Thermocouples are temperature sensors consists of two different metals joined together at one end. When the junction of the two metals is heated or cooled, a voltage is created which can be related to temperature.
A thermocouple is a simple, robust, cost-effective temperature sensor used in various temperature measurement processes. Various applications rely upon thermocouples. However, due to their wide range of models and technical specifications, it is vital to understand its basic structure, functionality, and ranges to better determine the right thermocouple type and material of thermocouple for an application.
Construction of Thermocouple
A thermocouple consists of two dissimilar metal conductors, which are electrically insulated except for the hot junction. At the hot junction, the conductors may be soldered or welded together. A refractory/metal sheath protects the thermocouple from mechanical damage. Two metal wires are extended as compensating cables or extension cables, which allows the meter or read-out device to be placed at a considerable distance away from the hot junction of the thermocouple.
Working of Thermocouple
The thermocouple is mainly used to measure high temperatures in furnaces. A junction kept near the hot surface is called the hot junction. The junction at which emf generated is measured is called cold junction. The magnitude of emf generated depends upon the temperature difference of two junctions and the type of metal used. Emf generated is directly calibrated in terms of temperature at the read-out device or meter reader to indicate the reading of temperature difference between two junctions.
Factors to consider when selecting Thermocouples for Temperature Measurement
- Construction, whether rigid.
- Atmospheric or environmental conditions.
- Type of insulation used.
Materials for thermocouples
Materials commonly used are copper, iron, platinum, rhodium, and alloys such as Constantan (60% Cu + 40% Ni) and Cromil (10% Cr + 90% Ni). The size of the wire depends upon the temperature range to be measured. For the measurement of high temperatures, heavier wires are used.
Pros of Thermocouple
- Wide measurement range of temperature from -200°C to 1600°C.
- Cheaper than a resistance thermometer (RTD).
- Accurate Measurements.
- Good reproducibility.
- Low cost. No need for external supply.
- Rigid and robust construction can be used in high vibration or adverse environments.
- Bridge circuits are not required for measurements.
- Ability to connect to suitable electrical equipment for measuring rapid or sudden changes in temperature.
Cons of Thermocouple
- Thermocouple does not allow any compromise in essential requirements during temperature measurements, such as Cold junction, Cable, and Lead compensation.
- Output is minimal. Requires signal amplification.
- At high temperatures, thermocouple shows a non-linear relationship of Temperature-EMF.
- Not suitable for areas subject to strong radiation fields.
- Slower response than RTD. Therefore, it cannot be used for high-sensitivity applications.
- Proper separation of extension cables from thermocouple wires is essential to avoid errors.
Applications of Thermocouple
- For monitoring liquid and gas temperatures in pipes, ducts, or in storage. Industrial furnaces, hot baths, and solutions in chemical reactors.
- For temperature measurement in the cryogenic range (-150°C to 273°C).
- Measurement of subzero temperature in the refrigeration system.
Choosing a Thermocouple?
Because a thermocouple can take many shapes and forms, it is crucial to understand how to select the suitable sensor correctly. The most common criteria used to make that choice are the temperature range, chemical resistance, abrasion and vibration resistance, and installation requirements. Installation requirements would also dictate the choice of a thermocouple probe. There are different types of thermocouples with various applications. For example, an exposed thermocouple will work best when high response times are required, but an ungrounded thermocouple is better in corrosive environments.
How do I know which type of junction to choose?
Sheathed thermocouple probes have three junction types: grounded, ungrounded, or exposed. At the end of a grounded junction probe, the thermocouple leads are physically attached to the inside of the probe wall. As a result, the probe wall transfers heat from the outside to the thermocouple junction effectively. The thermocouple junction is detached from the probe wall in an ungrounded probe. The response time is slower than the grounded style, but the ungrounded style provides electrical isolation.