PECM Issue 33 2018 | Page 98

MEASUREMENT & MONITROTING TEMPERATURE SENSORS LABFACILITY THERMOCOUPLE TEMPERATURE SENSORS 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 which provide a great degree of versatility and accuracy but, in many applications rely on a simple thermocouple (base-metal type usually) as the temperature sensor. Surprising really since instrumentation is typically accurate to a fraction of a degree centigrade whereas a base-metal thermocouple has uncertainties of ±2oC 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 40µV/oC), 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. 98 PECM Issue 33 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. The entire range of commonly used thermocouple types (J,K,T,E,N,R & S) span –200oC to 1800oC depending on their construction; they are generally rugged thanks to sheathing options and can operate in corrosive media; 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 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 47 years. B ognor Regis Tel: 01243 871280 Sheffield Tel: 01909 569446 [email protected] www.labfacility.com