Instrumentation

Types of RTD

In this Lesson, we will Discuss the Types of RTD.

RTD can be Categorized according to two types.

  1.  According to Connections.
  • Simplex
  • In Simplex Signal goes to one location only. 
  • Duplex
  • In Duplex Signal Goes to two Different Locations. for example: to PLC/DCS and to indicator
duplex rtd
duplex RTD

In Simplex only one Side is available with three wires.

2.   According to Wires.

  • 2 wire RTD.
  • RTD has 2 wires is known as 2 wire RTD.
  • 3 wire RTD.
  • RTD has 3 wires is known as 3 wire RTD.
  • 4 wire RTD.
  • RTD has 4 wires is known as 4 wire RTD.
  •       2 wire RTD is used for short distances.
  •       3 wire RTD is used for long distances. A third wire is used for Resistance compensation. Resistance compensation means when a signal is coming from large distance wire resistance is not added.

    • The various wiring arrangements are designed to reduce and/or eliminate any errors introduced due to resistance changes of the lead wires when they also undergo temperature changes.  RTDs used for electrical equipment generally use either a three-wire system or a four-wire system having paired lead wires.

The four-wire system is little affected by temperature-induced resistance changes in lead-wires, and, of all the arrangements, it is affected least by stray currents.  It, therefore, is used to measure temperature differences and is used generally for making very accurate measurements.

The three-wire system is generally satisfactory for industrial measurement using a secondary instrument that is remote, say, more than 3 meters distant from the RTD.

Although the error caused by temperature change in the leads is virtually eliminated in a 3-wire arrangement, a slight non-linearity in the resistance change is introduced with this scheme.

Commonly used RTD Materials:
Platinum (most popular and accurate)
• Nickel
• Copper
• Balco (rare)
• Tungsten (rare)

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