How can I sense very high temperatures with Arduino?

You probably want to use a thermocouple. It'll give you the ability to measure much higher temps than regular temperature sensor. You'll also need a IC like the MAX6675 to interface with it.

The MAX6675 performs cold-junction compensation and digitizes the signal from a type-K thermocouple. The data is output in a 12-bit resolution, SPI™-compatible, read-only format.

This converter resolves temperatures to 0.25°C, allows readings as high as +1024°C, and exhibits thermocouple accuracy of 8 LSBs for temperatures ranging from 0°C to +700°C.

Page 6 of the datasheet has the serial interface protocol information and. You can really just set the Chip Select line and it'll give you 16 bits of information, 12 of which will be the temperature.

One of the options to measure high temperatures would be to use thermocouples. This table provides a basic description of the types and the corresponding temperatures that they can sense.

A simple search led me to these components.

• Thermocouple Type-K Glass Braid Insulated - A mid range thermocouple
• Thermocouple Amplifier MAX31855 breakout board - Board to interface the thermocouple with the Arduino. This is needed as the voltage changes in the thermocouple are extremely small, and the rise is non-linear.

This is how the board is connected to the thermocouple.

There is also a tutorial on Adafruit that deals with using and interfacing thermocouples.

This is an image of the thermocouple interfaced with the Arduino.