Low temperature measurement

Low temperature measurement in cryogenic systems mainly includes temperature measurement, vacuum measurement, liquid level measurement and flow measurement.

The international unified standard for temperature measurement is the International Temperature Scale (ITS). my country has adopted this temperature scale as the national statutory temperature measurement scale for many years. The temperature scale currently used is the 1990 International Temperature Scale (ITS-1990).

Temperature measurement must select the measuring tool according to the range of the measured temperature, mainly the thermometer. Here are several 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 uses a platinum resistance thermometer. 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 room temperature, so the resistivity changes over time, and the stability of the thermometer is poor. Indium resistance thermometers have higher sensitivity than platinum resistance at low temperatures.

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

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 inverse of conductivity) is proportional to the exponential function of the inverse of the thermodynamic temperature, which characterizes the conductive properties of semiconductors. Under low temperature conditions, conduction is due to the presence of impurities (releasing or accepting electrons).

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

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

3. Thermocouple thermometer

Thermocouples can be used for temperature measurement in the normal temperature range and in low temperature environments. One junction of the thermocouple is placed in the temperature to be measured, and the other junction is placed in the reference temperature environment. For example: using an ice bath as a reference point and measuring low temperature with a thermocouple, if the required temperature resolution is about 0.1K, then when the measured thermoelectric potential is 5mV or above, the measuring instrument must have an accuracy of 1μV; if a liquid nitrogen bath (or liquid hydrogen bath) is used as a reference point, the total thermoelectric potential will be reduced. The advantage of using a low temperature reference point is that the conduction heat along the thermocouple wire can be greatly reduced.

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

There are three common low-temperature thermocouples: copper-constantan (T type), nickel-chromium-constantan (E type), and nickel-chromium-nickel-silicon (K type) thermocouples, which are commonly used in the temperature measurement range of -200℃ to ambient temperature (or above); nickel-chromium-gold-iron (iron mass fraction is 3%) thermocouples 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 pile. The thermoelectric potential of the thermocouple pile is the sum of the thermoelectric potentials of the stacked thermocouples, that is, the thermoelectric potential of n thermocouples is n times the thermoelectric potential of a single thermocouple.

The heat conduction along the thermocouple wire may cause serious errors in the thermocouple temperature measurement. 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 a very small diameter thermocouple wire can also greatly reduce the temperature measurement error.

4. Vapor pressure thermometer

Vapor pressure thermometers measure temperature by using the property that the saturated vapor pressure of a liquid is a fixed function of the temperature of the liquid in contact with the vapor. The thermometer consists of a temperature bulb filled with pure gas, which can be condensed within the temperature range. The temperature bulb is connected to the pressure measuring device through a capillary tube.

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

The purity of the fluid in the thermometer is critical, 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 package.