1. Fundamentals of Thermocouples
Thermocouples are the most commonly used temperature sensors.
A thermocouple is created when two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermo-electric voltage is known as Seebeck voltage, named after Thomas Seebeck, who discovered the phenomenon in 1821. The voltage is non-linear with respect to temperature; however for small changes,
DeltaV = S * DeltaT
where S is the Seebeck coefficient. However, S changes with temperature, causing the output voltages to be nonlinear over the operating range. By measuring a thermocouple's voltage, you can calculate temperature.
Thermocouples are designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, J-type thermocouple is made from iron as one conductor and constantan (copper-nickel alloy) as another.
2. Signal Conditioning Requirements
- Amplification for High-Resolution A/D Converter
Thermocouples generate very low-voltage signals (mV). To acquire these signals with a data acquisition (DAQ) board, you must amplify thermocouple signal to measure it accurately with a standard 12-bit DAQ device.
- Alternatively, you can use a DAQ device with a high-resolution A/D converter. A board with 16 bits of resolution and amplification capabilities or a board with 24 bits of resolution is recommended.
- Cold-Junction Compensation
Thermocouples require some form of temperature reference to compensate for unwanted parasitic thermocouples. Traditionally, junctions AC and BC were held at 0 °C in an ice bath. The National Institute of Standards and Technology (NIST) thermocouple reference tables are created using this setup. Although an ice bath reference is quite accurate, it is not always practical. A more practical approach is to measure the temperature of the reference junction with a direct-reading temperature sensor, such as a thermistor or an IC sensor, and then subtract the parasitic thermocouple thermoelectric contributions. This process is called cold-junction compensation.
A thermocouple can act very much like an antenna, making it very susceptible to noise from nearby 50/60 Hz power sources. Therefore, it is recommended to apply a 2 Hz or 4 Hz low-pass filter to your thermocouple signal to remove this noise.
A thermocouple's output voltage is not linear with temperature. Therefore, your system must perform linearization either through hardware or software.
3. Measurement Considerations
Thermocouples are the most popular amongst all temperature sensors. Unlike some other temperature sensors, they are:
- Cater to a wide temperature ranges
Other factors to consider when using thermocouples for temperature measurements:
- Thermocouples generate extremely low voltages, making them susceptible to noise.
- A thermocouple's temperature sensitivity is small, requiring accurate instrumentation.
- A cold-junction compensation sensor is required when using thermocouples.
- Thermocouples are not as stable as other available temperature sensors.
- Typical thermocouple accuracy is ~1°C.
4. Thermocouple Measurement Hardware
National Instruments offers hardware to measure many different types of temperature sensors. The table below shows NI temperature measurement hardware for a variety of needs. Click on an application type to see system components, prices, and more information on the hardware. For more temperature measurement information see the NI Temperature Measurement Resource Page.
Thermocouple Starter Sets
|NI USB TC01||1||Low cost,
Starting at $99 USD
|NI C Series||Up to 16||USB, wireless, ethernet
Starting at $588 USD with 24-bit resolution
|NI CompactDAQ||16 - 128||USB Data Acquisition,
Low-to-medium channel count
|NI SC Express||32 - 1,000+||High accuracy(~0.3°C), PXI Platform,
Medium-to-high channel count
|300 Vrms isolation,
Medium- to high-channel
Measuring Temperature Resource Page