Sensors and Actuators: Temperature Sensors - MCE 303, Lecture notes of Embedded Wireless Sensors

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MCE 303

SENSORS AND

ACTUATORS

Temperature Sensors

Temperature Sensors The temperature is sensed through heat transfer from the source to the measuring device. The physical (or chemical) change in the device that is caused by this heat transfer is the transducer stage of the sensing device. 2 RTD Thermistor Liquid thermometer Bimetal strip thermometer De Silva, C. W. (2007). Sensors and actuators: control system instrumentation. CRC Press. R a h i m e A l s a n ğ u r

Thermoelectric Effects The TE effect is the direct conversion of temperature differences to electric voltage and vice versa at the junctions of two different conductors. 4

Seebeck Effect

Thomas Johann Seebeck

Peltier Effect

Jean Charles Athanase Peltier William Thomson,^ Lord^ Kelvin

Thomson Effect

R a h i m e A l s a n ğ u r

Seebeck Effect 5

+ 𝑇 2 𝑆^ =^

S= Seebeck Coefficient (μV/K) https://www.msm.cam.ac.uk/utc/thermocouple/pages/ThermocouplesOperatingPrinciples.html R a h i m e A l s a n ğ u r

Thermocouple Types 7 The most common general use thermocouple! https://www.msm.cam.ac.uk/utc/thermocouple/pages/ThermocouplesOperatingPrinciples.html R a h i m e A l s a n ğ u r

8 Type K Thermocouple Reference Table: https://www.thermocoupleinfo.com/type-k-thermocouple.htm Important considerations in selecting a thermocouple (or any temperature sensor) include;

• temperature range,
• sensitivity,
• speed (time constant),
• robustness (to vibration, environment including chemicals, etc.),
• ease of use (installation, etc.). R a h i m e A l s a n ğ u r

Metals used in RTDs include platinum, nickel, copper, and various alloys. The useful temperature range of an RTD is about −200 °C to + 800°C. At high temperatures, these devices may tend to be less accurate than thermocouples. The speed of response can be lower as well(e.g., fraction of a second). 10 De Silva, C. W. (2007). Sensors and actuators: control system instrumentation. CRC Press. Resistance Temperature Detector (RTD) R a h i m e A l s a n ğ u r RTD PT

11 De Silva, C. W. (2007). Sensors and actuators: control system instrumentation. CRC Press. Resistance Temperature Detector (RTD) R a h i m e A l s a n ğ u r

Thermistor P ositive T emperature C oefficient ( PTC ) N egative T emperature C oefficient ( NTC ) 13 𝑅 = 𝑅 0 exp 𝛽

where temperature T is in kelvin (K). Typically, 𝑅 0 = 5000 Ω at 𝑇 0 = 298 °K (i.e., 25°C). The characteristic temperature β (about 4200°K) itself is temperature dependent. Thermistors are quite robust, and they provide a fast response and high sensitivity (compared with RTDs) particularly because of their high resistance (several kilohms) and hence high change in resistance. R a h i m e A l s a n ğ u r

Bimetal Strip Thermometer Unequal thermal expansion of different materials is used in the temperature measurement by a bimetal strip thermometer. If strips of the two materials (typically metals) are firmly bonded, thermal expansion causes this element to bend toward the material with the lower expansion. This motion can be measured using a displacement sensor or indicated using a needle and scale. Household thermostats commonly use this principle for temperature sensing and control (on–off ). R a h i m e A l s a n ğ u r 14

Semiconductor Based Temperature Sensors A semiconductor temperature sensor, also known as a solid-state temperature sensor, is an electronic device integrated into integrated circuits (ICs) capable of measuring temperatures accurately within a range of - 55 °C to 150 °C. It offers high performance, affordability, and ease of use, making them popular in automation and microprocessor-based measurement devices. R a h i m e A l s a n ğ u r 16

Semiconductor Based Temperature Sensors The semiconductor temperature sensor uses temperature - sensitive voltage in diodes to provide a linear response; however, its accuracy is relatively lower (1 - 5 °C), and its response time is slower (5 - 60 seconds) in a narrow temperature range. R a h i m e A l s a n ğ u r 17

Thermopile Temperature Sensors T h e t h e r m o c o u p l e m a t e r i al s u s e d i n t h e r m o p i l e s are u s u al l y b i s mu t h an d an ti m o n y, wh i c h h av e a re l a ti v e l y h i gh t h e r mo e l e c tr i c c o e f f i c i e n t ( a m e as u re o f th e m agn i tu d e o f th e i n d u c e d v o l tage i n re s p o n s e to th e t e m p e ra tu re d i f f e re n c e ). A n i n d i v i d u al th e r m o c o u p l e t yp i c al l y p ro d u c e s a l ow o u t p u t v o l tage , wh i c h re s u l ts i n a l ow d e t e c ti v i ty an d l i m i ts i ts u s e as a s e n s i n g d ev ic e. So , o n e w a y to i n c re as e t h e o u tp u t v o l tage i s to c o n n e c t m an y t h e r m o c o u p l e j u n c ti o n s ( ty p ic al l y 2 0 t o 1 2 0 ) i n s e r i e s. Al l t h e “ h o t ” j u n c ti o n s are p l ac e d c lo s e t o g e t h e r t o c o l l e c t t h e ra d i a t i o n. R a h i m e A l s a n ğ u r 19

Liquid Thermometers A liquid thermometer has two impor tant elements: (1) a temperature sensor (the bulb of a mercur y - in-glass thermometer) in which some change occurs with a change in temperature; and (2) some means of conver ting this change into a numerical value ( e.g. the visible scale that is marked on a mercur y - in-glass thermometer). R a h i m e A l s a n ğ u r 20