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RTD vs Thermocouple | Difference between RTD and Thermocouple

This page compares RTD vs Thermocouple and mentions difference between RTD and Thermocouple.

Introduction:
Both RTD and Thermocouple are most common temperature sensor types. They are used to measure temperature for process control in automation system.

RTD | Resistance Temperature Detector

RTD

• It is a device whose resistance changes over temperature in predictable, linear manner.
• Its operating temperature is upto 600oC.
• It is made from very fine wire (typically platinum) wrapped around ceramic core or created using thin film technology as shown in the figure-1.
• They provide accurate and repeatable results over the lone term with tolerance of about 0.15oC. They are stable for years.
• They are fragile and have long response time (2.5 to 10 seconds).
• They are bigger (about 3 to 6 mm) than thermocouples.
• Example: Pt100 (It is made from platinum. It is calibrated to 100 Ohms at 0 oC. Other RTDs are Pt1000, jPt100, Cu10, Cu25, Ni120 etc.

Thermocouple

Types of Thermocouple wires-J,K,T

• They are used for wide temperature ranges upto couple of 1000s degrees.
• They create a small voltage across a junction of two dissimilar metals when exposed to a temperature gradient. This produces less accurate reading with tolerance of about 2oC and can drift over time within hours.
• They don't use any ceramics and hence they are not fragile.
• They have faster response time (less than 1 second).
• They are smaller (less than 1.6 mm typically).
• Example types: Thermocouple types are J, K and T. J operates from -346 to 2193 oF, K operates from -454 to 2501 oF and T operates from -454 to 752oF. Figure-2 depicts thermocouple wires used for these types.
It is recommended not to use regular copper wire in order to extend the thermocouples as this will provide inaccurate readings.

Following table mentions summary of difference between RTD and thermocouple.


Specifications RTD Thermocouple
Accuracy More accurate Less accurate
Temperature range -200 to 600oC -200 to 2000oC
Cost More Less
Sensitivity Good (1" typical, other lengths are 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/oC,
Highly linear
Voltage,
10 to 40 microvolts/oC,
Approx. linear
Electrical noise resistance Less Susceptible More Susceptible
Size (Typical) ,dimension 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

Also refer advantages and disadvantages of RTD >> and advantages and disadvantages of Thermocouple >> for more information.



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