I ordered a PT100 temperature (2-wire) sensor at Ali Express, but I finally got a XiQi electrical thermocouple sensor type WRNT-03. I think this one is different to the PT100, as the resistance between the wires is about 12,4 Ohm (I think this should be 102 Ohm for a PT100?). Trying the sensor at the Adafruit_MAX31865 example together with a Max 31865 board gives almost the same resistance as when measuring with a multimeter and that resistance changes when heating up the sensor, however the calculated temperatures differ far from reality. Can someone tell me how to determine the right temperature with this sensor?
There is no relationship between a thermocouple sensor and a PT100 sensor, except that both are used to measure temperature.
The WRNT-03 is a K-type thermocouple, according to this ad, so a K-type thermocouple amplifier should work with it. Adafruit has them.
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Thermocouples are very different than PositiveTemperatureCoefficient varaiable resistors.
Thermocouples work by generating a small voltage difference by across a bi-metallic junction, and you need high good accuracy millivolt detection capability. WRNT-03 looks like a K-type thermocouple, with a 5mV difference across a 0°-122°C range:
You won't get meaningful results from measuring its resistance at different temperatures like a PT100.
PT100 should measure 100 homes at 25C, a PT1000 should measure 1K at 24C. The resistance will change with temperature, the higher the temperature the higher the resistance.
You wanted a PT100 sensor but what you have looks to be a Type K Thermocouple. The problem is as mentioned a thermocouple outputs a very small mV (milli-Volt) output. Fortunately you can buy a Type K thermocouple amplifier using a MAX6675 chip pretty inexpensive. There are plenty of simiilar which can be found using a simple Google. The AD4895 is another K thermocouple amplifier. Buy from a reputable supplier, as mentioned Adafruit is a good choice.
Just make sure since you ended up with a K Thermocouple that it meets your accuracy requirements.
Ron
Thank you all for answering. I did not get the PT100 like I thought already. I will try to get the sensor work with a MAX6675 board.
You will find plenty of code samples online for the 6675 chip. 
Ron
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