Instrumentation

How to check RTD


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  • In this lesson, we will study How to check RTD.
  • Resistance temperature Detector as the name indicates Detects Temperature and gives the output as resistance.
  • The short form of Resistance Temperature Detector is RTD.
  • RTD is a Temperature Sensor.
  • Resistance Temperature Detectors (RTDs) give a linear change in resistance with respect to temperature.
  • RTDs are a positive temperature coefficient. i.e. Resistance increases with an increase in temperature.
  • Platinum is Commonly used in RTDs because its temperature Resistance characteristic is more linear in comparison to copper and nickel.
  • RTD types are broadly classified according to the different sensing elements used.
  • Platinum, Nickel, and Copper are the most commonly used sensing elements.
  • Platinum is considered the best as it has the widest temperature range. This is shown in the resistance versus temperature graph below.
  • Platinum type RTD is also known for its best interchangeability than copper and nickel. It also has the highest time stability.

How to check an RTD

  • There are three wires in RTD. Out of the two wires are short-circuited as shown in the figure. The same color wires are short-circuited.

  • In the above figure, blue colour wires are short-circuited.
  • The different colour wire will show the same resistance with other same colour wires.
  • Now the resistance will be calculated by using the following formula

     Rt=R0(1+@T)

Where Rt is resistance at T degrees Celcius.

R0 is the resistance at 0 degrees Celcius.

T is the temperature that we need to calculate.

  @ is alpha that is temperature coefficient. Value of Alpha is .0385 for Platinum.

Now for PT 100 temperature will be Calculated by using formula

             [( Resistance Shown by Multimeter- 100)/0.385]

pt 100 rtd table
pt 100 rtd table

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