What is an RTD?RTD

A resistance temperature detector, also known as an RTD or resistance thermometer, is a type of temperature sensor. An RTD consists of a sensing element, wires to connect the sensing element to the measurement instrument and a support to position the sensing element in the process.

How do they work?

RTDs are designed to ensure precise and repeatable temperature versus resistance characteristics. The type of material used in RTD construction will determine the limits on the temperature the RTD can be exposed to. The sensing element in an RTD is an electrical resistor that changes the resistance value as the temperature changes. The change in resistance with temperature happens at a predictable rate and can be measured to provide the temperature of a material of process.

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. Outlines usable temperature ranges for each material.

RTD sensors

Table 1: Sensing Element Materials and Temperature Limits

Material

Usable Temperature Range

Platinum

-200°C to 650°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, tined 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 Teflon 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 Teflon Insulated

205°C

Silver Plated Copper/TFE Teflon Insulated

250°C

Nickel Plated Copper/TFE Teflon 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 a 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 for 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.