Where does the thermistor measure the temperature exactly ?

The thermistor actually measures the temperature inside the openQCM device. In fact the thermistor is embedded in the openQCM shield inside the hardware case, between the arduino micro and quartz crystal sensor. It is important to monitoring the temperature because the quartz sensor frequency depends on the variations in temperature.

You can read more about using a thermistor with Arduino to measure the temperature on the openQCM blog