Lecture 11 Temperature Sensing. ECE 5900/6900 Fundamentals of Sensor Design

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1 EE 4900: Fundamentals of Sensor Design Lecture 11 Temperature Sensing 1

2 Temperature Sensing Q: What are we measuring? A: Temperature 2 SI Units: Celcius ( C), Kelvin (K) British Units: Fahrenheit ( F) Celcius-Fahrenheit Approximate Conversion: y( F)=x( C)*2+30 Exact Conversion: y( F)=x( C)* Celcius-Kelvin Exact Conversion: y(k) = x( C)

3 Resistance Temperature Detector (RTD) Honeywell HEL-777-A-T-0 (Plantinum RTD) Temperature Sensors Thermistor NTCLE100E3 (NTC Thermistor) Thermocouple (Thermoelectric) Omron E52-CA1DYM6 3 Honeywell TD4A (Liquid Temperature RTD) PTCSL03T151DT1E (PTC Thermistor) Omron E52-CA1DYM6

4 Digital Thermometers (Thermistor) Applications of Temperature Sensors 4 Refrigeration and Food processing (RTD)

5 Gas Oven Safety Valve (Thermocouple) Applications of Temperature Sensors 5 Oil Refinery (Thermocouple)

6 Types of Temperature Sensors 6 Resistance Temperature Detector (RTD) NTC Thermistor Thermocouple

7 Resistance Temperature Detector (RTD) Sensors Thin film RTD 7 Thin Film RTDs are mainly platinum on Si substrate Platinum (Pt-RTD) exhibits a nearly linear temperatureresistance curve High stability, repeatability, and accuracy: good for extreme temperature conditions Pt-RTD can be used between -50 C and +500 C Ref:

8 Resistance Temperature Detector (RTD) Sensors Wire-wound RTD 8 Wire-wound RTDs contain Platinum wire wound on glass inside ceramic or glass tube More expensive than thin film RTDs Higher source currents Positive Temperature Coefficient: resistance increases with an increase in temperature. Typical PTC= /deg. C Typical resistance= ohms between -100 C and 400 C Ref: For both Wire-wound and thin-film RTDs

9 Resistance vs Temperature Curve 9 Stable, Linear Response of Platinum (Pt) C to 0 C: R R [1 At Bt Ct ( t 100)] 2 0 C to 630 C: R R [1 At Bt ] t t O O Ref:

10 Types of Temperature Sensors 10 Resistance Temperature Detector (RTD) NTC Thermistor Thermocouple

11 Thermistor Thermistor=thermal + resistor 11 Thermistor changes its resistance with change in temperature RTD are usually made from metals, thermistors can be ceramic or polymer based RTDs have wide temperature range, thermistors have limited temperature range Negative Temperature Coefficient (NTC): resistance decreases with increasing temperature

12 Thermistor Transfer Function 12 Resistance of thermistor at 25 C Stable, Linear Response of Platinum (Pt) Assume negligible self-heating (negligible rise in its temperature when the current is passing through the thermistor)

13 13 Thermistor Model Basic Equation ln T A T A T A A S where S=measured resitance of a thermistor at the unknown temperature T, and A 0, A 1, A 2, A 3 are constants derived empirically Simple Model (first order approximation) ln 1 ln 0 m o T T m S S T T e S S T A S m where β m =material characteristic temperature (K), T o =reference temperature (~25 C), S o =reference resistance Modified Steinhart-Hart Model (Vishay Spreadsheet) ln ln ln R R D R R C R R B A T where A,B,C,D=empirical constants (given) R o =reference resistance R= measured resistance

14 Thermistor Circuit Wheatstone Bridge with Instrumentation Amplifier 14 + Thermistor V o1 - + V o2 - Quarter Bridge INA126 Match R 1 =R 2 =R 3 =R=R 0 =resistance of thermistor at 25 C First figure out V o1 in relation to the resistance ratio, R 4 /R Use relation V o2 =GV o1 (where G= gain of the INA=>look up in the datsheet or derive from the measured data) and solve for unknown temperature T

15 Types of Temperature Sensors 15 Resistance Temperature Detector (RTD) NTC Thermistor Thermocouple

16 Type K (Chromel-Alumel) Thermocouple 16 Thermocouple Amplifier Thermocouple Junction Ref: Thermocouple application by Texas Instruments

17 Thermocouple Sensors Thermocouple (or Thermoelectric) Sensor 17 No voltage is developed at the thermocouple junction! Thermocouple junction is formed when to dissimilar metals or metal alloys are joined together (often the leads are also made of dissimilar metals Thermocouple junction put in contact with the hot or cold point and the leads are kept at room temperature The temperature gradient is developed along the length of the leads which results in two separate Seebeck EMFs Ref: Thermocouple application by Texas Instruments

18 Thermoelectric Circuit Thermoelectric (Seebeck) Effect These can also be two different metals 18 When one end of the conductor is hotter than the other end, electrons at the hot end are thermally energized and diffuse toward the cooler end N-type: Diffusion of electrons creates positive charge at hot end and negative charge (abundance of electrons) at the cool end As a result potential difference or thermocouple EMF is generated

19 Seebeck Coefficients 19 Possible wire pairings: Type J(T): Iron(copper) for the positive terminal and constantan for the negative terminal Most Common is Type K Type K: Chromel-Alumel Chromel: 90% Ni, 10% Cr Alumel: 95% Ni, 2% Mn, 2% Al and 1% Si Ref: Thermocouple application by Texas Instruments

20 Thermocouple: Voltage to Temperature Curves 20 T x T r V p p ; where, T x = unknown temperature, T r =reference temerpature, V p =thermocouple emf, α p =thermocouple sensitivity [μv/ C]