By Anupam Prakash (3RD YR EIE) ROLL NO.:-18700510014 Temperature Sensors By Anupam Prakash (3RD YR EIE) ROLL NO.:-18700510014

Brief Overview Types of Temperature Sensors and how they work Applications Advantages Disadvantages and Conclusion

Types of Temperature Sensors Thermocouples Resistance Temperature Detectors (RTDs) Thermistors Semiconductors

Thermocouples Two wires of different metal alloys. Converts thermal energy into electrical energy. Requires a temperature difference between measuring junction and reference junction. Easy to use and obtain.

Principle Of Operation Thermocouple follows Seebeck effect The emf produced in a thermocouple is given by E= a ( Ɵ) + b ( Ɵ)*( Ɵ) where Ɵ = difference in temp. b/w the hot junction and reference junction of thermocouple. & a , b are contants.

Types of Thermocouple Types Composition Temp. Range B Pt 30% Roh (+) 2500-3100 deg. F C Tu 5% Rhe (+) Tu 26% Rhe (-) 3000-4200 deg. F E Chromel (+) Constantan (-) 200 -1650 deg. F J Iron (+) 200-1400 deg. F K Alumel (-) 200- 2300 deg. F

Types Composition Temp. Range M Ni (+) Ni (-) 3200-2250 deg. F N Nicrosil (+) Nisil (-) 1200-2300 deg. F R Pt 13% Rh (+) Pt (-) 1600-2640 deg. F S Pt 10% Rh(+) Pt(-) 1800-2640 deg. F T Copper (+) Constantan (-) -330 – 660 deg. F

Thermocouple Applications Plastic injection molding machinery Food processing equipment Deicing Semiconductor processing Heat treating Medical equipment Industrial heat treating Packaging equipment

Thermocouples Advantages Disadvantages Simple, Rugged High temperature operation Low cost No resistance lead wire problems Point temperature sensing Fastest response to temperature changes Least stable, least repeatable Low sensitivity to small temperature changes Extension wire must be of the same thermocouple type Wire may pick up radiated electrical noise if not shielded Lowest accuracy

Resistance Temperature Detectors (RTDs) Wire wound and thin film devices. Nearly linear over a wide range of temperatures. Can be made small enough to have response times of a fraction of a second. Require an electrical current to produce a voltage drop across the sensor

Resistance with temp. relationship R=Ro (1+a1T+a2(T*T)+…… +an(T*T*T… n times)) where Ro = resistance at temp. T=0 Types of RTD 2 wire RTD 3 wire RTD

RTD Applications Air conditioning and refrigeration servicing Furnace servicing Foodservice processing Medical research Textile production

RTDs Advantages Disadvantages High cost Most stable over time Slowest response time Low sensitivity to small temperature changes Sensitive to vibration (strains the platinum element wire) Decalibration if used beyond sensor’s temperature ratings Somewhat fragile Most stable over time Most accurate Most repeatable temperature measurement Very resistant to contamination/ corrosion of the RTD element

Thermistors A semiconductor used as a temperature sensor. Mixture of metal oxides pressed into a bead, wafer or other shape. Beads can be very small, less than 1 mm in some cases. The resistance decreases as temperature increases, negative temperature coefficient (NTC) thermistor.

Application of Thermistors Resistance temp. characterstics RT1= RT2 exp ( B ( 1/T1 -1/T2) where RT1 = resistance at temp. T1 RT2 = resistance at temp. T2 B = constant Application of Thermistors Most are seen in medical equipment markets. Thermistors are also used are for engine coolant, oil, and air temperature measurement in the transportation industry.

Thermistors Limited temperature range Advantages Disadvantages Limited temperature range Fragile Some initial accuracy “drift” Decalibration if used beyond the sensor’s temperature ratings Lack of standards for replacement High sensitivity to small temperature changes Temperature measurements become more stable with use Copper or nickel extension wires can be used

Semiconductors Are small and result from the fact that semiconductor diodes have voltage-current characteristics that are temperature sensitive. Temperature measurement ranges that are small compared to thermocouples and RTDs, but can be quite accurate and inexpensive.

Semiconductor Applications Hard Disk Drives Personal Computers Electronic Test Equipment Office Equipment Domestic Appliances Process Control Cellular Phones

Determining Factors of a Temp. Sensor Low Power Serial Interface Small Accurate Wide temperature range Extras I2C Interface Temperature Alarms

CONCLUSION In this application note , the concept and usage of different temperature sensors are discussed . Among these temp. sensors 1. Thermocouple gives the fastest response to temp. change 2. RTD is most suitable over time , most accurate temp. sensor. 3. Thermistor has the highest sensitivity to small temp. change.

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