EE 4900: Fundamentals of Sensor Design 1faculty.weber.edu/snaik/ECE5900_ECE6900/09Lec09_Temperature.pdf · ECE 5900/6900 Fundamentals of Sensor Design Dr. Suketu Naik EE 4900: Fundamentals

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ECE 5900/6900 Fundamentals of Sensor Design Dr. Suketu Naik

EE 4900: Fundamentals of Sensor Design

Lecture 11Temperature Sensing

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Temperature Sensing

Q: What are we measuring?

A: Temperature

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)*1.8+32

Celcius-Kelvin

Exact Conversion: y(K) = x(°C) + 273.15

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Temperature Sensors

Resistance

Temperature

Detector (RTD)Honeywell HEL-777-A-T-0

(Plantinum RTD)

Honeywell TD4A

(Liquid Temperature RTD)

Thermistor

NTCLE100E3

(NTC Thermistor)

Thermocouple

(Thermoelectric)

Omron

E52-CA1DYM6

PTCSL03T151DT1E

(PTC Thermistor)

Omron

E52-CA1DYM6

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Applications of Temperature Sensors

Digital

Thermometers

(Thermistor)

Refrigeration

and

Food processing

(RTD)

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Applications of Temperature Sensors

Gas Oven Safety

Valve

(Thermocouple)

Oil Refinery

(Thermocouple)

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Types of Temperature Sensors

Resistance Temperature Detector (RTD)

NTC Thermistor

Thermocouple

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Resistance Temperature Detector (RTD) Sensors

Thin film RTD

Thin Film RTDs are mainly platinum on Si substrate

Platinum (Pt-RTD) exhibits a nearly linear temperature-

resistance curve

High stability, repeatability, and accuracy: good for extreme

temperature conditions

Pt-RTD can be used between -50°C and +500°CRef: http://www.ussensor.com/thin-film-platinum-rtds, http://www.pyromation.com/Downloads/Doc/Training_RTD_Theory.pdf

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Resistance Temperature Detector (RTD) Sensors

Wire-wound RTD

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= .00385 /deg. C

Typical resistance=100-300 ohms between -100°C and 400°CRef: http://www.ussensor.com/thin-film-platinum-rtds, http://www.pyromation.com/Downloads/Doc/Training_RTD_Theory.pdf

For both Wire-wound

and thin-film RTDs

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Resistance vs Temperature Curve

Ref: http://archives.sensorsmag.com/articles/0503/52/main.shtml

-200°C to 0°C: )]100(1[ 32 tCtBtAtRR Ot

0 °C to 630°C: ]1[ 2BtAtRR Ot

Stable, Linear

Response of

Platinum (Pt)

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NTC Thermistor

Thermocouple

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ThermistorThermistor=thermal + resistor

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

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Thermistor Transfer Function

Stable, Linear

Response of

Platinum (Pt)Resistance of

thermistor at

25 °C

Assume negligible self-heating (negligible rise in its temperature

when the current is passing through the thermistor)

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Thermistor Model

Basic Equation

3

3

2

210ln

T

A

T

A

T

AAS

where S=measured resitance of a thermistor at the unknown temperature

T, and A0, A1, A2, A3 are constants derived empirically

Simple Model (first order approximation) 1

0

11

0

ln1

ln 0

m

oTTm S

S

TTeSS

TAS

m

where βm=material characteristic temperature (K), To=reference

temperature (~25 °C), So=reference resistance

Modified Steinhart-Hart Model (Vishay Spreadsheet)

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0

2

00

lnlnln

R

RD

R

RC

R

RBAT

where

A,B,C,D=empirical

constants (given)

Ro=reference resistance

R= measured resistance

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Thermistor Circuit

Wheatstone Bridge with Instrumentation Amplifier

Thermistor

Quarter Bridge INA126

+

Vo1

-

+

Vo2

-

Match R1=R2=R3=R=R0=resistance of thermistor at 25°C

First figure out Vo1 in relation to the resistance ratio, R4/R

Use relation Vo2=GVo1 (where G= gain of the INA=>look up in the

datsheet or derive from the measured data) and solve for unknown

temperature T

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NTC Thermistor

Thermocouple

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Type K (Chromel-Alumel) Thermocouple

Thermocouple

Amplifier

Thermocouple

Junction

Ref: Thermocouple application by Texas Instruments

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Thermocouple SensorsThermocouple (or Thermoelectric) Sensor

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 EMFsRef: Thermocouple application by Texas Instruments

No voltage is developed at the

thermocouple junction!

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Thermoelectric (Seebeck) Effect

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

These

can also

be two

different

metals

Thermoelectric Circuit

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Seebeck Coefficients

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% SiRef: Thermocouple application by Texas Instruments

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Thermocouple: Voltage to Temperature Curves

p

p

rx

VTT

; where, Tx= unknown temperature, Tr=reference temerpature,

Vp=thermocouple emf, αp=thermocouple sensitivity [μV/°C]

EE 4900: Fundamentals of Sensor Design 1faculty.weber.edu/snaik/ECE5900_ECE6900/09Lec09_Temperature.pdf · ECE 5900/6900 Fundamentals of Sensor Design Dr. Suketu Naik EE 4900: Fundamentals

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