How Does a Thermocouple Work?

Thermocouple Basics

Thermocouples operate based on the Seebeck effect, a phenomenon within the broader scope of the thermoelectric effect:

This principle states that when two different conductive materials are connected at one end and subjected to a temperature difference, a voltage is generated along the junction. This voltage is directly proportional to the temperature difference between the joined end (hot junction, T₁) and the other ends (cold junctions, T₂). According to international convention, at 0°C, all thermocouples are calibrated to show no thermo-voltage, meaning they read 0V.

Due to their simple construction, thermocouples present low equivalent impedance and require minimal, ideally no, current in the measurement circuit. This simplicity, however, makes them susceptible to electromagnetic interference (EMI) and potential errors from ground loops, which can affect measurement accuracy.

This is discussed further in the Thermocouple Limitations section below

Voltage to Temperature Conversion

The relationship between thermocouple voltage and temperature is proportional but non-linear. Because of this, converting a voltage into a temperature reading typically involves using standard lookup tables or polynomial equations.

One well-trusted source for this information is the National Institute of Standards and Technology (NIST). LabJack has created built-in temperature conversion functions (AIN_EF and TCVoltsToTemp) that use the polynomials described by the Inverse Coefficients tables on the NIST website. These tables also describe the expected error due to the conversion.

Thermocouple Junctions and Cold Junction Compensation (CJC)

Except at 0 °C, a voltage will be established at any points in a thermocouple connection where one conductive material transitions to another. This includes the junction between the two thermocouple materials at the remote end (hot junction) and in the connection between each thermocouple material and extension wire or terminal connection (cold junctions). The only voltage of interest is the one at the hot junction, so the voltage at the cold junction(s) will appear as an error in the voltage.

To obtain accurate temperature readings, it's crucial to account for the voltage at the cold junctions, which can introduce errors. Cold junction effects are commonly reduced or eliminated by placing a temperature sensor near the cold junction to measure the cold junction temperature. The temperature is converted to an equivalent thermocouple voltage to be removed from the thermocouple voltage measurement. This is cold junction compensation (CJC).

In the ”olden days”, a cold junction would literally be placed in an ice bath to keep it at 0 °C. No thermo-voltage will be generated at 0 °C, thereby eliminating any cold junction effects. Thank goodness for silicon temperature sensors!

LabJack devices streamline this process with built-in temperature sensors designed for effective CJC. See your device datasheet for specific CJC capabilities.

Thermocouple Advantages and Limitations

Thermocouples are widely used in temperature measurement due to their simplicity and broad applicability across various environments. However, while they offer several advantages, there are inherent limitations that users must consider to ensure accuracy and reliability in their measurements.

Advantages of Thermocouples

• Wide Temperature Range: Thermocouples can measure a vast range of temperatures, which is not feasible with some other types of sensors.

• Fast Response Time: Due to their minimal mass at the junction point, thermocouples can provide quick temperature readings, making them ideal for dynamic temperature monitoring.

• Robustness and Durability: Thermocouples are suitable for harsh environments, including those with high temperatures and corrosive atmospheres.

• Versatility: Available in various types and configurations, thermocouples can be selected to suit specific applications, enhancing their utility across different sectors.

• Direct Measurement: As passive sensors, thermocouples generate voltage directly related to temperature, eliminating the need for external power.

Limitations of Thermocouples

1. Small Output Voltage: Thermocouples generate a low voltage signal (e.g., around 40 µV/°C for K-type), which can make achieving high temperature resolution challenging without adequate amplification and resolution capabilities from the DAQ. 

This is the reason the U3/T4 requires an LJTick-InAmp, whereas the U6/T7 devices have the necessary resolution and amplification built-in.

2. Cold Junction Effects: The voltage at the cold junction can vary with environmental temperature changes, introducing potential errors. Effective cold junction compensation (CJC) techniques are necessary for accurate readings.

Our thermocouple AIN_EF (available only on the T7 & T8) and TCVoltsToTemp functions can handle calculations for CJC given a cold junction temperature.

If connecting to a breakout board, such as the CB37, rather than directly to the LabJack terminals, you might want to use an external silicon based sensor such as the LM34CAZ to measure the cold junction temperature.

3. Non-Linear Output: The relationship between temperature and voltage in a thermocouple is non-linear, requiring calibration or computational methods (e.g., using NIST lookup tables or polynomials) for accurate conversion.  

LabJack’s UD and LJM libraries provide convenient functions that use the NIST equations to handle the conversion, and DAQFactory has built-in conversion functions of its own.

4. Poor Inherent Accuracy: Variabilities in thermocouple material can introduce errors. International standards such as IEC 584-2 specify tolerances for these errors. These are typically ±1.0 to ±2.5 °C at moderate temperatures, but can be as high as ±9.0 °C at 1200 °C. 

See the Thermocouple Comparison Table on the thermocouple Wikipedia page

5. Bad Ground Loops: This is a common and complex problem people encounter when using thermocouples. It occurs when bare thermocouple wire is used, or a metal thermocouple probe where one of the thermocouple wires inside is connected to the probe (often called a "grounded" probe). If multiple of these are connected to a common conductor (e.g. a system of connected metal pipes), and connected single-ended to the LabJack, you can get ground loops and unexpected thermocouple junctions. 

The typical fix is to use differential inputs with a resistor (100k is typical) from the negative input to GND. As noted in the Differential Readings Application Note, the resistor is sized to be low enough to provide a path for bias currents but high enough to prevent ground loop issues. Differential connections usually prevent this problem, but sometimes further steps are needed (e.g. channel-to-channel isolation). 

To prevent this, install thermocouples so that this situation cannot occur:

a. Use some sort of substance, between the thermocouple metal and test specimen, that has good thermal conduction but poor electrical conduction.  Epoxy, tape, etc.

b. Place the thermocouple near, but not actually touching, the test specimen.

c. Use probes that are called "ungrounded". A very common option.

d. Use thermocouples with an electrically insulating layer between the thermocouple metal and test specimen.  One example is the SA1 series of self-adhesive thermocouples from Omega.

6. Ground Offsets: Care must be taken to avoid ground offset errors with single-ended measurements.  Error will result if the negative lead of the thermocouple is connected to a GND terminal that is at a different voltage than Ground at the A/D chip.  

See the "Ground Offsets" section on the Mux80 Datasheet.  A quick summary of this information would be that differential measurements (with ~100 kΩ resistor from negative AINx to GND) are recommended if using thermocouples with the Mux80.

7. EMI Susceptibility: The weak signal generated by thermocouples makes them prone to electromagnetic interference. EMI can easily introduce noise on long thermocouple wires. For example, an arc furnace near the thermocouple could cause noise issues.