Thermistors are thermally sensitive resistors whose electrical resistance changes in a very precise way when exposed to a corresponding change in body temperature. Negative Temperature Coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and Positive Temperature Coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature.
A thermocouple consists of two dissimilar conductors which in contact produce voltage which dependents on the difference of temperature of the junction to other parts of the circuit. Thermocouples are rather inexpensive and widely used for measurement and control.
Principle of a thermocouple (Pictures by Francois Tardieu)
IR Temperature Sensors
Contactless infrared temperature sensors measure surface temperature by converting thermal energy radiated within the spectral range of 8-14 μm from any surface in its field-of-view (FOV) to an electrical signal often with a response time of less than 1 second. The measurement of surface temperature is based on the Stefan-Boltzmann law which states that the total energy radiated per unit surface area is directly proportional to the fourth power of the surface temperature T, a constant (ρ, Stafan Boltzmann-constant) and the emissivity (ε) of the surface.
The critical considerations for the use of these sensors include:
- type of surface measured: emissivity may be substantially different between different surfaces ε for plant leaves ~0.98 (ε for different metallic surfaces often <0.2),
- the field of view: the target size increases with the distance,
Thermal imaging measures the surface temperature based on the same principle as the IR temperature sensors by using thermographic cameras which detect radiation in the infrared range and provide images at the image of a barley plant one below. Currently thermography cameras with a relatively high thermal and spatial resolution are more and more used in plant sciences, new detectors lead to reduced costs for the use of these approaches. However, thermal imaging and the interpretation of thermal imaging in particular in plant sciences is rather complicated which requires accurate calibration and the extraction of physiological processes from surface temperature of leaves is particularly complex.
Temperature of a barley plant (Picture by Hendrik Albrecht)