Basics of Thermocouples

Using the Seebeck effect (explained above), a thermocouple generates a voltage according to the temperature difference T between the junction of the two types of metals (temperature measuring junction) T1 and the instrument-side junction (reference junction) T0.
Temperature measurement using a thermocouple measures this voltage with an instrument.

• Directly reading the temperature with the reference junction kept at 0°C (32°F) (cold junction compensation)
• Measuring the temperature of the reference junction (reference junction compensation) and adding it to the temperature difference ΔT

It is difficult to keep the cold junction at 0°C (32°F) during measurement. Instead, the temperature of the temperature measuring junction can be obtained by measuring the temperature around the terminal and adding to it a thermoelectromotive force with 0°C (32°F) as the reference. This is known as reference junction compensation.

To use the thermocouple effectively for your application, you first have to consider the right type for your specific temperature range and environmental conditions—as discussed above, not all thermocouples are made equal.

Each thermocouple type is composed of different materials, which provides them with different temperature ranges and responses, so it's really important to consider your application. Once you've selected the appropriate thermocouple, you need to install it properly. This means that the thermal junction of the thermocouple should be in direct thermal contact with the surface or medium whose temperature we're measuring.

The thermocouple's wires should be connected to the appropriate measuring unit, such as a digital thermometer, a data logger, or a control system that can interpret the millivolt signal generated by the changes in temperature. Whatever the measuring unit might be, it's most likely equipped with a cold junction compensation, which is crucial for accurate temperature readings.

You, as the user or operator, must ensure that the instrument is properly calibrated, including the required compensation that matches the type of thermocouple used. These have to be properly configured to ensure correct temperature measurements and readings.
Adequately calibrated equipment and accurate readings enable you to monitor, record, and even control the temperature based on the needs of your specific application.

Take a thermocouple that is put in a glass of hot liquid.
The liquid is assumed to have a uniform temperature of 100°C (212°F) (no temperature gradient).
At this time, no thermoelectromotive force is generated in the thermocouple section in the liquid. A thermoelectromotive force is generated only where there is a temperature gradient.
Since the sensor section of the thermocouple is where a thermoelectromotive force is generated, this section with a temperature gradient is the sensor section.

Thermocouples generate a small voltage that's directly related to the temperature difference between the thermocouple measurement juncture, also known as the hot juncture, and the reference (cold) juncture. This voltage is generated due to the Seebeck effect, which is caused when a circuit made of two dissimilar metals produces a voltage when there is a temperature difference between the two junctures.

The voltage generated by the thermocouple is usually in the microvolt to millivolt range and varies depending on the type of the thermocouple, the metals used in the junctions, and the temperature difference between the two spots. For example, a Type K thermocouple—made of chromel and alumel—generates approximately 41 microvolts per degree of Celsius on temperature difference.

The measurement instrument takes the generated voltage and converts it into a temperature reading by accounting for the reference junction, which is often done using a built-in junction sensor. The resulting voltage is then interpreted according to the type of thermocouple that is being used to measure temperature.

The following eight types of thermocouples are available according to the combination of two types of metallic conductors.

B, R, and S thermocouples are called noble metal thermocouples while N, K, E, J, and T thermocouples are called base metal thermocouples.
Noble metal thermocouples—which contain metals having a high melting point, such as platinum and rhodium—are generally used for measurement of temperatures of +1000°C (+1832°F) or higher, and base metal thermocouples are often used for measurement of temperatures below +1000°C (+1832°F).
The table below lists the characteristics of each type of thermocouple.

The core fibers of a thermocouple are generally shielded from outside air to ensure resistance to oxidation and durability in corrosive environments.
A thermocouple in which the area between its metallic sheath and a pair of thermocouple core fibers has been filled with powdery inorganic insulation is called a “sheath-type thermocouple.”

Due to the above features, the use of these thermocouples has been expanded gradually since they were put into practical use more than 10 years ago.

Select the ideal junction type according to your application.

Compensation lead wires refer to conductors used to connect thermocouples and temperature-measuring instruments.
As they have an almost equivalent thermoelectromotive force characteristic to that of thermocouples in their operating temperature range (0°C to +60°C / 32°F to +140°F), they are mainly used to extend thermocouples.

Let’s assume the following temperature gradient.

Because the temperature sensing section has a temperature gradient, a thermoelectromotive force corresponding to that temperature difference is also generated on the compensation lead wire. The instrument calculates the total of the generated thermoelectromotive forces and displays the results as the temperature.

If a copper lead wire, instead of a compensation lead wire, is used as shown above, no thermoelectromotive force is generated even on a section with a temperature gradient. This issue generates an error in the results of the temperature measurement.

In cases where there is no temperature gradient, no thermoelectromotive force is generated either. Therefore, copper lead wires may be used for the extension of a section having no temperature gradient where no thermoelectromotive force is generated.

For a connection between a thermocouple and a compensation lead wire, a general terminal block may be used without any problems on the condition that there is no temperature gradient in the connecting section. However, it becomes impossible to perform an accurate measurement if a temperature difference occurs. In this case, use a dedicated connector that has a thermoelectromotive force characteristic that is equivalent to that of the relevant thermocouple.

Thermocouples can be used even after being extended by 1 km or more. However, instruments generally have a specific “input signal resistance,” that is, the maximum resistance of input signals that can be wired. Note that accurate measurement becomes impossible if the total resistance value of the thermocouples exceeds this value.

Calibration of thermocouples refers to the operation that determines the relationship between the value indicated by the thermocouple being used and the true temperature. In general, calibration is performed once every half a year. Methods of calibration are largely classified into the fixed point method and the comparison method.

KEYENCE’s products are designed for precise measurements and can help eliminate common problems such as incorrect polarity, mismatched types, and usage of incorrect materials.

Contact us today to learn more about our innovative solutions for your temperature-sensing needs.