Thermowell

Thermometers are well-known liquid expansion devices. Generally speaking, they come in two main classifications: the mercury type and the organic, usually red, liquid type. The distinction between the two is notable, because mercury devices have certain limitations when it comes to how they can be safely transported or shipped. 

For example, mercury is considered an environmental contaminant, so breakage can be hazardous. Be sure to check the current restrictions for air transportation of mercury products before shipping.

The commonly-known thermometer is yet another temperature sensor. 

Whatever sensor you select, it will not likely be operating by itself. Since most sensor choices overlap in temperature range and accuracy, selection of the sensor will depend on how it will be integrated into a system.

 

Thermocouples. Probably the most used method. Thermocouples are basically two wires, made of different metals and joined at one end. Changes in the temperature at the joined end induce a change in electromotive force (emf) between the other ends. See our page on Thermocouple Types for a description of the more usual materials used in thermocouple construction, and the benefits of each combination. 
Resistive Temperature Devices. Resistive temperature devices work on the principle that the electrical resistance of a material changes as its temperature changes. There are two main type of Resistive temperature device: 
     - Metallic devices (commonly referred to as RTDs). RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature, 
    - Thermistors. Thermistors rely on resistance change in a ceramic semiconductor with the resistance dropping non linearly with temperature rise. 
Bimetallic devices. Bimetallic devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side will expand more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. 
Infrared sensors. Infrared sensors are non-contacting devices. They infer temperature by measuring the thermal radiation emitted by a material. They tend to be used when temperature measurements are extremely high, so are not seen offshore too often.