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Application Note: RTD, Thermocouple or Thermistor?

Resistance Temperature Detectors (RTD's)

An RTD sensing element consists of a wire coil or deposited film of pure metal. The element's resistance increases with temperature in a known and repeatable manner. RTD's exhibit excellent accuracy over a wide temperature range and represent the fastest growing segment among industrial temperature sensors. Their advantages include:

  • Temperature range: -260 to 850°C (-436 to 1582°F)
  • Repeatability and stability: The platinum resistance thermometer is the primary interpolation instrument used by the National Institute of Standards and Technology(NIST)from -260 to 962°C. Laboratory models stable within 0.0025°C are available. Even ordinary RTD's typically drift less than 0.1°C/year.
  • Sensitivity: The voltage drop across an RTD provides a much larger output than a thermocouple.
  • Linearity: Platinum and copper RTD’s produce a more linear response than thermocouples or thermistors. RTD non-linearities can be corrected through proper design of resistive bridge networks.
  • Low cost system: RTD's use ordinary copper extension leads and require no cold junction compensation
  • Standardization: Manufacturers offer RTD’s to industry standard curves, most commonly 100Ω platinum to IEC 751.

Thermocouples

A thermocouple consists of two wires of dissimilar metals welded together into a junction. At the other end of the signal wires, usually as part of the input instrument, is another junction called the reference junction. Heating the sensing junction generates a thermoelectric potential (emf) proportional to the temperature difference between the two junctions. This millivolt-level emf, when compensated for the known temperature of the reference junction, indicates the temperature at the sensing tip. Published millivolt tables assume the reference junction is at 0°C. Thermocouples are simple and familiar. Designing them into systems however is  complicated by the need for special extension wires and reference junction compensation. Thermocouples advantages include:

  • Extremely high temperature capability: Thermocouples with precious metal junctions may be rated as high as 1800°C(3272°F).
  • Ruggedness: The inherent simplicity of thermocouples makes them resistant to shock and vibration.
  • Small size/fast response: A fine-wire thermocouple junction takes up little space and has low mass, making it suitable for point sensing and fast response. Note, however, that many Minco RTD’s have time constants faster than equivalent thermocouples.

Thermocouple Data Loggers

Thermistors

A thermistor is a resistive device composed of metal oxides formed into a bead and encapsulate in epoxy or glass. A typical thermistor shows a large negative temperature coefficient. Resistance drops dramatically and non-linearly with temperature. Sensitivity is many times that of RTD's but useful temperature range is limited. Some manufacturers offer thermistors with positive coefficients. Linearized models are also available.  

There are wide variations of performance and price between thermistors from different sources. Typical benefits are:

Low sensor cost: Basic thermistors are quite inexpensive. However, models with tighter interchangeability or extended temperature ranges often cost more than RTD's. 

High sensitivity: A thermistor may change resistance by tens of ohms per degree temperature change, versus a fraction of an ohm for RTD's. 

Point sensing: A thermistor bead can be made the size of a pin head for small area sensing.

Many of the data loggers featured on Microdaq.com use a thermistor. See each logger's specifications for sensor type.  

Sensor Comparison Chart

  RTD Thermocouple Thermistor
Temperature range -260 to 850°C -270 to 1800°C -80 to 150°C (typical)
Sensor Cost Moderate Low Low
System Cost Moderate High Moderate
Stability Best Low Moderate
Sensitivity Moderate Low Best
Linearity Best Moderate Poor
Specify for: General purpose sensing
Highest accuracy
Temperature averaging		 Highest temperatures Best sensitivity
Narrow ranges(e.g. medical)
Point sensing

Information provided by Minco.com, a supplier of RTD, thermocouple and thermistor temperature sensors.

 

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