How does a thermocouple measure temperature?
A thermocouple measures temperature based on the Seebeck effect, which is the principle that a voltage (or electromotive force, EMF) is generated when there is a temperature difference between two different metal wires or conductors. Here's a step-by-step explanation of how this works:
1. Composition of a Thermocouple
A thermocouple consists of two different metal wires or alloys, known as the thermoelectric materials, which are connected at one end. This joined end is called the measuring junction (or hot junction). The other ends of the wires are connected to a measurement device, which is called the reference junction (or cold junction).
The two metals used in a thermocouple create a potential difference (voltage) when they are subjected to a temperature gradient.
2. The Seebeck Effect
The Seebeck effect is the phenomenon where a voltage is generated when there is a temperature difference between two dissimilar metals. This voltage is proportional to the temperature difference between the measuring junction and the reference junction.
When the measuring junction (where the two metals meet) is heated or exposed to a temperature, the electrons in the metals gain energy and move, creating a voltage across the wires.
The reference junction (usually kept at a known or stable temperature, often 0°C or room temperature) serves as a baseline for the measurement.
The greater the temperature difference between the two junctions, the larger the voltage generated.
3. Voltage Generation
The voltage generated by the thermocouple is very small, usually in the millivolt range, and varies with temperature. The relationship between the temperature difference and the voltage is typically nonlinear. However, the voltage can be measured accurately by specialized instruments (such as a voltmeter or digital thermometer) to determine the temperature at the measuring junction.
4. Reference Junction Compensation
Since the temperature at the reference junction affects the voltage reading, it must be known or compensated for. There are two common ways to handle this:
Cold Junction Compensation (CJC): This method measures the temperature at the reference junction and adjusts the voltage accordingly.
Ice Point Reference: In some cases, the reference junction is kept at a constant known temperature (such as 0°C), usually by placing it in an ice bath.
5. Calculating Temperature
The voltage generated by the thermocouple is then compared to a reference table or an equation, which correlates the voltage to a specific temperature based on the thermocouple's material properties. These tables, known as thermocouple reference tables, are specific to the type of thermocouple (e.g., Type K, Type J, Type T, etc.) and contain the known voltage-temperature relationship.
