A basic temperature sensor, like a thermocouple, measures temperature based on the principle of the Seebeck effect. The Seebeck effect states that when two dissimilar metals are connected at two different points and there is a temperature gradient between those points, a voltage is generated across the junctions.
In the case of a thermocouple, it consists of two different types of metals (usually wires) that are joined together at one end to form a junction. When this junction is exposed to a temperature difference, a potential difference (voltage) is produced across the junction. This voltage is directly related to the temperature difference between the hot and cold junctions.
The basic steps of how a thermocouple works to measure temperature are as follows:
Temperature gradient: The thermocouple's two ends, commonly referred to as the hot junction and the cold junction, are exposed to two different temperatures. For example, the hot junction may be placed inside a furnace where the temperature is being measured, while the cold junction remains at room temperature.
Seebeck effect: The temperature difference between the two junctions causes electrons in the metals to move, leading to the generation of a small electromotive force (EMF) or voltage across the junctions.
Voltage measurement: The voltage generated by the thermocouple is proportional to the temperature difference between the hot and cold junctions. To measure this voltage, the thermocouple is connected to a measurement circuit or a temperature controller.
Reference temperature: To get an accurate temperature reading, the cold junction's temperature needs to be known precisely. This temperature is considered the reference point, and its value is added or subtracted from the measured voltage, depending on the thermocouple type and calibration.
Temperature calculation: Once the voltage is measured and the reference temperature is accounted for, the temperature can be calculated using the thermocouple's specific calibration curve or a look-up table that relates voltage to temperature.
It's important to note that different types of thermocouples use different combinations of metals, which result in various temperature ranges and sensitivities. Common thermocouple types include Type K (chromel/alumel), Type J (iron/constantan), Type T (copper/constantan), and more. The choice of thermocouple depends on the temperature range and the application's requirements.