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REFERENCE:

Thermocouple Theory
Specifying Thermocouples
Specifying RTDs
Improving Temperature Measurements
Specifying Temperature Sensors in Hazardous Areas
Thermowells - Why They Fail
Thermowell Selection Guide
Choosing the Right Temperature Sensor
 
 
 
 
 

Choosing the Right Temperature Sensor

Criteria Thermocouples vs RTDs TC RTD
Range

Although new and improved manufacturing techniques have increased the range of RTDs, this category belongs to Thermocouples. Better than 95% of RTDs are used in temperatures below 1000 F. Thermocouples can be used up to 2700 F.

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Sensitivity

Grounded Thermocouples are inheritantly tip sensitive; while RTD elements are isolated from their sheaths. A grounded Thermocouple will respond to a 63% step change in temperature nearly 3 times faster than a RTD counterpart.

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Cost

Comparing a 12 inch, SS sheath .25'', Type J grounded Thermocouple, with a 100 Ohm platinum RTD.00385 Alpha, prices the thermocouple at 2.5 to 3 times less than an RTD. Installed cost make up some of this difference since RTDs use inexpensive copper lead wire to transmit the signal back to the DCS.

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Accuracy

There are many factors to determine accuracy; linearity, stability, and repeatability to name a few that can affect accuracy. While a Thermocouple's stand alone accuracy can approach that of an RTD, the superior advantages in these other areas make the RTD the choice.

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Linearity

Temperature vs. resistance nearly plot a straight line for an RTD, while a Thermocouple shows an almost "S" like curve.

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Ruggedness

Thermocouples can essentially be one piece. RTD elements both thin film and wire wound must be connected to copper wire.

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Stability

Due to their linearity and virtually drift free output, RTDs are more stable than Thermocouples.

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