Resistance temperature Detector


  • The resistance of an Resistance temperature Detector varies directly with temperature:

– As temperature increases, resistance increases.

– As temperature decreases, resistance decreases.

  • RTDs are constructed using a fine, pure, metallic, spring-like wire surrounded by an insulator and enclosed in a metal sheath.
  • A change in temperature will cause an RTD to heat or cool, producing a proportional change in resistance. The change in resistance is measured by a precision device that is calibrated to give the proper temperature reading.

A Resistance temperature Detector is widely used in process industries for application of temperature measurement. Here, A RTD is shown in figure. Size of RTD (length, sheath-dia) may change as per requirement/ pipe size


These RTDs can be used in process for temperature measurement such as-

  • Pipeline fluid
  • Heat exchangers
  • Vessel
  • Columns

A schematic diagram for RTDs is shown in figure-

RTD comes  with Two sensors:-

1:-Simplex RTD– A simplex RTD Consist of single pair RTD/PT100 element at its hot end.

2:-Duplex RTD –  A duplex RTD consist of a pair  RTD/PT100 elements at its hot end instead of a single element in a single enclosure. The main reason such a configuration is to enable Redundancy.

Duplex RTD

For RTDs element protection from direct temperature/ fluid exposure and replacement purposes (When RTD is faulty), Thermowell are placed for mounting RTDs. Some different type of thermowell are shown below-

Why Thermowell Used??

Protective Barrier

A thermowell’s main job is to protect a RTD from external conditions, which it does by housing the RTD, as shown in Figure. With the RTD safe inside, the thermowell can handle an assortment of corrosive or abrasive process media and withstand turbulent or high pressure conditions. The thermowell prevents the RTD’s sensing element from being exposed to extreme hot or cold temperatures.

Increased Serviceability

If you need to remove or replace a RTD, thermowells simplify the process by providing easy access. They stay installed in the piping system while a RTD is being serviced. As a result, operations continue, which means there is no unnecessary downtime, and the process media continues to flow without worry of escaping the system.

Reduced Operational Costs

RTDs last longer and do not need to be serviced as often when thermowells are used to protect them. In effect, thermowells help decrease the total cost of RTDs by minimizing the time spent maintaining and servicing the instruments. The fact that you won’t need to replace RTDs as often or maintain a large inventory of them will also save you money in the end.

Some Technical Word For Thermometer That YOU should Know

Thermowell root dimension (Q)

Thermowell Bore size

Thermowell Immersion (“U”) Length

Thermowell Lagging Extension (“T”) Length

Note:- A special type RTD is used to measure bearing temperature in Rotating Machine  such as compressor, turbine  called as PIN type RTD. Working and operation are same but design is  different. Pin type RTD also comes in  simplex and duplex.

Pin type RTD shown in Figure:-


RTD Connection With Transmitter

2 Wire RTD

3 Wire RTD
4 Wire RTD ​

RTD connection with indicator

2 wire RTD
3 wire RTD
4 wire RTD