Resistance Temperature Detector ( RTD )

Resistance Temperature Detectors (RTD) are temperature measuring transducers used in industrial applications. RTDs are simple in construction with higher accuracy and repeatability and which are used in the applications below 600°C.

Principle:

As the name suggests the RTDs are temperature sensitive resistors. The resistance temperature detector (RTD) measures the electrical conductivity as it varies with temperature. The electrical resistance generally increases with
temperature, and the device is defined as having a positive temperature coefficient.

The sensor is the combination of the transducer and electronics that measure the resistance. The resistance change is measured by the electronic unit of the sensor. The resistance measured will be the function of the temperature.

The electrical resistance of which changes with temperature as approximated by the following formula:

RT= Resistance of RTD at given temperature T (ohms).

Rref = Resistance of RTD at the reference temperature Tref (ohms)

α = Temperature coefficient of resistance (ohms per ohm/degree)

The magnitude of the temperature coefficient determines the sensitivity of the RTD, it depends on the metal used in the RTD

Construction:

The main component of the RTD is the metal used in it. Platinum(PT) is the widely used metal in industries, apart from PT other metal used are Nickel and Copper. Platinum is the most common and has the best linear characteristics of the three, although Nickel has a higher temperature coefficient giving it greater sensitivity.

RTDs are either a metal film deposited on a form or are wire-wound resistors, which are then sealed in a glass-ceramic composite material. The coil is wound to be noninductive. The space between the element and the case is filled with a ceramic powder for good thermal conduction. The element has three leads, so that correction can be made for voltage drops in the lead wires.

Lead compensation techniques are used in RTD connection to reduces the effect of lead wires/ connecting wires.

 

The RTD connected in the thermowell is shown above, the sheaths are made of materials such as productive tube pyrex glass, Porcelain, quartz or nickel depending on the range of temperature and nature of fluid whose temperature is to be measured.

Requirements of conductor material used in RTD:

  • Change in resistance of material per unit change in temperature should be as large as possible
  • Material should have high value of resistivity so that minimum volume of material is used in its construction
  • Resistance of the material should have a continuous and stable relationship with temperature

What are PT100 and PT1000 in RTD?

In PT100 the ‘PT’ defines that the metal is Platinum and the ‘100’ is the resistance in ohms at ice point (or 0°C). These are generally wire wound and are quite common in industrial uses.

PT1000 exhibits 1000Ω resistance at 0°C Celsius temperature. These are generally thin film devices and are more expensive.

200 and 500 ohm Platinum RTD’s are available but are more expensive and less common.

Types of RTD :

Platinum is most popular for RTD’s, it has good calibrated accuracy, is quite stable and has good repeatability, but is quite expensive. They are, however, not as sensitive as the Nickel and Balco devices. Nickel is not quite as repeatable but is less expensive.

Advantages:

  • Good sensitivity
  • Linear over wide operating range
  • Uses standard copper wire
  • High temperature operating range
  • Copper RTD’s minimise thermocouple effect
  • Interchangeability over wide range
  • Works in wide temperature ranges

Disadvantages:

  • Bulky in size and fragile
  • Slow thermal response time due to bulk
  • Self-heating problems
  • More susceptible to electrical noise
  • More expensive to test and diagnose

 

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