Thermocouple assemblies, also called thermocouples, temperature sensors or temperature probes, instruments that both sense heat and control temperature. Consisting of two connected, dissimilar metal wires, their operation is based on the Seebeck Effect, which theorizes that a voltage is always created between two dissimilar metals, and that said voltage changes in proportion to exterior temperature changes. Thermocouple assemblies are used for industrial, residential and commercial purposes alike. Systems and devices that use them include: water tanks, plastic extrusion machines, heat exchangers, parts washers, kilns, ovens, water heaters, furnaces, pressure chambers and thermostats.
A basic thermocouple consists of two metals connected at their bases and connected again at their tips with a bead. From here, there are a few different types of thermocouples. The most universal thermocouple type, for example, is the Type K, which consists of two alloy wires, alumel (aluminum and nickel) and chromel (chromium and nickel). Type K’s have a high temperature resistance to be used with everyday devices. Type K thermocouple assemblies are part of a larger group called noble metal thermocouples, of which all measure up to 2000°C.
Other noble metal thermocouples include K, N, R and S Types. They are all, of course, noble metals, which are metals with high resistance to corrosion from chemicals and organic acids. Types T and J are base metal thermocouples, meaning they may be composed of any metal except a noble, or precious, metal. Commonly, they’re composed of tin, nickel, copper, aluminum or lead. They measure temperatures under 1000°C. Type C refractory metal thermocouples are named after refractory metals, which are coated in material with a high melting pointing. Handling temperatures above 2600°C, they have the highest heat resistance.
Manufacturers decide what combination of metals and calibrations to give to a thermocouple assembly to build based on an application’s required temperature range and intended environment. For instance, thermocouple wires with very thin walls have a narrower temperature range capacity than those with thicker wires. So, if an application calls for a broader temperature range, its thermocouple must have thicker wire walls.
In addition, different metal materials respond differently to environmental factors like mechanical vibrations, abrasions or chemical exposure; these factors must be taken into account when fashioning a thermocouple. In addition, if a thermocouple is being installed into a pre-existing system, manufacturers must design it for compatibility.
Most thermocouple assemblies are covered in a protective layer insulative tubing called a sheath. In response to this, thermocouples may have one of three junction types: grounded, ungrounded or exposed. For the fastest response and reading time, thermocouples are exposed, meaning it protrudes beyond the sheath so that it is directly exposed to the environment around it. However, this method is only viable in non-pressurized and non-corrosive environments.
Usually, multiple thermocouples work together to gather information that will commute a larger measurement. Often, this information is connected to automated computer technology that quickly and accurately converts into digital form for further analysis. Similarly, many thermocouples are accompanied by thermocouple wires or thermowells, which are accessories that remove them from the damaging influence of heat, while extending their reach.
Alternatives to thermocouples include thermistors and resistance temperature detectors (RTDs). Both of these devices use the electrical resistance of certain metals to measure temperature change. Thermistors are made from metal oxides, which have an inverse resistance to rising temperature; as temperatures increase, thermistor resistance falls. For this reason, thermistors fall into the category of “negative temperature coefficient” (NTC) sensors. As they only accurately measure environments up to 200℃, thermistors are only appropriate for low to mid-temperature applications.
For higher temperatures, RTDs are a much better choice; they accurately (within +0.5%) measure temperatures between -200 and 800℃. While RTDs tend to have produce tighter accuracies than thermocouple assemblies, thermocouples are have a much higher heat capacity and are much less expensive to make and use. Thermocouple assemblies are the most popular temperature measuring system because they are simply constructed, easily installed and inexpensive. In addition, they boast short response times, good readability, wide temperature ranges and more application options than thermistors and RTDs.