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