MEMS Thermal Sensors .pptx

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1. MEMS Thermal Sensors Dr.D.Sindhanaiselvi PTU

2. MEMS Non-Contact Thermal Sensor Digital Temperature Sensor Thermal Imaging sensor Infrared Sensors Digital Thermometer Temperature sensor on PCB LM35 Temperature sensor

3. Classification CONTACT Thermocouple Thermistor NTC PTC RTD NON CONTACT Radiation Thermometers Infrared thermal imaging Spot radiometers Scanners Thermal Imagers

4. TEMPERATURE COEFFICIENT OF RESISTANCE: The temperature coefficient of resistance is COEFFICIENT OF RESISTANCE generally defined as the change in electrical resistance of a substance with respect to per degree change in temperature. The electrical resistance of conductors such as gold, aluminium , silver, copper,  it all depends upon the process of collision between the electrons within the material . When the temperature increases, the process of electron collision becomes rapid and faster. As a result, the resistance will increase with the rise in temperature of the conductor.

5. Consider a conductor whose resistance at 0°C is R 0  and the resistance at a temperature T°C is R T . The relation between temperature and resistances R 0  and R T  is approximately given as R T =  R 0  [1+ α (T-T )]; R T =  R 0  [1+ α (∆T )] The above equation that the change in  electrical resistance  of any substance due to temperature depends mainly on three factors – The value of resistance at an initial temperature. The rise in temperature. The temperature coefficient of resistance α. The value of α can vary depending on the type of material. In metals, as the temperature increases the electrons attain more kinetic energy, thus more speed to undergo frequent collisions. We know that the resistivity of any substance is given by ρ = m/nq 2  τ RELATION BETWEEN TEMPERATURE AND RESISTANCE

6. So , the resistivity depends on the number of charge carriers per unit volume n and the relaxation time τ between collisions. When the temperature of the metal is increased, the average velocity of the current carriers i.e the electrons increases and result in more collisions. This means that the average time between successive collisions τ decreases. But the change in the value of n due to the increase in temperature is negligible which further means that the value of resistivity now is dependent only on the change in τ.

7. TYPES OF TEMPERATURE COEFFICIENT Positive temperature coefficient The resistivity and the resistance of the material increases due to decrease in τ. Hence the value of the temperature coefficient of metal is positive.

8. Negative temperature coefficient In the case of  semiconductors  and insulators, the number of charge carriers per unit volume increases with an increase in temperature. The decrease in τ is compensated well by the increase in n such that the value of resistivity and resistance decreases with an increase in temperature. Hence , the value of the temperature coefficient of resistivity in semiconductors and insulators is negative. 

9. Thermistor material have a temperature coefficient of resistance α given by Where Δ R is the change in resistance due to change in temperature Δ R . R s is the material resistance at the reference temperature.

10. Equation of Resistance

11. At 20° Celsius, we get 12.5 volts across the load and a total of 1.5 volts (0.75 + 0.75) dropped across the wire resistance. If the temperature were to rise to 35° Celsius, we could easily determine the change of resistance for each piece of wire. Assuming the use of copper wire (α = 0.004041) we get:

12. THERMOELECTRICITY Thermoelectricity is the direct and thermodynamically reversible conversion of heat to electricity and vice versa. It is encompassed by three differentiated effects that are: Seeback effect Peltier effect Thompson effect

13. Seeback Effect & Peltier Effect Seeback effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. Peltier effect, the cooling of one junction and the heating of the other when electric current is maintained in a circuit of material consisting of two dissimilar conductors, the effect is even stronger in circuits containing dissimilar semiconductors.

14. Thomson effect Thomson effect , the evolution or absorption of heat when electric current passes through a circuit composed of a single material that has a temperature difference along its length. This transfer of heat is superimposed on the common production of heat associated with the electrical resistance to currents in conductors.

15. THERMOCOUPLE A thermocouple is defined as a thermal junction that functions based on the phenomenon of the thermoelectric effect, i.e. the direct conversion of temperature differences to an electric voltage . It is an electrical device or sensor used to measure temperature. A thermocouple can measure a wide range of temperatures. It is a simple, robust, and cost-effective temperature sensor used in various industrial applications, home, office, and commercial applications.

