Thermocouples are simple temperature sensors composed of two different materials that generate a voltage proportional to the heat they measure. They are available in different material combinations standardized into types. Amplifiers may be needed to read temperatures into a computer, and accuracy can be affected by several factors. With proper maintenance, thermocouples provide a cost-effective and reliable method of obtaining temperature data.
Thermocouples are very simple and robust temperature sensors. They are composed of two different materials joined on one side and separated on the other. The separated ends are considered the output and generate a voltage proportional to the heat they are measuring or monitoring. That is, the higher 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 ambient temperature and monitoring the presence of a pilot flame.
Thermocouples are available with different material combinations which allow for a very wide range of applications. The different compositions are standardized into thermocouple types. The different types are given letter names that are standardized across the industry. This way the consumer can purchase a type “J” thermocouple reader from one manufacturer and “J” thermocouples from another manufacturer and be able to put together a system that will work. The thermocouple manufacturer will provide tables indicating the best type of thermocouple for a particular temperature range. The manufacturer will also provide data on the relationship between temperature and voltage 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 needed if you want to read temperatures into a computer. The relationship of voltage to temperature is not always a simple linear relationship, so a “lookup” table may be needed to convert voltage readings to temperature readings. This information is available from the thermocouple manufacturer. Most thermocouple manufacturers will also make devices that convert voltages into “engineering” units. This makes using a thermocouple-based temperature acquisition system even easier to implement.
However, the simplicity of the thermocouple can be deceiving. Many factors can affect the accuracy of the system. Care must be taken when terminating the thermocouple at the measuring device so that no errors are introduced at that point. The voltage across the thermocouple terminals is proportional to the temperature of the junction, so when using a thermocouple, it is very important that the thermocouple junction be in close contact with the object being measured. Since the thermocouple is an electrically conductive 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 thermocouple wires can break down 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 of obtaining temperature data.
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