Let’s Discuss the Different Types of RTDs

Types of RTDs

The Platinum-RTDs are generally offered in 2 different categories; 1st is the ‘Class A’ & the other is ‘Class B.’ 

  • Class A: This one is presumed to be high in accuracy, and also, this one has ±0.06 ohms ice-point-tolerance. 
  • Class B: This one is known to have standard accuracy, and also, this one has ±0.12 ohm ice-point-tolerance. This type B is used popularly in most of the industry verticals.

Other RTDs Types

  • PRT or the Platinum-Resistance-Temperature Sensor: These provide awesome accuracy over a vast range of temperature (ranging from -200°C to even up to 850°C). 
  • Other options for resistance value: The RTD elements are even provided with resistances of 200 Ω, 500 Ω and 1000 Ω at zero degrees Celsius. These RTD types are commonly called PT200, PT 500 & PT 1000 respectively. 
  • SPRT or the Standard-Platinum-RTD: ITS-90 (1990 International-Temperature-Scale that is utilized as a global practical scale for temperatures in the laboratories of national metrology such as NIST & NPL etc.) consists of several fixed reference points as well as various interpolation devices, which are made use of for determining the scale amongst the points.

Standards of RTD

Platinum RTDs have basically 2 standards

  1. European standard (this is also called the DIN/IEC standard) 
  2. American standard

RTD Construction

RTD elements almost permanently need insulated conductors. At a temperature below about 250 degree Celsius, the PVC, silicon rubber or the PTFE-insulator are majorly made use of. Fiberglass or ceramic is used over it. The measuring point, & largely most wires, needs a housing/sleeve protection, which is usually made from metal alloys, which are chemically inert to this process that is under monitoring. 

  • Platinum Thin Film: RTD’s thin film style is perhaps the most known model due to its rugged design & less price. For making a thin film element, a small ceramic chip is covered with a very thin (0.0001″) platinum film & then a resistive path is cut or chemically etched into the platinum film. After that, the element is covered with quite a thin glass layer for protecting it from harmful chemicals as well as gases. 
  • Internal Coil Wire Wound: This type is usually made of platinum wire. A very small platinum wire (0.02 mm) is twisted & after that slipped into a small 2-hole ceramic insulator. After that, big extension wires are welded to the platinum wire ends & cemented in. Some of the manufacturers fill the insulating holes with ceramic powder after the coils are installed. This prevents the coils from moving & shorting against one another. The one end, which is right opposite the extension wires, is also coated with ceramic cement. 
  • External Wound Element: An external wound element of RTD is formed by winding the sensor element wire around the central mandrill that is generally formed from ceramic. After that, this coil or winding is covered with glass/other material for insulating so as to protect & secure the winding. These wires are then welded to the extension cords & after that secured to the frame with ceramic epoxy/cement.

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