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. |
|
* |
|
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. |
|
* |
|
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. |
|
* |
|
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. |
|
|
* |
Linearity
|
| Temperature vs. resistance nearly
plot a straight line for an RTD, while a Thermocouple
shows an almost "S" like curve. |
|
|
* |
Ruggedness
|
| Thermocouples can essentially
be one piece. RTD elements both thin film and
wire wound must be connected to copper wire. |
|
* |
|
Stability
|
| Due to their linearity and virtually
drift free output, RTDs are more stable than
Thermocouples. |
|
|
* |
