6 Key differences between RTD's and Thermocouples

Resistance temperature detectors, or RTD, and thermocouples are very common types of sensors used for measuring temperature. They are used as measuring devices since they are easy to use. The basic difference between them is their operating principles and manufacturing processes. Here we will cover the difference between the RTD and thermocouples. So let’s get started with the six key differences between RTDs and thermocouples.

What are thermocouples?

Thermocouples come with different types of metals connected at one point to form junctions. If the junction of two metals is heated or cooled, a voltage is generated that can provide the temperature measured. These sensors are made in different configurations, like thermocouple probes. These probes are connectors, transition joint thermocouple probes, thin film thermocouples, bare wire, or in some cases, thermocouple wire.

There are different applications where thermocouples are used due to their modes and features. But it is best to discuss their structure, functions, and range to find the right thermocouple type for applications.

Thermocouples Limitations

• Low accuracy
• Less sensitive
• Relatively high drift-over-time

What are RTDs?

RTD is a temperature sensor that comes with a resistor that varies resistance with change in temperatures. Mostly used RTD is Pt100 which is an industrial probe used for different laboratory and industrial uses and proffered due to its repetitions, stable operational and accurate results.

Some RTD elements come with a length of finely coiled wire wrapped around a ceramic core. The element is normally fragile, so it is put in a sheathed probe for protection. The RTD component is made with pure materials that have variable resistance at different temperatures. The material comes with a change in resistance when temperature varies; this change helps to measure the temperature.

Limitations

• Less temperature range
• High prices

6 Key differences between RTD’s and Thermocouples

Temperature Range

Thermocouples are best for high temperatures. Different manufacturing methods are used to increase the range of RTD probes, but about 90 percent of RTD is made for temperatures below 400 °C. While some thermocouples can be used to measure about 25000 centigrade, For accurate measurement, a thermocouple input and a thermocouple controller are used.

Cost

The price of RTD is higher than that of thermocouples. The RTD has prices that are about two to three times higher than thermocouples with the same temperature. Savings are made on RTD installation that is less expensive as less wire is used.

Sensitivity

These sensors operate fast for temperature changes, but the thermocouple operates at high speeds. The grounded fast-acting thermocouples respond about three times faster than Pr100 RTD. The high-speed temperature sensor is exposed to thermocouples. But the design features have improved the response time of thin-film PT100 probes.

Accuracy

RTDs are more accurate than thermocouples. RTD comes with an accuracy of 0.1 centigrade, and then 1 centigrade for some thermocouples. But some thermocouple modules can match RTD accuracy. The different parameters that can affect sensor accuracy are their stable operation, linearity, etc.

Linearity

The temperature resistance relation in the RTD sensor is linear in the sensor range, while the thermocouple has an S-type plot.

Stability

The RTD probe reading is stable and repeated for a longer duration. Thermocouple readings will drift since chemicals change in the sensor. RTDs linearity features and drift absence make them stable for longer operation.

Feature	RTD	Thermocouple
Operating Temperature Range	-200 to 600 centigrade	-200 to 2000 centigrade
Response Time	1 to 50 seconds	0.1 to 10 seconds
Sensitivity	Less sensitive	More sensitive
Cost	High cost	Low cost
Size	Larger	Smaller
Accuracy	High accuracy	Less accuracy
Stability	More stable	Less stable
Self-heating	Exists	Negligible
Output	Linear	Non-linear

Advantages and disadvantages of RTDs and thermocouples

RTD Advantages

It measures low values.
It is a highly accurate device.
Its output is linear.

RTD Disadvantages

It is a high cost.
It is not used for temperatures greater than +850 centigrade.
Its reaction time is low, then different temperatures change.

Advantages thermocouple

It is less expensive than RTD.
It measures a high range.
It is high-speed.

Disadvantages of thermocouples

It is a non-linear device.
Poor accurate
It has poor stability with output results.

Types of RTDs

There are two main types of resistance thermocouples:

Thin-film resistors were created with the use of an accurate layer of platinum deposited on ceramic and sealed with glass.
The wire wound resistor comes with wire wrapped and embedded in a glass casing.

The use of these RTDS is based on accuracy, temperature ranges measured, and vibration.

Some types of RTD on different types of wire are here.

2-wire RTDs

It is a common type and comes with two lead wires to make a circuit. This circuit added the resistance of lead wires to the resistance of RTD, which decreased accuracy.

3-wire RTDs

It has extra wire to compensate for lead wire resistance, resulting in highly accurate temperature values.

4-wire RTDs

It is a highly accurate RTD since it is used for constant compensation. This RTD is a temperature sensor used in laboratories.

Types of thermocouples

The main types of thermocouples are listed here.

Iron and copper-nickel (type J)
Nickel-chromium and nickel-aluminum (type K)
Copper and copper-nickel (type T)

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Faqs

Mostly, RTD is limited to the highest temperatures of 1000 degrees Fahrenheit. While some thermocouples are used for measuring about 2700 degrees Fahrenheit,.