I recently bought this MAX6675 sensor with a K-type thermocouple, and I'm wondering what's wrong with it. I'm been testing it against a DS18B20 based sensor (metallic tip), and MAX6675 is giving me readings way off the mark.
I tested it putting both sensors together against a hot plate and taking readings every 15 seconds. Plate start from room temperature. Previously, sensors were bring to start temp by immersing it in ice water.
- Signal to Arduino Pin 8. - Ground to Ground. - Vcc to Vcc (plus a 4.7K resistor between Vcc and Ground).
- SCK to Arduino Pin 4. - CS to Arduino Pin 5. - SO to Arduino Pin 6. - Vcc to Arduino Pin 3 (HIGH). - Gnd to Arduino Pin 2 (LOW).
This is the result from a run test (temps in °C):
Basically, the greater the temp, the greater the difference between MAX6675 reading and "true" temp (as defined by DS18B20). In previous tests, when real temp reach 100°C, MAX6675 was giving ~150°C readings. This is not a offset or noise, but something else, almost exponential.
What is wrong here? Bad thermocouple? Throw it away and buy something else?