What Is a Thermocouple?
A thermocouple is a temperature sensor that produces a small voltage proportional to temperature at the point where two dissimilar metals join. It can measure over a very wide range.
How Does It Work? (The Seebeck Effect)
When the junction of two dissimilar metals heats up, a small voltage forms between the ends; this is called the Seebeck effect. This voltage (at the millivolt level) is measured and converted to temperature.
Types
| Type | Range (approx.) |
|---|---|
| Type K | −200…+1260 °C |
| Type J | −40…+750 °C |
| Type T | −200…+350 °C |
Cold-Junction Compensation
The measurement relies on the difference between the hot junction and the reference (cold) junction; for an accurate reading the cold junction's temperature must be known and compensated. Special thermocouple interface chips (e.g. the MAX6675) do this.
Where Is It Used?
Furnaces, boilers, engine exhaust, heat treatment and industrial measurements needing very high temperature.
Interface Chips
Special interface chips are used to read a thermocouple's millivolt signal (e.g. MAX31855, MAX6675). These both amplify the signal and do the cold-junction compensation, and give the temperature directly over SPI.
Common Mistakes
- Skipping cold-junction compensation: without compensation the reading drifts.
- Reading without amplification: since the signal is at the mV level, an amplifier is needed.
- Wrong-type extension cable: a cable incompatible with the thermocouple type adds error.
Frequently Asked Questions
Why does a thermocouple measure a wide range?+
Thermistor or thermocouple?+
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