Low temperature measurement

    The low temperature measurement in the cryogenic system mainly includes temperature measurement, vacuum measurement, liquid level measurement and flow measurement.

    The international uniform standard for temperature measurement is the International Temperature Scale (ITS), which has been adopted as the national legal temperature scale for temperature measurement in my country over the years. The current temperature scale is the 1990 International Temperature Scale (ITS-1990).

    For temperature measurement, the measurement tool must be selected according to the range of temperature measurement, mainly a thermometer. Here are some commonly used thermometers.

    1. Metal resistance thermometer

    The resistivity of a conductor or semiconductor changes with temperature, which is the basis of a simple and reliable resistance thermometer. Below 961.78℃, the international temperature scale adopts platinum resistance thermometers. In addition to platinum, other materials, such as copper, nickel, and indium, have been used as resistance thermometer materials. Nickel is prone to deformation at normal temperature, so the resistivity will change with time, and the stability of the thermometer is poor. Indium resistance thermometers have higher sensitivity than platinum resistance thermometers at low temperatures.

    A very important part of installing a resistance thermometer is to eliminate thermal and mechanical stress, because stress will generate additional resistance. Armoured platinum resistance thermometers are often used in industrial production. These thermometers are usually enclosed in a cylindrical metal shell with a diameter of about 4 mm and a length of about 10 to 12 mm.

    2. Semiconductor resistance thermometer

    Since the conductivity of semiconductors is related to temperature, this material can be used as a resistance thermometer. Under high temperature conditions, the resistivity (the reciprocal of the electrical conductivity) is proportional to the exponential function of the reciprocal of the thermodynamic temperature, which characterizes the conductivity of semiconductors. Under low temperature conditions, conduction is carried out due to the presence of impurities (emission of electrons or acceptance of electrons).

    Germanium resistance thermometer is a widely used semiconductor resistance thermometer, which is doped with arsenic, gallium or antimony to obtain the required resistance characteristics. The germanium is sealed in a metal box and four leads are connected. The typical operating range of industrial germanium resistors is between 1.5 and 100 K. Germanium resistance thermometers are usually calibrated at seven temperature points.

    Due to the characteristics of high sensitivity, low price, small size, and relatively simple resistance temperature curve of carbon resistance, carbon resistance thermometers have also been widely used as thermometers for low temperature measurement.

    3. Thermocouple thermometer

    Thermocouples can be used for temperature measurement in the normal temperature range, and can also be used for temperature measurement in low-temperature environments. One junction of the thermocouple is placed in the measured temperature, and the other junction is placed in the reference temperature environment. For example: Use an ice bath as a reference point and use a thermocouple to measure low temperature. If you need to determine the temperature resolution is about 0.1K, when the measured thermoelectric potential is 5mV or more, the measuring instrument must have an accuracy of 1μV; if used Liquid nitrogen bath (or liquid hydrogen bath) as a reference point, the total thermoelectric potential will decrease. The advantage of using a low temperature reference point is that it can greatly reduce the conduction heat along the thermocouple wire.

    The temperature indication can be determined by the conversion of the measured thermoelectric potential from the calibration curve relationship (corresponding to a certain reference temperature).

    There are three commonly used low-temperature thermocouples: copper-constantan (T type), nickel chromium constantan (E type), and nickel chromium nickel silicon (K type) thermocouples, which are commonly used for measurement from -200°C to ambient temperature (or above) Temperature range: Ni-Cr-Au-Fe (3% iron) thermocouple can be used in the low-temperature range of 10~180K.

    One disadvantage of thermocouples is that the thermoelectric potential is too small, usually in the millivolt level. This problem can be solved by using several thermocouples to form a thermocouple stack. The thermoelectric potential of a thermocouple stack is the sum of the thermoelectric potentials of stacked thermocouples, that is, superimposing n thermocouples, and the thermoelectric potential is n times the thermoelectric potential of a single thermocouple.

    Conductive heat along the thermocouple wire may cause serious errors in the temperature measurement of the thermocouple. To reduce this situation, let a certain length of thermocouple wire pass through a lower temperature isothermal environment (heat sink), so that the error will be reduced. In addition, the use of thermocouple wires with a small diameter can also greatly reduce temperature measurement errors.

    4. Vapor pressure thermometer

    The vapor pressure thermometer uses the property of a fixed function of the saturated vapor pressure of the liquid and the temperature of the liquid in contact with the vapor to measure the temperature. The thermometer includes a temperature bulb filled with pure gas, which can be condensed within the temperature sensing range. The bulb is connected to the pressure measuring device through a capillary tube.

    The advantage of the vapor pressure thermometer is that it has a higher sensitivity in the application temperature range. For example, in the temperature range of 63 to 80 K, the sensitivity of a nitrogen thermometer is 15 to 3 kPa/K. Similarly, vapor pressure thermometers are not like constant-volume gas thermometers, which require corrections for fixed-volume or gas imperfections. A major disadvantage of this kind of thermometer is that the temperature measurement range is too small, and a single liquid cannot be used to measure all the temperatures from the liquid He to the ambient temperature. Although vapor pressure thermometers can be used to measure the temperature range from the triple point to the critical point, these thermometers are usually only used for temperature measurement in the vapor pressure range of 5 to 250 kPa.

    The purity of the fluid in the thermometer is very important, and a small amount of impurities will cause the vapor pressure to deviate from the relationship between the saturated vapor pressure and temperature of the pure substance. At the same time, it should be noted that the fixed volume should be small enough, and the temperature of the connecting pipe should not be lower than the temperature of the temperature sensing bulb.

(Huiputech)