What is an RTD?
RTD stands for resistance temperature detector. An RTD is a temperature sensor that uses electrical resistance to indicate temperature.
Elements of an RTD
Types of RTDs
RTD’s are categorized into different groups based on the metal used inside to create the resistance. Most modern industrial RTD’s use platinum as the resistance metal due to its inherent stability, temperature range and regularity of its temperature curve. Reotemp also offers less common RTD types to match and replace legacy and niche sensors.
RTD Accuracy
A common RTD misconception is that many people think that RTD accuracies are a fixed percentage across the board. This is not true. To find the accuracy for an RTD you will have to use a formula to calculate an accuracy at a specific point. Reotemp offers platinum 100Ω RTDs that conform to internationally recognized classes. For each class you will use a different formula to calculate an accuracy at the temperature that you plan to measure. These classes are specified in IEC60751 and ASTM E1137. These standards specify the accuracy formulas for an industrial platinum RTDs. For customers who require a higher accuracy than offered in the B, A or AA classes, Reotemp offers 1/10 B RTDs. 1/10 B RTDs are 10 times more accurate than class B RTDs. For these type of sensors there currently is no established international standard.
| Tolerance Class | Operating Rance °C | Tolerance Range °C | Tolerance Values °C |
| B | -196 to +600 | -196 to +600 | ± (0.3 + 0.005 | t | ) |
| A | -196 to +600 | -30 to +300* | ± (0.15 + 0.002 | t | ) |
| AA | -196 to +600 | 0 to +150* | ± (0.1 + 0.0017 | t | ) |
| 1/10 B | -196 to +600 | 0 to +50* | ± (0.03 + 0.0005 | t | ) |
| *Class B tolerance for rest of operating range. | |||
Accuracy Comparison: Class A vs B
| Temperature °C | Class A RTD °C | Class B RTD °C |
| -190 | 1.3 | 1.3 |
| -100 | 0.8 | 0.8 |
| -50 | 0.6 | 0.6 |
| 0 | 0.15 | 0.3 |
| 50 | 0.3 | 0.6 |
| 100 | 0.4 | 0.8 |
| 200 | 0.6 | 1.3 |
| 300 | 0.8 | 1.8 |
| 500 | 2.8 | 2.8 |
When to use an RTD vs a Thermocouple
The primary reason RTDs are chosen over thermocouples is their superior accuracy. For example, see the chart below.
| Type K Thermocouple | Class B RTD | |
| 32°F | ± 4°F | ± 0.5°F |
| 392°F | ± 4°F | ± 2.3°F |
RTDs also offer better stability with less temperature drift over time as they age. Less temperature drift results in a longer service life and less replacement with better processing results. RTDs are also a better choice for low temperature applications as they have much better accuracies below 0°C. At temperatures below 0°C, thermocouples are less stable and less reliable. RTD’s offer better repeatability. When temperature moves from a point and then returns RTDs stay much closer to the original point. RTDs offer greater interchangeability. When replacing or swapping sensors the match between each sensor is much closer. In cases where long lead wires are required RTDs are often recommended. This is because thermocouples require a more expensive lead wire consisting of the same metal as the thermocouple sensor. With RTDs only common copper hook up wire is needed.