Basics of Thermocouples & RTD's

Thermocouples are used to measure industrial process temperatures from -184°C (-300°F) to 1482°C (2700°F). They sense temperature at the tip when two dissimilar metal wires are joined together to form a junction. At the (+) & (-) terminals a millivoltage increases as the temperature goes up.

Thermocouples are "application specific" to exactly meet each customer's process needs... made to order and rarely off the shelf.

Since there are millions of industrial processes, there are millions of thermocouple variations. If the thermocouple is specified incorrectly, it may last for only a few minutes in the process. For harsh applications, some thermocouples last only a few months. It is difficult to predict service life for all applications.

As a manufacturer, TempoTech selects the "best fit" thermocouple for longest service life based on...

  • Maximum operating temperature
  • Environment (liquid, gas, atmosphere, chemical etc )
  • Mechanical Mounting Requirements

The most common thermocouple calibration types are J & K and one lead is magnetic. The lead wires and outer jacket are colour coded

A thermocouple must be hooked up to instrumentation with thermocouple extension wire and the red lead is always negative.

An RTD ( Resistance Temperature Detector ) looks similar to a thermocouple, but works on the principal of is resistance increasing with temperature. 100 Ohms @ O°C, 138.51 Ohms @ 100°C etc.

RTDs are much more accurate and linear than thermocouples and may be connected to instrumentation using copper wire.

RTDs have a Platinum element and therefore are more expensive than equivalent J or K thermocouples.

They come in (2), (3) or (4) wire versions, colour coded R&W, RR&W or RR&WW.

Generally RTDs are installed up to 450°C (842°F).

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