5.2 Contact sensors

For thermal sensors that are allowed to make contact with the subject or medium to be measured there exist a number of possible sensors

They are subdivided in several types:

  • Thermocouples
  • RTD's (thermistors, PT100's, diodes, Semiconductor Temperature Sensors)
  • Other types (expansion, bimetallic, optic, etc.)

 

For the first 3 types the most striking (dis)advantages are summarised in the table below.

 

The most commonly used temperature sensors and their properties (www.picotech.com)
  Thermocouple RTD (Pt100) Thermistor
Operating Range -200 °C to 2000 °C -250 °C to 850 °C -100 °C to 300 °C
Accuracy Low
1 °C common
Very High
0.03 °C common
High
0.1 °C common
Linearity*  Medium High  Low
Thermal Response** Fast  Slow  Medium
Cost  Low  High  Low to moderate
Noise Problems  High  Medium  Low
Long term stability  Low  High  Medium
Cost of measuring instrument  Medium  High  Low
* Linearity is not an issue if using modern digital measuring instruments, as look-up tables stored in memory provide compensation.
** Thermal response is considered for the measuring element only, not its enclosure.