APPLICATION

Sensor Selection

Thermocouples

essentially comprise a thermoelement (a juction of two specified dissimilar metal) and an appropriate two wire extension lead. A thermocouple operates on the basis ao the juction located in the process producing a small voltage which increase with temperature. It does so, on reasonably stable and repeatable basis.

Resistance Thermometers

utilise a precision resistor, the Ohms value of which increases with temperature (in the case of a positive temperature coefisient). such variations are very stable and precisely repaetale.

Thermistor

are an alternative group of temperature sensors which display a large value of temperature coeficient of resistance (usually negative, sometimes positive). They provide high sensitivity over a limited range. In practical terms, the alternative types of assembly utillise similar (in some cases identical) construction but must be used different ways depending on the application.

  THERMOCOUPLES RTDs THERMISTORS

GRAPHIC

ss-thermocuples-graphic ss-rtds-graphic ss-thermistors-graphic

ADVANTAGES

  • No resistance leadwire problems
  • Fastest response to temperature changes
  • Simple, rugged
  • Inexpensive
  • High Temperature Operation
  • Point temperature sensing
  • Most stable, accurate
  • Contamination resistant
  • More linear than thermocouple
  • Area temperature sensing
  • Most repeatable temperature measurement
  • High ouput, fast
  • Two wire ohms measurement
    Economic
  • Point temperature sensing
  • High resistance
  • High sensitivity to small temperature changes

DISADVANTAGES

  • Non-linear
  • Low Voltage
  • Least stable, repeatable
  • Least sensitive to small temperature changes
  • Expensive
  • Current source required
  • Self-heating
  • Slow response time
  • Low sensitivity to small temperature changes
  • Non-linear
  • Limited temperature range
  • Fragile
  • Current source required
  • Self-heating
Which temperature sensor is the best choice for my application?

Thermocouple are best suited to high temperatures, environmental extremes, or applications requiring microscopic size sensors. They are also recomended for high vibration environments.
 

RTDs are best most industrial measurements over a wide temperature range, especially when sensor stability is esential for proper control.
 

Thermistros are best for low temperature applictations over limited temperature ranges.