Research On Thermometer In Dilution Refrigeration System

This paper introduces the definition of thermodynamic temperature scale,the development of international temperature scale,the importance of extremely low temperature measurement,the principle,classification,selection and installation of thermometers,and also introduces the cryogenic fluids in detail,especially the properties of ⁴He and ³He,the way to obtain low temperature,the principle of dilution refrigeration,the working principle of dilution refrigeration system adopted in the experiment,and the experimental environment of the thermometer studied in this paper.The characteristics,measurement principle,detection and control,calibration,measurement range,response time and measurement accuracy,long-term and short-term stability,and the realization of temperature control function of cernox and RuO₂ thermometers used in the dilution refrigeration system to measure the extremely low temperature of about 10mK are comprehensively studied.The specific work is summarized as follows:（1）In the selection of cryogenic thermometer,we should take the actual situation of experimental equipment into comprehensive consideration,such as the measurement range,size,stability,and whether it would be disturbed by the magnetic field,etc.In most cases,the resistance thermometer is used,because it is easy to read the number,organize and analyze the data obtained.In the actual installation and application,special attention should be paid to its thermal contact,self-heating,the heat leakage of leads and thermal radiation,so as not to cause great interference to the experimental system.During the operation of non-liquid helium dilution refrigeration system,we should be fully aware of the principle,operation steps and emergency treatment of the whole system.（2）The cernox thermometer used in the system has the characteristics of wide temperature range,small magnetoresistance under magnetic field,high sensitivity when measuring low temperature,short response time and excellent stability.The resistance is measured by the four wire method,which is more accurate than the two wire one,with the excitation voltage in the order of millivolt.The temperature range of cernox thermometer is wide,ranging from 1.5K to 300K;and the response time is 4.2K/1.5ms,77K/50ms,273K/135ms,which is short.The cernox thermometer has a high precision with the uncertainty being in the order of mK,and short-term stability fluctuating within the range of 10mK.Its long-term stability is good in low temperature.We can use cernox thermometer to control the temperature above 1.5K,which is stable and ideal with the average fluctuation at 0.1%.（3）The RuO₂ thermometer used in the system has the advantages of being able to measure the extremely low temperature below 10mK and is not very sensitive to the magnetic field.It also has a unique packaging design.The resistance is measured by four wire method,and the excitation voltage is in the order of microvolt;The calibration of RuO₂ thermometer adopts the method of nuclear thermometer Co₆₀calibration.The measuring range of RuO₂ thermometer is 10mK～1.5K,which can meet the needs of measuring extremely low temperature.The response time is short,4.2K/0.5s,77K/2.5s.The repeatability of RuO₂thermometer is within 15mK.The short-term stability is about 1mK,and the long-term stability is±30mK.The RuO₂ thermometer can also be used to control the temperature below 1.5K,and the temperature fluctuates within±1.5mK,with the average fluctuation of 0.3%.