The sensing element in an RTD typically contains either a coil of wire or a substrate with an etched film of platinum.  The use of extension wires attached to the sending element allows the electrical resistance to be measured from a distance away from the process or material being measured. A protective sheath (normally stainless steel) houses the sensing element. Platinum is typically the material of choice in an RTD as this allows for a larger temperature range and it is a more stable material. Nickel and copper can also be used in RTD construction if a lower temperature range is required. Table 1. Outline usable temperature ranges for each material.

Table 1: Sensing Element Materials anRTD GPA WEB vert imaged Temperature Limits
Material Usable Temperature Range
Platinum -200°C to 850°C
Nickel -100°C to 315°C
Copper -75°C to 150°C

The type of wire insulation used in an RTD also influences the temperature an RTD can be exposed to. Table 2A. refers to the most commonly used wire and insulation materials and their temperature limits. Table 2B. refers to the wires that connect the sensing element to the control instrument or readout and temperature limits. Typical choices for wire insulation include nickel, nickel alloys, tinned copper, silver-plated copper or nickel-plated copper.

Table 2A: Connecting Wire Temperature Limits – Construction
Wire / Insulation Materials Maximum Usage Temperature
Nickel Plated Copper/TFE PTFE  Insulated 250°C
Solid Nickel Wire 650oC
Solid Copper Wire 300oC
Table 2B: Connecting Wire Temperature Limits – Extension
Wire / Insulation Materials Maximum Usage Temperature
Tinned Copper/PVC Insulated 105°C
Silver Plated Copper/FEP PTFE Insulated 205°C
Silver Plated Copper/TFE PTFE Insulated 250°C
Nickel Plated Copper/TFE PTFE Insulated 250°C
Nickel Plated Copper/Fibreglass Insulated 480°C

 

For RTD construction, there are two methods most used. The most common is to place the RTD element and attached wires into a metal tube with a closed-end. The tube is packed with a vibration dampening and/or heat transfer material, alumina powder in most cases, and seal the open end with silicone, epoxy, or ceramic cement.

An alternate construction method is the use of a mineral insulated metal sheath (MIMS) cable. The RTD element is inserted into a drilled hole and attached to nickel or copper wires insulated by Magnesium Oxide (MgO). The end is also insulated with MgO and welded closed. The other end has extension wires attached prior to sealing.

Once the temperature range and application has been determined, Pyrosales will select the most suitable materials and method of construction of the RTD.

2, 3 or 4 wire configuration

2-wire construction is the least accurate. 2-wire RTDs tend to be used with short lead wires or where close accuracy is not required.

3-wire construction is the most popular choice for industrial applications, whilst 4-wire construction is used in laboratories where close accuracy is imperative.

RTD wire configuration

There are many options to consider when choosing the correct RTD element for your requirements:

  1. Temperature rating
  2. Tolerance, accuracy, and interchangeability
  3. Time response
  4. Distance from control or measuring equipment

RTDs are suitable for many applications across a wide range of industries including, air conditioning and refrigeration, stoves and grills, textile production, food processing, plastics, petrochemical, and temperature measurement of air, gases, and liquids. Use an RTD when you require accuracy and stability and where accuracy must extend over a wide temperature range.