A basic temperature sensor, such as a thermocouple, measures temperature by exploiting the principle of the Seebeck effect, which is the phenomenon where a temperature gradient across a conductor generates a voltage difference between its two ends. In the context of a thermocouple, this voltage difference is used to infer the temperature at the measurement point.
A thermocouple consists of two different types of metal wires (or conductors) that are joined at one end to form a junction. When there is a temperature difference between the two ends of the thermocouple, a voltage is generated due to the difference in the Seebeck coefficients of the two metals. The Seebeck coefficient is a material-specific property that relates the temperature difference to the generated voltage.
Here's a step-by-step explanation of how a basic thermocouple measures temperature:
Cold Junction Reference: One end of the thermocouple, often referred to as the "cold junction," is kept at a known and constant temperature. This temperature is usually measured and compensated for in the temperature calculation to ensure accuracy.
Temperature Gradient: The other end of the thermocouple, called the "hot junction," is exposed to the temperature that needs to be measured. This temperature creates a temperature gradient along the length of the thermocouple.
Seebeck Effect: The temperature difference between the hot and cold junctions causes a voltage difference to develop across the thermocouple due to the Seebeck effect. This voltage difference is often very small and is in the millivolt range.
Voltage Measurement: The generated voltage is measured using a voltmeter or other appropriate instrumentation. The voltage reading is then used to calculate the temperature at the hot junction.
Thermocouple Tables or Calibration: To convert the measured voltage into a corresponding temperature value, thermocouple tables or calibration curves are used. These tables are based on extensive experimentation and provide a reference for the relationship between voltage and temperature for different thermocouple types.
Compensation for Cold Junction: Since the reference (cold junction) is typically not at absolute zero but at a known temperature, the voltage generated at the cold junction also affects the measured voltage. This is accounted for using the cold junction compensation techniques.
Calculation: By considering the measured voltage, the known cold junction temperature, and the thermocouple's characteristics from the thermocouple tables, the temperature at the hot junction can be calculated.
It's important to note that different types of thermocouples are made from different combinations of metals (such as Type K, Type J, Type T, etc.), each with its own unique Seebeck coefficient and temperature range. Additionally, thermocouples are widely used due to their ruggedness, simplicity, and ability to measure high temperatures, but they may have limitations in terms of accuracy and precision compared to other temperature sensing methods in certain applications.