Question

Please write down the invasive, semi-invasive and non-invasive techniques in temperature measurement. Explain their working principles briefly.

Answer 1

Ans :

Invasive Temperature Measurement

In this technique of temperature measurement, thermoelectric devices such as thermocouples, thermistors and platinum resistance thermometers are used for measuring temperature. Thermocouples operate by the principle of Seebeck Effect, in which two dissimilar conductors produce a difference in voltage when exposed to a temperature gradient along the wire, from tip to the end, according to the Seebeck co-efficient of the material. The electromotive force (emf) that is created through a temperature gradient is due to the diffusion of electrons in the material from the warm to cooler areas.

Semi-Invasive Temperature Measurement

In this technique liquid crystals is used to measure temperature as their colour can change reversibly depending upon the temperature at the sensor. While being liquid they also exhibit characteristics of crystals. Therefore their appearance will alter from being completely transparent to showing a solid colour within the range of the sensor. Often TLC (Thermochromic Liquid Crystal) is used in this method. It is mounted on a black background to get clear temperature readings. Since this liquid crystal also exhibit characteristics of crystals, when the temperature reaches a certain level, the liquid crystals go from an amorphous arrangement to a more crystalline state and reflects different wavelengths of light. These colours can be interpreted using a digital camera system linked to calibrated image processing software.

Non-Invasive Temperature Measurement

In this technique, Radiation thermometers is used to measure thermal radiation emitted from the surface of a material. IR (Infrared) radiation thermometers can measure the emitted thermal radiation within a small radius, dictated by the beam size and divergence of the laser. The measured radius is a function of distance from the measurand. Multiple probes is used in assembly processes by mounting using jigs and fixtures to monitor temperatures of the assembly components non-invasively.