserbeam of 10.6μm wavelength is used to measure the surface temperature time rise excited bythe CO2 laser. This dynamic calibration system was used to test the response times of severaldifferent kinds of thermocouples. The experimental results have shown that this calibrationsystem can be used to calibrate transient surface temperature sensor with a time response ofthe sub-millisecond order.Using the continual beam and the pulsed beam sent out by the same laser in the samesystem to carry on the static calibration of the infrared detector and the dynamic calibration ofthe temperature sensor to be checked, the differences in the environment of the sensorinstalling and the error caused by the change of thermo physical property can be avoided.Thus the difficult problem of traceable temperature dynamic calibration is solved. Throughthe analysis of geometrical optical model of the image focusing characteristic of an ellipsoidalmirror, a spherical mirror can be used to replace the ellipsoidal one within the aberrationpermission scope. The difference between them is not too much in the requested radius(namely 177mm).The ZEMAX software is used to perform the simulation on the upper-mentioned opticalsystem with the spherical mirror, it can be seen that the diameter of the image focused on theinfrared detector is 3.65mm. The result shows that the mathematical model is rational. Aspherical reflector with large focusing aperture is used to raise the signal-to-noise ratio of theinfrared detector. The high speed TeCdHg IR detector is placed at one conjugate focus, thesurface temperature sensor to be calibrated is placed at the other focus. Using the outputsignal of the high speed IR detector as the input signal, the latter temperature sensor will becalibrated.