Extremely low temperatures are typically measured using specialized devices and techniques. The most common method is to use a thermometer or sensor that is specifically designed to work at low temperatures. Here are a few techniques used for measuring extremely low temperatures:
Thermocouples: Thermocouples are temperature sensors that rely on the principle of the Seebeck effect. They consist of two dissimilar metals joined together at one end. When there is a temperature gradient along the length of the thermocouple, a voltage is generated that is proportional to the temperature difference. Thermocouples can be calibrated to measure temperatures as low as a few Kelvin (close to absolute zero).
Resistance thermometers: Resistance thermometers, also known as resistance temperature detectors (RTDs), use the principle that the electrical resistance of certain materials changes with temperature. These sensors are made from materials with a known temperature-resistance relationship, such as platinum. By measuring the change in resistance, the temperature can be determined. RTDs can be used to measure temperatures down to a few Kelvin as well.
Gas thermometers: Gas thermometers rely on the properties of gases at low temperatures. These thermometers use gases like helium or hydrogen, which exhibit unique characteristics at extremely low temperatures. The pressure or volume of the gas is measured to determine the temperature. Gas thermometers can be used to measure temperatures close to absolute zero.
Cryogenic temperature sensors: Specialized sensors and probes are designed to operate at cryogenic temperatures. These sensors often use superconducting materials, which lose all electrical resistance at very low temperatures. By measuring the electrical properties of the superconducting material, the temperature can be determined. These sensors are commonly used in research and industrial applications involving cryogenics.
It's important to note that different techniques have different temperature ranges and sensitivities. The choice of measurement method depends on the specific temperature range being investigated and the level of accuracy required for the application.