How to accurately know the thermometer is accurate?

Thermometer is a common used to measure the floorboard of the instruments of all kinds of medium temperature. According to the working principle of usage scenarios, such as different, we can put the thermometer can be divided into many types. How to accurately know the thermometer is accurate? In fact also to concrete analysis according to different thermometer. Usage scenarios, for example, if we choose the measuring room temperature with a thermometer, we can at the same time many different or the same thermometer in the same room, we can observe the value of the thermometer, if one of the values of the thermometer and other the difference value of the thermometer jiaotong university, explain the thermometer exist error. If is the bimetallic thermometer, this approach is not feasible. Because in the bimetallic thermometer is mainly used to measure the temperature field of solid, gas and liquid medium temperature. The constant temperature conditions, different position of the same kind of medium temperature is different. So, if you want to accurately know the bimetallic thermometer is accurate, can have two kinds of methods: