T&M RTD vs Thermocouple: A Detailed Comparison of Temperature Sensors
This article explores the differences between two common temperature sensor types: Resistance Temperature Detectors (RTDs) and Thermocouples. Both are widely used in automation systems for process control. Let’s delve into the specifics of each.
Resistance Temperature Detector (RTD)
An RTD, or Resistance Temperature Detector, is a sensor whose electrical resistance changes predictably and linearly with temperature. Here’s a breakdown:
- How it works: RTDs utilize the principle that a metal’s resistance changes with temperature.
- Operating Temperature: They typically operate up to 600°C.
- Construction: They are made from fine wire, often platinum, wrapped around a ceramic core. Thin-film technology is also used.
- Accuracy and Stability: RTDs offer accurate and consistent measurements over time, with a tolerance of around 0.15°C. They are known for their long-term stability.
- Fragility and Response Time: They are somewhat fragile and have a slower response time, typically between 2.5 to 10 seconds.
- Size: RTDs are generally larger than thermocouples, ranging from about 3 to 6 mm.
- Examples: Common types include Pt100 (100 Ohms at 0°C), Pt1000, jPt100, Cu10, Cu25, and Ni120.
Thermocouple
Thermocouples are another common temperature sensor, which operate on a different principle.
-
Operating Temperature: Thermocouples are known for their wide temperature range, capable of measuring temperatures up to several thousand degrees Celsius.
-
How it works: They generate a small voltage across the junction of two dissimilar metals when exposed to a temperature difference.
-
Accuracy: Thermocouples are less accurate than RTDs, with a tolerance of about 2°C, and their readings can drift over time.
-
Robustness and Response Time: They are not fragile and have a quick response time, usually less than 1 second.
-
Size: Thermocouples are typically smaller than RTDs, often less than 1.6 mm.
-
Examples: Common types include J, K, and T thermocouples. J operates from -346 to 2193 °F, K from -454 to 2501°F, and T from -454 to 752 °F.
Important Note: It is not recommended to use standard copper wire to extend thermocouple wires as this will cause inaccurate readings.
RTD vs Thermocouple: Key Differences
Here’s a comparison table summarizing the key differences:
| Specifications | RTD | Thermocouple |
|---|---|---|
| Accuracy | More accurate | Less accurate |
| Temperature range | -200 to 600 °C | -200 to 2000 °C |
| Cost | More | Less |
| Sensitivity | Good (1” typical, other lengths available) | Low (Point sensing only) |
| Response time | 1 to 7 seconds | Less than 0.1 seconds |
| Robustness | Good | Good, subject to drift |
| Reference junction | Not required | Required |
| Long term stability | Excellent | Good, but subject to drift |
| Power Supply | Required | Not required |
| Output type | Resistance, 0.4 Ohm/Ohm/°C, Highly linear | Voltage, 10 to 40 microvolts/°C, Approx. linear |
| Electrical noise resistance | Less Susceptible | More Susceptible |
| Size (Typical) | Medium to small, >0.5 mm | Small to large, <0.5 mm |
| Self heating | Low | No |
| Lead Effect | Medium | High |
| Advantages | • Good stability • Excellent accuracy | • Inexpensive • Fastest response • High temperature operation |
| Disadvantages | • Marginally high cost • Current source required | • Least sensitive • Non-linear • Low voltage • Least stable, repeatable |
For further information, refer to the advantages and disadvantages of RTDs and the advantages and disadvantages of Thermocouples.
