Question

Briefly discuss the different physical principles that are used in temperature measuring instruments and give examples of instruments that use each of these principles.

1. The thermoelectric effect

2. Resistance change

3. Sensitivity of semiconductor device

4. Radiative heat emission

5. Thermography

6. Thermal expansion

7. Sensitivity of fiber-optic devices

8. Color change

9. Change of state of material

Answer 1

There are four basic types of temperature measuring devices, each of which uses a different principle.

1. Mechanical devices (liquid-in-glass thermometers, bimetallic strips, bulb & capillary, pressure type etc.)

2. Thermojunctive (thermocouples)

3. Thermoresistive (RTDs and thermistors)

4. Radiative (infrared and optical pyrometers)

principle of operation:

A change in temperature causes some kind of mechanical motion, typically due to the fact that most materials expand with a rise in temperature. Mechanical thermometers can be constructed to use liquids, solids, or even gases as the temperature-sensitive material.

1. Thermocouple effect : The instrument which uses the thermocouple for the measurement of the temperature, current and voltage such type of instrument is known as the thermocouple instrument. It is used for both the AC and DC measurement. The thermocouple is an electric device which uses two wires of the different metal.

2. Resistance change : RTD (resistance tempreture detector) working based upon resistance change. An RTD is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature. In practice, an electrical current is transmitted through a piece of metal (the RTD element or resistor) located in proximity to the area where temperature is to be measured. The resistance value of the RTD element is then measured by an instrument. This resistance value is then correlated to temperature based upon the known resistance characteristics of the RTD element.

3. Sensitivity of semiconductor device : The temperature measurement of a power semiconductor device is of great interest. This important parameter is used in two main ways. The first one is for thermal management of power modules. In this case, the chip temperature is used to get the thermal resistance or thermal impedance of the component that is usually provided by manufacturers in the data-sheet. The second use concerns the monitoring of the aging of components for reliability reasons. For this purpose, temperature is monitored and used either directly or indirectly through the thermal resistance or thermal impedance. If the temperature is acquired during the functional operation of the converter, it can be used as a damage indicator of the power module. An increase of this parameter may be due to a rise of a thermal resistance and used as a detector of a solder layer delamination process [1]. The measurement is then used to validate global dimensioning in operating conditions.

Three main methods are currently used to evaluate the temperature of power semiconductor devices [

Optical methods,

Physical contact methods,

Electrical methods.

4. Radiative heat emmision :

Radio micrometer is used to measure the heat radiation. A radio micrometer is an instrument used in detecting radiant energy of heat or light form.

5. Thermography : Every object with a temperature over the absolute zero point (0 Kelvin = -273,15 °C) emits infrared (IR) radiation. The human eye cannot perceive this, as it is more or less blind at this wavelength. Not the thermal imager. Its central element, the detector, is sensitive to IR radiation. Based on the intensity of the IR radiation, it determines the temperature of the object's surface, and makes it visible for the human eye with a thermal image. This process is referred to as thermography. Thermography camera is normally used to detect tempreture of an object.

6. Thermal expansion : A dilatometer is a precision instrument for the measurement of dimensional changes in material as a function of temperature.

7. Sensitivity of optic fibre device : Fibre optic power meter is used for measuring all parameters like sorce, output, reciever signal etc.

8. Color change : A pyrometer is a type of remote-sensing thermometer used to measure the temperature of distant objects. it is a device capable of measuring the temperature of an object by its incandescence, visible light emitted by a body which is at least red-hot.

9. Change of state of material: A thermometer is a device that used this principle to measure the tempreture of a an object. the mercury present in the thermometer expand as tempreture increase  and vice versa.