Resistance Temperature Detectors (RTDs) and thermocouples are the two most common types of temperature sensors in industrial processes. Sometimes it is difficult to assess all the facts and choose the one that better matches the needs of your application.
Resistance Temperature Detectors
An Resistance Temperature Detector is a fine wire coiled inside a probe. The wire’s electrical resistance changes with temperature. The relationship between the variation in resistance and temperature is known and is used to determine temperature. Common materials for Resistance Temperature Detector elements are metals such as platinum, nickel, or copper. Platinum is preferred as it is chemically inert and has a very linear resistance-temperature relationship for a wide temperature range. There are three types of Resistance Temperature Detectors sensors: thin film, wire-wound, and coiled.
- A thin film Resistance Temperature Detector has a very thin layer of metal deposited on a ceramic substrate.
- A wire-wound element has the coil wrapped around an insulating core.
- A coiled Resistance Temperature Detector is like a light bulb — the wire is held in place by some mechanical support but is coiled by itself.
Which type is better suited for a certain application depends mainly on desired stability, temperature range, and durability.
| Pros | Cons |
| Good accuracy | Limited temperature range (-200° to +850°C) (-328°F to +1562°F) |
| Long term stability | Slow response |
| Repeatable | Current source required |
| No extension wire required | Self-heating error |
| Less rugged | |
| More expensive |
Thermocouples
A thermocouple is made of two pieces of different conductors or semiconductors that are in contact. The difference in temperature between two contact points creates a voltage. The relationship between temperature and voltage is known and is used to determine temperature. Nickel, chromium, aluminum, and copper alloys are some of the most popular materials. The choice of material will depend on cost, melting point, chemical properties, and stability, among other factors.
| Pros | Cons |
| Wide temperature range (-200°C to +2300°C) (-328°F to +4172°F) | Accuracy degrades with time |
| Inexpensive | Less stable |
| Fast response | Non linear |
| Rugged | Low voltage output |
| Tip sensitive | Reference required |
| Self-powered | Extension wire required |
| Small diameter | |
| Multi-point sensing |
Resistance Temperature Detector or Thermocouple: Which One to Use
The choice of using an Resistance Temperature Detector or Thermocouple will be driven by your application requirements. In general, thermocouples are better for high temperature processes, applications that require fast response times, and those with limited space. Resistance Temperature Detectors offer better accuracy, repeatability, and stability. The first step is to establish the requirements of your application in terms of:
- Temperature range
- Pressure range
- Humidity
- Shock and vibration
- Medium (solid, liquid, or gaseous; corrosive; hazardous)
- Flow rate
In many cases you will find more than one option. Eliminate the guess work with the help of WIKA’s experts. They have the knowledge and many years of field experience. They can help you analyze your application and find the best products in the market that suit your particular needs. Give WIKA’s experts a call today.
