A thermocouple is a type of temperature sensor that converts thermal energy into electrical voltage, which can be used to measure temperature. They are valued for their simplicity, durability, and wide range of operating options, making them suitable for most applications.
Thermocouples consist of two dissimilar metal wires. When one junction of these wires experiences a different temperature than the other, a small voltage is generated. This voltage changes predictably with temperature, allowing thermocouples to provide a reliable way to monitor temperatures across various industries, including high-temperature furnaces and more.
How does a thermocouple work?
As mentioned, a thermocouple consists of two different metal wires joined at one end to create a temperature-sensing junction. When this junction experiences a temperature change, it generates a small electrical voltage that corresponds to the temperature difference between the junction and the wire ends.
Thermocouples operate based on the Seebeck effect, first discovered by Thomas Johann Seebeck in the early 19th century. While any two dissimilar metals can produce a voltage, only specific types of thermocouples offer stable, accurate, and predictable temperature readings in various environments.
When one end of the metal wire is heated, the free electrons diffuse toward the colder end. Because different metals react to heat in distinct ways, the electrons in each wire move at different rates. This creates a difference in electrical charge between the hot end and the cold end.
A thermocouple circuit consists of two junctions: the Hot Junction and the Cold Junction.
The hot junction is the joined end placed in the environment or on the surface being measured, while the cold junction is the unjoined end connected to a measuring instrument at a known, constant temperature.
The voltage generated is directly proportional to the temperature difference between the hot junction and the cold junction. By knowing the temperature of the cold junction and measuring the generated voltage, the measuring instrument can calculate the exact temperature at the hot tip.
Why choose thermocouples for temperature measurement?
Thermocouples are preferred for temperature measurement because they offer a wide temperature range, fast response times, high durability, and low cost.
They can measure temperatures accurately, even under extreme conditions. Intense heat, constant vibration, corrosive chemicals, and high pressures do not disrupt their readings (provided the appropriate thermocouple is selected). They continuously operate, measure, send data, and support processes that depend on strict thermal control. Conax thermocouples can be used in extremes ranging from -200˚C to 2315˚C. Moreover, they have fast response times and are one of the more affordable options compared to more precise alternatives, such as Resistance Temperature Detectors (RTDs).
Thermocouple types
There are many different thermocouple types available for a wide range of applications. Each thermocouple alloy combination determines its temperature range and sensitivity. These combinations are standardised and represented by letter designations.
| SERIES | DESCRIPTION | RANGE F.S | REMARKS |
|---|---|---|---|
| J Type | Iron vs Copper Nickel (Constantan) | 0° C to +760° C | For use in reducing atmospheres. Iron may oxidise if unprotected in oxidising atmospheres. Limited use possible in oxidising atmospheres at high temperatures; not recommended at low temperatures. |
| K Type | Nickel-Chromium vs Nickel-Aluminium | -200° C to +1260° C | For use in oxidising atmospheres. Not recommended for reducing atmospheres. |
| E Type | Nickel-Chromium vs Copper-Nickel | -200° C to +870° C | Good for use in oxidising atmospheres. The highest EMF output of the common thermocouples. |
| T Type | Copper vs Copper-Nickel (Constantan) | -200° C to +370° C | For use in oxidising, reducing and inert atmospheres. Capable of cryogenic temperature service. Good where moisture is present. |
| N Type | Nickel-Chromium-Silicon vs Nickel-Silicon-Magnesium | 0° C to +1260° C | Less affected by the order/disorder transformation that causes calibration shifts in Type K. For use in oxidising atmospheres. |
| S/R Type | Platinum-Rhodium vs Platinum | 0° C to +1480° C | For use in oxidising atmospheres. Alumina protection tubes are recommended to resist contamination at elevated temperatures. |
| B Type | Platinum-30% Rhodium vs Platinum-6% Rhodium | +870° C to +1700° C | For use in oxidising, inert, or vacuum atmospheres. Alumina protection tubes are recommended to resist contamination at elevated temperatures. |
| C Type | Tungsten-5% Rhenium vs Tugsten-26% Rhenium | 0° C to +2315° C | For use in hydrogen, inert or vacuum atmospheres. |
Applications
Conax thermocouples are heavy-duty, ultra-high-temperature solutions designed to withstand extreme environmental challenges and are suitable for a wide range of applications.
Here is a list of some of the most popular industries:
Gas Turbines & Jet Engines
- Used for exhaust gas monitoring in large industrial gas turbines.
- Flight-certified for commercial aviation applications.
Semiconductor
- In diffusion furnaces, Conax thermocouples provide precise vertical and horizontal multi-zone temperature profiles crucial for monitoring heat during silicon wafer processing.
Pressure Vessels, Autoclaves and Vacuum Furnaces
- Bulkhead feedthroughs: These specialised seal fittings are used to pass sensor wires, power electrodes, or tubes through a vessel wall without compromising vacuum or high-pressure integrity.
- Employed in high-temperature heat treating within industrial kilns, blast furnaces, and aerospace vacuum chambers.
Oil and Gas
- Hazardous area sealing: Custom thermocouples are utilised to prevent gases or liquids from travelling down sensor conduits into control rooms, ensuring safety in critical environments.
Advantages and Disadvantages
Advantages:
– Wide Temperature Range: Capable of measuring a vast spectrum of temperatures, from cryogenic levels to extremely high temperatures.
– Versatility: Available in various types to cater to specific needs and environments.
– Cost-Effective: Generally more affordable compared to other temperature sensors.
– Fast Response Time: Some types can quickly respond to temperature changes, which is critical in dynamic processes.
– Durability: Built to withstand harsh environments when protected by appropriate sheaths.
Disadvantages:
– Accuracy: Requires careful handling and regular calibration to maintain accuracy.
– Limited Chemical Resistance: Certain thermocouple materials can corrode, restricting use in chemically aggressive environments.
– Installation Considerations: Proper installation is crucial for reliable performance; incorrect installation can lead to errors and reduced lifespan.
– Susceptibility to Electrical Noise: Measurements can be influenced by electrical interference, necessitating proper shielding and grounding.
If you’re searching for your next temperature measurement sensor, contact us today to learn more about Conax Thermocouples. In the meantime, check out our page on thermocouples: https://www.technimeasure.co.uk/thermocouples/.
