Thermal sensor By: Sanchari guha 15/IC/48

Why we use thermal sensor? Different properties of an object change due to change in temperature. Various changes in physical dimension. Variation in electrical and optical properties. Generation of thermo-electricity. Change in frequency of vibration of piezoelectric crystals. Change in velocity of sound. Change of intensity of emitted radiations.

Types of thermal sensor Expansion thermometer. Electrical temperature instrument. radiation instrument

Expansion thermometer Expansion of liquid Expansion of gas Expansion of solid This type of sensors is mainly used for recording environmental temperature locally. Component Brass & Invar generally. Range : -75℃ to 540 ℃ This type of sensor is generally used for laboratory purpose and body temperature measurement. Various types of liquids are used in it. Range : -120℃ to 320℃ This system is used to measure the temperature of open tank liquid to cooking kettle and steam line, pipe line of the fluid flow. Range : -120 ℃ to 427 ℃

Electrical measurement instrument Resistance temperature detector (RTD) Platinum RTD: Pt 100, at 0℃ resistance of Pt is 100

Wire wound type RTD 2 wire wound 3 wire wound 4 wire wound (R2/R1)=(R3/(L1+L2+RT)) If R1=R2 RT+L1+L2=R3 Lead wire error : L1 & L2 (R2/R1)=((R3+L3)/(L1+RT)) If R1=R2 RT+L1=R3+L3 L1=L3 ; RT=R3 Lead wire error : L1-L3 (R2/R3)=((R1+A)/(RT+B)) RT=(R1a+R1b)/2 Lead wire error is A & B

thermistor SHAPE OF THERMISTOR Positive temperature co-eff (PTC) : Made of doped polycrystalline ceramic containing barium, lead & stongtium titanate. Negative temperature co-eff (NTC): Specific mixture of oxides of Si, Ge, Ni, Mn etc. SHAPE OF THERMISTOR Rod Plate Disc Glass probe

Working formula of thermistor Rt = Ro exp β (1/t – 1/to) β= temperature resistance co-eff of thermistor. ADVANTAGES Small size, fast response, available in different shapes. Suitable for narrow range (-60℃ to 150℃) Low cost. Wire resistance and contact resistance are negligible wrt thermistor resistance. DISADVANTAGE R & P relation is non-linear & not used for wide temp. range

Materials of thermocouple J= iron constantan (-200 to 1000)℃ T= copper-constantan(-200 to 350) ℃ K= chromel-alumel(-190 to 1200) ℃ R= pt-ptRh(13%) (0 to 1600) ℃ S= pt-ptRh(10%) (0 to 1600) ℃ B= ptRh(30%) –ptRh(6%) (0 to 1820) ℃

Industrial use of thermocouple ICE BATH TECHNIQUE :

RADIATION INSTRUMENT radiational PYROMETER The shielding element protects thermocouple junction from direct radiation and by this thermocouple hot junction we can measure temperature of radiation receiver which is proportional to radiation energy of the target. Range: 700℃ to 3000 ℃. FORMULA RT=σT^4

Optical pyrometer Brightness of target (Bt) =brightness of filament (Bf) Target temperature (Tt) =filament temperature (Tf) Intensity of radiant energy is proportional to source of known intensity

reference Sensors and Transducers, D.Patranabis. Instrument Transducers, H.K.P.Nubert Measurement system by Tata McGraw Hill. Class notes by P.Rakshit ma’am. Photos from Google.

conclusion We conclude from this chapter that thermal sensors are used in our daily life and in industrial purposes in various ways. Thermal sensors are widely used sensors in industrial purposes as it is easily available and easy to use. It is available in various shapes for various purposes. It serves our national security and used in various military purposes. Still now a lot of researches are going on this field to serve mankind in various ways.

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