16. Working of Thermocouple A thermocouple consists of two plates of different metals. Both plates are connected at one end and make a junction. The junction is placed on the element or surface where we want to measure the temperature. This junction is known as a hot junction. And the second end of the plate is kept at a lower temperature (room temperature). This junction is known as a cold junction or reference junction.

17. According to the Seebeck effect , the temperature difference between the two different metals induces the potential differences between two points of the thermocouple plates. If the circuit is closed, a very small amount of current will flow through the circuit. A voltmeter is connected to the circuit. The voltage measured by the voltmeter is a function of a temperature difference between two junctions.

18. Types of Thermocouples According to different types of combinations of alloys, the thermocouples are available in different types. The type of thermocouple is chosen according to the application, cost, availability, stability, chemical properties, output, and temperature ranges. Type J Thermocouple This type of thermocouple is a low-cost and most used thermocouple. The positive lead is made of iron and a negative lead is made of constantan (45% nickel and 55% copper ). Thermocouple grade wire, -346 to 1,400F (-210 to 760C) Extension wire, 32 to 392F (0 to 200C)

19. The positive lead is colored white and the negative terminal is colored red. And the overall jacket is colored black . The temperature range of type J thermocouple is between -210˚C to 750˚C (-346F to 1400F). This type of thermocouple has a smaller temperature range and short life span compared to type K thermocouple. But this type of thermocouple is well suited for oxidizing atmospheres. The accuracy of this type of thermocouple is ±2.2˚C (0.75%). This type of thermocouple is not recommended for lower-temperature applications. And the sensitivity of this type of thermocouple is approximately 50μV/˚C.

20. Type K Thermocouple The K-type thermocouple is the most common type of thermocouple, and it has the widest temperature measuring range. The positive lead of Type K thermocouple is composed of approximately 90% nickel and 10% chromium. The negative lead is composed of approximately 95% nickel, 2% aluminum, 2% manganese, and 1% silicon. The positive lead is colored yellow and it is a non-magnetic material. The negative lead is colored red and it is a magnetic material. And the overall jacket is colored yellow.

21. The temperature range of type K thermocouple is -200˚C to +1260˚C (-328 F to +2300 F). It is inexpensive and widely used in general-purpose applications where temperature sensitivity requires approximately 41μV/˚C. The accuracy of type K thermocouple is ±2.2 C% (0.75%). The accuracy of thermocouples also depends on the deviation in alloys

22. APPLICATIONS OF THERMOCOUPLE Thermocouples are used widely across the different industrial and scientific applications. Some of the most common application areas of thermocouples are; Industrial sectors Power generation Steel mills Biotech Pharmaceutical Cement Oil & Gas Industry

23. Comparison of RTD, T/C, Thermistor

24. D6T Thermal Sensor for Human Detection The D6T series MEMS thermal sensor from is a super-sensitive IR temperature sensor that uses MEMS sensing technology. This sensor is capable of detecting the presence of motionless humans by detecting heat from the body and is also used automatically to turn off needless lighting, and AC when peoples are not there. This sensor is also used to monitor the room’s temperature and also maintains optimal room temperature levels, continually it detects strange changes in temperature so detecting factory line stoppages otherwise find out overheating areas for early fire outbreaks prevention.

25. MEMS THERMOPILE

26. Semiconductor-based temperature sensors  A semiconductor-based temperature sensor is usually incorporated into integrated circuits (ICs ). These sensors utilize two identical diodes with temperature-sensitive voltage vs current characteristics that are used to monitor changes in temperature. They offer a linear response but have the lowest accuracy of the basic sensor types. These temperature sensors also have the slowest responsiveness across the narrowest temperature range (-70  ° C to 150  ° C).

27. https://byjus.com/jee/temperature-coefficient-of-resistance / https://www.precisionmass.com/types-and-applications-of-thermocouple / https://www.ametherm.com/blog/thermistors/temperature-sensor-types