Ken Jefferies of Labfacility argues that thermocouples, despite their humble origins, are still a versatile and valuable way of measuring temperature, especially in industrial applications.
Where does the humble thermocouple stand in the high-tech world of measurement and control? The funny thing is that the temperature instrumentation market is dominated by complex, sophisticated measurement and control systems that provide a high degree of versatility and accuracy but, in many applications, they rely on a simple thermocouple (base-metal type usually) as the temperature sensor. This is surprising really, since instrumentation is typically accurate to a fraction of a degree centigrade, whereas a base-metal thermocouple has uncertainties of +/-2degC before it even sees the process.
Since measurement accuracy of the entire process is only as good as that of the sensor, why rely on something that, in principle, at least, has not significantly changed in nearly 200 years (Thomas Seebeck discovered the principle of the thermocouple in 1821)? Why indeed, especially when you are presented with a feeble output (typically 40uV/degC), a non-linear transfer characteristic and the need for automatic 'cold-junction' compensation or some external ice reference; also, one of several types must be selected for each application, complete with a dedicated type of extension cable.
Harsh environments and wide ranges
The answer to these questions is a surprisingly simple one: nobody has yet been able to come up with anything better to cope with harsh environments and wide temperature ranges.
Commonly used thermocouple types (J,K,T,E,N,R & S) span –200 to 1800degC, depending on their construction; they are generally rugged thanks to sheathing options, and can operate in corrosive media; they are replaceable world-wide, reliable and, in mineral insulated form, flexible and versatile. All thermocouple types are supported by the majority of microprocessor-based instruments; they do not require excitation, since they are thermoelectric and can operate with reasonably long cable runs.
There are, of course, many alternative types of temperature sensor on the market, including resistance thermometers, thermistors, infra-red devices; all are valuable in particular applications. However, none of these alternatives to the thermocouple can compete in terms of temperature range, versatility and cost-effectiveness.
Labfacility has been manufacturing thermocouple and resistance thermometer assemblies for over 35 years. Contact the company for more technical information about thermocouples or to discuss particular applications.
