Thermocouples are very simple and durable temperature sensors. They are comprised of two different materials joined at one end and separated at the other. The separated ends are considered the output, and they generate voltage which is proportional to the heat they are measuring or monitoring. That is, the hotter the temperature, the higher the voltage. The fact that two metals generate voltage is known as the Seebeck effect. Two common applications of thermocouples are measuring room temperature and monitoring the presence of a pilot light.
Thermocouples come with different pairings of materials allowing for a very wide range of applications. The different compositions are standardized into thermocouple types. The different types are given letter names which are standardized across the industry. In this way the consumer can purchase a "J" type thermocouple reader from one manufacturer and "J" thermocouples from a different manufacturer and be able to put together a system that will work. The manufacturer of thermocouples will provide tables indicating the best thermocouple type for a particular temperature range. The manufacturer will also provide data regarding the temperature and voltage relationship for each type of thermocouple.
Thermocouples are easy to use and require no batteries or fancy electronics. However, the voltage signals generated are very small so an amplifier may be required if you wish to read the temperatures into a computer. The voltage relationship with the temperature is not always a simple linear relationship so a "look up" table may be needed to convert the voltage readings into temperature readings. This information is available from the thermocouple manufacturer. Most manufacturers of thermocouples will also make devices which convert the voltages into "engineering" units. This makes using a thermocouple based temperature acquisition system even easier to implement.
The simplicity of the thermocouple may be deceiving though. Many factors can affect the accuracy of the system. Care needs to be taken when terminating the thermocouple at the measuring device so that no errors are introduced at that point. The voltage at the thermocouple terminals is proportional to the temperature of the junction so when using a thermocouple, it is very important that the junction of the thermocouple be in very close contact with the object that is being measured. Since the thermocouple is an electrically conducting wire, care must be taken so that there is no possibility of contacting other exposed electrical conductors. Finally, over time the insulation between the two wires of the thermocouple can breakdown and cause errors in the temperature reading.
With a little planning and periodic maintenance a thermocouple based temperature acquisition system will provide a cost effective and reliable method for obtaining temperature data.