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| TECHNICAL ARTICLE - 'THE HUMBLE THERMOCOUPLE' |
| 24/01/06 |
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TECHNICAL ARTICLE
Where does the humble thermocouple stand in the high-tech world of measurement and control?.
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 +/-2C 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 presented with a feeble output (typically 40uV/C), 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.
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 and S) span -200 to +1800C depending on their construction.
They are generally rugged thanks to sheathing options, can operate in corrosive media and are replaceable world-wide; reliable.
In mineral insulated form, they are flexible and versatile.
All thermocouple types are supported by most 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 and infra-red devices, which are all valuable in particular applications.
However, none of these alternatives to the thermocouple can compete in terms of temperature range, versatility and cost-effectiveness
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