Thermocouples are temperature sensing devices that are commonly used in various industries and applications. They are based on the principle of the Seebeck effect, which states that when two dissimilar metals are joined at two different temperatures, a voltage is generated across the junction. This voltage is directly proportional to the temperature difference between the two junctions.

Thermocouples are widely used for measuring temperature in a variety of environments and materials, including gases, liquids, and solids. However, there are certain limitations to their use, which are important to consider when selecting a temperature measurement device.

Thermocouples cannot be used to measure:

1. Temperature of gas: While thermocouples can measure the temperature of a gas, there are certain factors that need to be taken into account. Gases have low thermal conductivity, which means that the temperature measured by a thermocouple may not accurately represent the true temperature of the gas. Additionally, the presence of impurities or contaminants in the gas can affect the accuracy of the measurement.

2. Temperature of liquid: Thermocouples can measure the temperature of liquids, but there are considerations to be made. The type and composition of the liquid can affect the accuracy of the measurement. For example, if the liquid is corrosive or has a high viscosity, it can damage the thermocouple or affect its response time.

3. IR radiation: Thermocouples are not suitable for measuring infrared (IR) radiation. IR radiation is a form of electromagnetic radiation with wavelengths longer than those of visible light. Thermocouples are designed to measure temperature based on the thermal energy of a material, not the radiation emitted by it. To measure IR radiation, specialized devices such as IR sensors or pyrometers are used.

In conclusion, while thermocouples are versatile temperature measurement devices, they have limitations in accurately measuring the temperature of gases, liquids, and IR radiation. It is important to consider these limitations and select the appropriate temperature measurement device based on the specific application requirements.