Comparison between RTD and Thermocouple

interviewquestions

#1

RTD and Thermocouples are temperature sensing devices widely used in industries. Both transducers are useful for many applications, choosing the transducer for the right purpose depends on many factors what we are going to discuss here.

Listed down are the main factors which influence in choosing between RTD and Thermocouple.

Principle of work:

Thermocouple: Thermocouple is made by joining two dissimilar metals when the junction of metals are exposed to temperature an emf is induced across the metals. When the temperature increases the voltage induced also increases.

RTD: Resistance Temperature Detector, for certain elements when the temperature increases the resistance of the metal increase, such as Platinum. The metal is exposed to the temperature that should be measured and the change in resistance is measured.

Placement:

There are two types of temperature measuring instruments available.

  • Surface mount
  • Immerse type

Both the RTD and Thermocouple are available as Surface mount type and Immerse type.

Surface mount types are mounted above the surface of the pipe or tank for which the temperature should be measured. Immerse types are immersed in the vessel or pipe thus have direct contact with the process fluid, needed thermowell.

Types:

Thermocouple:

Thermocouples are classified based on metal combination used and based on the grounded and ungrounded junction.

Based on grounding:

Based on metals used:

Type T: Copper-Constantan
Type J: Iron-Constantan
Type E: Chromel-Constantan
Type K: Chromel-Alumel

RTD:

Based on structure:

  • Wire wound - external and coil type
  • Thin film type.

Based on lead wire compensation:

The RTD needs compensation techniques used to neglect the additional resistance of lead wires used. They are:

  • 2 wire RTD circuit: Which has no compensation wires

  • 3 wire RTD circuit: All three lead have same resistance

  • 4 wire RTD circuit: eliminates lead wire error.

Temperature range:

Thermocouple:

  • Type T: -200°C to 350° C

  • Type J: 0°C to 750°C

  • Type E: -200°C to 870°C

  • Type K: 0°C to 1180°C

Metal combination Platinum-Rhodium thermocouple has temperature range up to 2640°F and Tungsten-Rhenium has maximum temperature 4200°F.

RTD:

  • Wire wound type RTD: -200°C to 500°C

  • Thin film type RTD: -50°C to 200°C

Calibration:

Thermocouple: On-site calibration only

RTD: Can easily recalibrate for longer life and traceability.

Performance:

Thermocouple:

Time response- For bare wire 1 milliseconds. For packaging 2 to 3 seconds

Stability- Depend on process condition

Accuracy- Up to +/- 4°F

RTD:

Time response- 2.5 seconds, for packaging 4-6 seconds

Stability: Show drift 0.06°C after 1000 hours at 400°C

Accuracy: Maximum +/- 0.27°F for grade A device and +/-0.54°F for grade B device