Thermocouple type K or PT100?

Both measure temperature. They are good at different things.

Thermocouple type K

Two different metal wires joined at the tip. Cheap, tough and fast, and it works from about −200 to about 1,250 °C. The catch is accuracy: a standard type K is good to about ±2.2 °C, or 0.75% of the reading at high temperatures. Use type K extension cable all the way to the controller, not ordinary copper wire.

PT100

A small platinum resistor: 100 ohms at 0 °C. It is more accurate (a class B PT100 is about ±0.3 °C at 0 °C) and stays accurate for years, but most PT100s only go up to about 400 to 600 °C, and they cost more. Use the 3-wire type, so the length of the cable does not change the reading.

The simple rule

• Hotter than about 500 °C (furnaces, kilns, exhaust): thermocouple.
• Below that, and you care about one or two degrees (food, plastics, labs, HVAC): PT100.
• Below that, and a few degrees do not matter: either; type K is cheaper.

Whatever you choose, tell the seller the probe length, the diameter and the thread you need (for example M8 or 1/4 inch), and whether it needs a protective sleeve (thermowell).