A basic temperature sensor, such as a thermocouple, measures temperature based on the principle of the Seebeck effect. The Seebeck effect states that when two different metals are joined together at two different temperatures, a voltage is generated across the junction of the metals. This voltage is proportional to the temperature difference between the two junctions.
Here's how a basic thermocouple temperature sensor works:
Metal Junctions: A thermocouple consists of two different types of metal wires, often referred to as the "legs" of the thermocouple. These metals are chosen for their specific thermoelectric properties. The point where the two metal wires are joined is called the "hot" junction, and the other ends of the wires are referred to as the "cold" junction.
Temperature Difference: The "hot" junction is exposed to the temperature that needs to be measured, while the "cold" junction is kept at a reference temperature. This temperature difference between the two junctions is what drives the generation of a voltage.
Voltage Generation: Due to the Seebeck effect, a voltage is produced across the junctions of the two different metals. The magnitude of this voltage is directly proportional to the temperature difference between the hot and cold junctions. This voltage is typically very small and is measured in millivolts (mV).
Measurement and Calibration: The voltage generated by the thermocouple is then measured using a device called a voltmeter or a thermocouple reader. The voltage reading is converted into temperature units using a calibration curve or a mathematical equation specific to the type of thermocouple being used. Different types of thermocouples have unique characteristics and voltage-temperature relationships, which is why calibration is essential for accurate temperature measurements.
Compensation: To ensure accurate measurements, the cold junction (reference junction) temperature must be known. This is typically accomplished using a separate temperature sensor, such as a thermistor or an integrated circuit, placed at the reference temperature point. This compensates for any temperature variations at the cold junction.
Thermocouples are widely used temperature sensors due to their simplicity, ruggedness, and ability to measure a wide range of temperatures. However, they do have some limitations, such as limited accuracy and sensitivity compared to other temperature sensing methods like resistance temperature detectors (RTDs) or thermistors. The choice of the specific type of thermocouple depends on factors like the desired temperature range, accuracy requirements, and environmental conditions.