Difference Between Thermocouple And Thermopile

Thermopiles and thermocouples are both devices used to measure temperature differences based on the principle of the Seebeck effect. Learn the difference between the two, including their construction, operation, applications, advantages and disadvantages.

Thermocouple

A thermocouple also known as a “thermoelectrical thermometer” is a temperature measurement device that operates based on the Seebeck effect, which is the generation of a voltage difference when two different metals are joined at one end and subjected to a temperature gradient.

Construction

A thermocouple is composed of two different metal wires (or conductors) that are joined together at one end to form a measurement junction. This junction is exposed to the temperature being measured, while the other ends of the wires are kept at a reference temperature. The point where the two different metals are joined is referred to as the “hot” or “measurement” junction, while the other ends are known as the “cold” or “reference” junctions.

Operation

When there is a temperature difference between the measurement junction and the reference junctions, it creates an electromotive force (EMF) or voltage difference due to the Seebeck effect. This voltage is proportional to the temperature difference and the properties of the metals used in the thermocouple.

Types of Thermocouples

• Type K: Made of chromel (nickel-chromium alloy) and alumel (nickel-aluminum alloy), it’s one of the most widely used thermocouples, offering a wide temperature range and good accuracy.
• Type J: Comprising iron and constantan (copper-nickel alloy), it’s suitable for a limited temperature range and is often used in vacuum applications.
• Type T: Created from copper and constantan, it’s also good when it comes to accuracy. It is used in low-temperature applications.
• Type E: Made of chromel and constantan, it offers high accuracy and is good for moderate temperature ranges.
• Type N: Consisting of Nicrosil (nickel-chromium-silicon alloy) and Nisil (nickel-silicon alloy), it provides good accuracy at high temperatures.

Advantages And Disadvantages of Thermocouples

Advantages

• They are simple in construction, reliable and can operate in extreme conditions.
• They cover a wide temperature range, from cryogenic temperatures to high temperatures.
• They produce a voltage output directly proportional to the temperature difference, simplifying temperature measurement.

Disadvantages

• They may require calibration over time due to drift and aging of the thermocouple materials.
• The voltage generated is small, requiring amplification and signal conditioning for accurate temperature readings.
• They have lower sensitivity compared to some other temperature sensors.

Applications

• Monitoring temperature in manufacturing processes, ovens, furnaces and chemical reactions.
• Controlling heating, ventilation and air conditioning systems of HVAC systems.
• Monitoring engine temperatures and exhaust gases in automotive.
• Ensuring safe operating temperatures in aircraft engines and critical components.
• Accurate temperature measurement in research and experimental setups in laboratories.

Thermopile

A thermopile is an electronic device that is used to measure temperature differences or to convert thermal energy into electrical energy. It is made of several thermocouples connected usually in series or less commonly in parallel to increase the overall output voltage

In other words, a thermopile is a device that consists of multiple thermocouples connected in series or parallel. A thermopile with N thermocouples will output a voltage N times bigger than the one produced by a single thermocouple.

Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a temperature difference or temperature gradient.

Construction

Each thermocouple in the thermopile consists of two different metal wires (known as “legs” or “junctions”) that are joined at one end to form a measurement junction. The other ends are usually referred to as the reference junctions. The measurement junction is exposed to the temperature being measured, while the reference junctions are maintained at a constant reference temperature.

Operation

When there is a temperature difference between the measurement junction and the reference junctions, it creates a voltage difference due to the Seebeck effect. Each individual thermocouple in the thermopile generates a small voltage output, and when these voltages are combined, they result in a larger total output voltage. This aggregated output voltage is then measured and can be used to calculate the temperature difference between the measurement junction and the reference junctions.

Advantages And Disadvantages of Thermopile

Advantages

• They provide a higher output voltage compared to single thermocouples and thus can work well for applications requiring amplification of the output signal.
• They can cover a broader temperature range.
• They are useful for detecting temperature differences rather than absolute temperatures.

Disadvantages

• They are generally less sensitive to temperature changes than individual thermocouples.
• They may have slower response times compared to single thermocouples due to the nature of their construction.

Applications

• They are used in non-contact temperature measurement devices such as infrared thermometers. They detect the infrared radiation emitted by objects and convert it into an electrical signal.
• They are used in gas appliances like fireplaces and water heaters to detect the presence of a flame. The heat generated by the flame creates a temperature difference that produces a voltage, which is then used to control the gas valve.
• They are employed in pyroelectric sensors that can detect rapid temperature changes. These sensors find applications in security systems, motion detectors and even some imaging devices.
• In some cases, they are used to convert waste heat into electrical energy. This is known as thermoelectric power generation, where the temperature gradient between a hot source and a cooler sink generates a voltage that can be used to power electronic devices.

Key Differences: Thermopile vs Thermocouple

Structure

• Thermopile: A thermopile consists of multiple thermocouples connected in series or parallel to increase the output voltage. It generates a larger output voltage compared to a single thermocouple.
• Thermocouple: A thermocouple is a simple device consisting of two different metal wires joined at one end to form a measurement junction. The temperature difference between the measurement junction and the reference junction generates a voltage.

Output Voltage

• Thermopile: Produces a higher output voltage due to the arrangement of multiple thermocouples.
• Thermocouple: It generates a lower output voltage compared to a thermopile.

Sensitivity

• Thermopile: Generally less sensitive to temperature changes than a single thermocouple.
• Thermocouple: More sensitive to temperature changes due to the direct measurement of the voltage across the junction.

Applications

• Thermopile: Used in applications where a higher voltage output is required, such as infrared radiation detection and some types of temperature sensors.
• Thermocouple: Widely used for measuring temperature in various industrial, scientific, and consumer applications.

Accuracy

• Thermopile: Provides a relatively lower accuracy compared to some types of thermocouples.
• Thermocouple: Can offer higher accuracy, especially when calibrated and used correctly.

Response Time

• Thermopile: Generally slower in responding to temperature changes due to its construction.
• Thermocouple: Offers faster response times due to the direct contact measurement of temperature.

Size

• Thermopile: It is larger in size due to the presence of multiple thermocouples.
• Thermocouple: Compact and can be designed for various sizes and configurations.

Stability

• Thermopile: Generally more stable over time in terms of output voltage.
• Thermocouple: Can experience drift and degradation in performance over extended usage.

Cost

• Thermopile: Often more expensive due to its larger size and complexity.
• Thermocouple: Generally more cost-effective and widely available.

Output Signal

• Thermopile: Generates a continuous voltage output that can be amplified for signal processing.
• Thermocouple: Produces a voltage output that needs to be amplified and converted to a readable temperature value using a reference junction.

Key Takeaway: Thermopile vs Thermocouple