According to the usage of temperature measuring instrument, it can be divided into contact method and non-contact method.
1. Contact method, such as bimetallic thermometer
The contact temperature measurement method is to make the temperature sensitive element contact with the measured object. When the measured temperature and the temperature sensitive element reach the heat balance, the temperature of the temperature sensitive element and the measured object is equal.If one of them is a thermometer, it can be used to measure the temperature of another object，This method of measuring temperature is called contact method.
Advantages: simple, reliable and high measurement accuracy
Disadvantages: need good thermal contact, hysteresis, damage to the temperature field of the object, and discomfort with high temperature measurement.
2. Non-contact method, such as infrared thermometer
The temperature measuring element does not contact with the measured object directly, but uses the principle that the heat radiation energy (or bright spot) of the object changes with the temperature to measure the temperature of the object, which is called non-contact method.
Advantages: the temperature field of the object is not changed, the temperature range is wide, there is no upper limit, and the reaction is fast.
Disadvantages: large error, easily affected by external factors such as distance, smoke and water vapor.