A thermocouple is a kind of temperature sensor.
Unlike semiconductor temperature sensors such as the TMP36 , thermocouples have no electronics inside them, they are simply made by welding together two metal wires. Because of a physical effect of two joined metals, there is a slight but measurable voltage across the wires that increases with temperature. The type of metals used affect the voltage range, cost and sensitivity, which is why we have a few different kinds of thermocouples. The main improvement of using a thermocouple over a semiconductor sensor or thermistor is that the temperature range is very much increased. For example, the TMP36 can go from -50 to 150°C, after that the chip itself can be damaged. Common thermocouples on the other hand, can go from -200°C to 1350°C (K type) and there are ones that can go above 2300°C!
Thermocouples are often used in HVAC systems, heaters and boilers, kilns, etc. There are a few different kinds but this tutorial will discuss K type, which are very common and easier to interface with.
One difficulty in using them is that the voltage to be measured is very small, with changes of about 50 uV per °C (a uV is 1/1000000 Volts). While it is possible to read these voltages using a clean power supply and nice op-amps, there are other complications such as a non-linear response (its not always 50uV/°C) and cold-temperature compensation (the effect measured is only a differential and there must be a reference, just as ground is a reference for voltage). For that reason, we suggest only using an interface chip that will do the heavy lifting for you, allow you to easily integrate the sensor without as much pain. In this tutorial we will use a MAX6675 K-thermocouple interface chip which doesn't even require an ADC, spitting out a nice digital data signal of the temperature.
- Size: 24 gauge, 1 meter long (you can cut it down if desired)
- Price: $10 at the adafruit store
- Temperature range: -100°C to 500°C / -150 to 900°F (After this the glass overbraiding may be damaged)
- Output range: -6 to +20mV
- Precision: +-2°C
- Requires an amplifier such as MAX31855
- Interface: MAX6675 (discontinued) MAX31855, or AD595 (analog)
- K Thermocouple Datasheet
- MAX6675 Datasheet
- MAX31855 Datasheet