Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length. They are "simple", rugged, need no batteries, measure over very wide temperature ranges and more. They have their quirks, too, like everything else. The solution to thermocouple uses and problems lies in the details of a given application.
Have a read through, click on the key topics below to get more detail about the thermocouple (TC), how it works, color codes, recommended use limits and thermocouple standards. There is a link to an excellent article on how they work and notes on using thermocouples from a well-known expert on them.
Don't forget to check our temperature references pages for the many additional publications that provide a wealth of information on the theory and application of these very popular and rugged temperature sensors.
The applications are where the seasoned TC user and the novice part company and the one with know-how usually acheives the desired result. If all else fails to impress you to be thorough and careful with thermocouples, consider these few facts:
- Thermocouples measure their own temperature. You must infer the temperature of the obect of interest by being certain there is no heat flow between them when you take the measurement. That's easier than it sounds in some case.
- Thermocouples can err in reading their own temperature, especially after being used for a while, or if the insulation between the wires loses its resistance due to moisture or thermal conditions, or there are chemical, nuclear radiation or mechanical effects with the immediate surroundings.
- Beware of electrical hazards using thermocouples, they are electrical conductors, even refractory oxide sheathed models at high temperatures; should they contact another source of electricity, they can kill you!
- Oh, Yes..Thermocouples DO NOT MEASURE AT THE JUNCTIONS! They can't, it is physically impossible to have a temperature gradient at a point. Also the Electric Field Strength (i.e. volts/meter) at such an impossible condition would be infinite, sufficient to tear the materials apart. So, if you want to understand TCs better, start with the very basics; learn about the Seebeck Effect and how thermocouples really work!
Also, if you use thermocouples, you need to have some way to interpret their small and non-linear output voltages. There's lots of ways to do it, the simplest being a measurement of the output voltage and looking up the value in a table of millivolts dc versus temperature AND correcting for the cooler junction not being at 0 °C, or 32°F, according to your units inclination. The high end is to hook the thermocouples up to a modern readout display or a DAQ module plugged into a PC and read away!
However, it is not a fool proof business and there are many subtle things about thermocouples and their uses that have made well-intentioned engineers, who thought they understood them, look like fools. The hooking up and reading out are the easy parts of a measurement.
The selection, installation details and the conditions of use play a big role in obtaining a measurement that is accurate and reliable. It's like many measurement subjects, the devil is in the details and "simple" thermocouples have a lot of details!
Their context of use is perhaps the biggest, especially where relatively high temperature-above a few hundred degrees (on anyone's scale) are the object of measurement. Above a few thousand degrees, there are a whole range of additional problems.
Thanks for visiting and if you plan to use them, be careful, don't get fooled by their apparent simplicity. Learn the details, start with the basics (What measurement precision do I need?-Is it realistic for using thermocouples, etc.etc?) In other words always apply a systematic, measurement engineeering approach, please
What is a Thermocouple?
Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient.
Thermocouple Types and More
- Thermocouple Types, Letter Designations, Construction The ISA in the USA started the standards on Thermocouple nomenclature that includes the letter conventions used to describe certain alloy wire pairs. This is now carried on around the World and ASTM Committee E20 on Temperature provides the standard (ASTM E 230) that is now the American National Standard for the letter designation, the calibration tables, the color coding and recommended use limits of the most common types of thermocouple wire pairs
- Calibration Tables-Downloadable Courtesy of Pyromation in Fort Wayne, Indiana USA
- Calibration Tables-The NIST Manual for ITS-90 Calibration Tables From the NIST website.
- Recommended Use Limits and Tolerances Maximum temperatures by thermocopule type and construction and wire size, part of the information from the ASTM E230 Standard
- Thermocouple Wire Color Codes: USA and Others Courtesy of ISE, Instrument Service & Equipment, Cleveland Ohio, USA.
- Standards for thermocouplesand other temperature sensors.
- Calibration & Traceability Includes links to vendors of calibration equipment and services.
- How thermocouples work and notes on using them A great article by Prof. Moffat of Stanford University courtesy of Electronics Cooling Magazine.
- Training Courses and Other Resources Various support and educational resources where one can learn more about technical and applications details.
- Thermocouple Suppliers A growing list of TC vendors around the world. Also visit TempSensor.net, The Temperature Community website, where TC and other temperature sensor vendors can enter their own listings of products and news at no charge.
- Miscellaneous Information
Thermocouples are used in many places with many things like indicators and controllers to do something useful, such as control a heating system to heat a product through a temperature-time profile that causes it to soften or cook or set or transform from a stressed condition to an annealed one or any number of physio-chemical changes that produce a desired end result. This page has a growing collection of Web sites offers a look at some of those things and places. - Thermocouple Applications
Information and resources on successful uses of thermocouples, some with more detail than others.
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