A basic temperature sensor, like a thermocouple, measures temperature based on the principle of the Seebeck effect. The Seebeck effect describes the phenomenon of generating an electromotive force (EMF) or voltage across two different types of metals when they are joined together at two junctions and there is a temperature gradient between the junctions.
Here's how a thermocouple works:
Composition: A thermocouple consists of two different metal wires joined together to form two junctions. These metals are chosen based on their specific thermoelectric properties. Common thermocouple types include Type K (nickel-chromium and nickel-aluminum), Type J (iron and constantan), and Type T (copper and constantan).
Temperature gradient: When one of the junctions (called the "hot" junction) is exposed to a higher temperature, and the other junction (called the "cold" junction) is at a lower temperature, a temperature gradient is established along the length of the thermocouple.
Seebeck effect: Due to the temperature difference between the two junctions, the different metals experience a difference in their electron energies. This difference in energy levels results in the flow of electrons from one metal to the other, causing the generation of an electromotive force (EMF) or voltage across the two junctions.
Measuring the voltage: The voltage generated by the thermocouple is directly proportional to the temperature difference between the hot and cold junctions. A temperature-to-voltage conversion table, also known as a calibration table, is used to relate the EMF to the corresponding temperature. This table is provided by the thermocouple manufacturer or can be found in reference materials.
Reference junction: To accurately measure temperature, you need to consider the temperature of the reference junction (the one that is not exposed to the varying temperature). Usually, this reference junction is maintained at a constant temperature, often at 0°C (32°F), using a reference junction compensation technique or a separate temperature sensor like a cold junction compensation circuit.
Readout: The EMF or voltage measured across the thermocouple's junctions is typically read using a voltmeter or connected to a temperature measurement device, such as a temperature controller or a data acquisition system. These devices then convert the voltage to temperature using the calibration table, providing the temperature reading in Celsius, Fahrenheit, or Kelvin.
It's essential to note that thermocouples are simple, robust, and widely used temperature sensors in various applications due to their wide temperature range and reliability. However, they might have certain limitations, such as relatively lower accuracy compared to other types of temperature sensors like RTDs (resistance temperature detectors) or thermistors. The accuracy of thermocouples can vary depending on the thermocouple type, temperature range, and calibration.