Development Of Automatic Calibration System For Low Temperature Thermometers Based On Cryocooler And Thermal Switch

With the rapid development of cryogenic physics and aerospace engineering,the demand for cryogenic temperature sensors is continuously increasing.To achieve high measurement accuracy,cryogenic thermometers should be calibrated before they are applied.For past low temperature thermometer calibration systems,most of them used cryogenic thermostat as the cold source,which took too much time to cooling down.And such thermostats are designed as open system and always consume a large amount of cryogenic fluid,which makes it necessary for liquid supplement.As a result,this kind of system has poor calibration efficiency and high cost.However,small cryocoolers,such as G-M and pulse tube refrigerators,have the advantages of compact structure,no liquid consuming,fast cooling and long running time,which makes it an ideal cold source in thermometer calibrating.To improve the precision and efficiency of thermometer calibration and reduce the cost,an automatic calibration system for low temperature thermometers based on a G-M refrigerator and LabVIEW programing was developed.In addition,a thermal switch is introduced to overcome the large temperature fluctuation and narrow controllable temperature range of the calibration system cooled by a G-M refrigerator only.Some conclusions were obtained after the research on working performance of this system.Firstly,the temperature fluctuations of calibration copper block coming from the refrigerator cold head can be effectively reduced through increasing the thermal resistance between the 2^nd stage cold head and the block.The larger the thermal resistance is,the better suppression effect on the fluctuation of temperature will be.And the stabilization of the temperature on the block is beneficial to the improvement of calibration accuracy of thermometers.Secondly,the static method has a higher accuracy than the dynamic one.Dynamic calibration method can be regarded as a quasi-static process which is difficult to achieve.So,deviation caused by dynamic calibration is hardly neglected.Therefore,the thermometer should be calibrated with the steady-state method to ensure calibration accuracy.Although the introduction of thermal resistance can suppress temperature fluctuations,it brings penalty of seriously slowing down the cooling rate.The application of low temperature thermal switch technology to cryogenic thermometer calibration system can effectively solve the contradiction between cooling time and temperature control accuracy.An electromagnetic driven thermal switch was designed and integrated,which helps realize active control in the full low temperature range from 3.5K to room temperature.With this technology,the calibration block can be cooled from room temperature down to below 4.2K within 3h when the switch is on.When the thermal switch is off,huge thermal resistance is created between the cold head and the isothermal block.And the block can be control to 280 K with good temperature stability while the heating power is very low.Not only the thermometer calibration accuracy is guaranteed,but also the efficiency of calibration is improved.For practical calibration tasks covering a large range of temperature,plenty of steady-state temperature states are required.If each control point will takes long time to reach the stable state,the total process must be very inconvenient and intractable by manpower.This paper completed an automatic system for low temperature thermometer calibration via LabVIEW program and communications between PC and devices.It enables completely automatic finishing of the procedures of data acquisition,temperature control stages setting,data processing and report generation.Therefore,the calibration cost is reduced and efficiency is raised,without human resource consumption.Finally,three different types of thermometers were calibrated with the automatic calibration system.The calibration precision of the platinum resistance thermometer calibrated is less than 20mK in the temperature range of 30³00K.The diode and the carburized ceramic thermometer reaches higher precision within 3mK below 10K.While above 10K,the calibration accuracy of carburized ceramic thermometer and diode thermometer are 50mK and 75mK respectively.Meanwhile,the contribution of measurement and equation fitting error to the calibration accuracy of the system was evaluated.Improvement of the measurement instrument can effectively improve the calibration accuracy of the system.