Thermocouples (cont’d) and Variable Resistors (Thermistors)

23 Feb 2000

Introduction to Temperature Sensors

Kirchoff’s Current Law Convention

+_

+_

Using – + = +, we have +Vb - IR = 0or Vb = IR

Using + – = +, we have - Vb + IR = 0or - Vb = - IRor Vb = IR

Same Result!Just be consistent.

clockwise current flowVb

I R

Language of Thermocouples

Temperature Range -270oC to 2320oC depends on the type of the

thermocouple Passive

Does not require external power source Non-intrusive

Does not affect the system much Linearity

In general, it is a nonlinear function of temperature Sensitivity

From 0.011 mV/oC for type R to 0.076 mV/oC for type E

Millivolt Output of Common Thermocouples (Reference Junction at 0oC)

-20

-10

0

10

20

30

40

50

60

70

80

90

-250 250 750 1250 1750

Temperature (C)

Ou

tpu

t (m

V)

K

T

J

E

R

S

Type Materials Lead wirecolor

Operatingrange(oC)

Approximatesensitivity(mV/oC)

T Copper/constantan Blue -250 to 400 0.052

E Chromel/constantan Purple -270 to 1000 0.076

J Iron/constantan Black -210 to 760 0.050

K Chromel/alumel Yellow -270 to 1372 0.039

R Platinum/platinum-13% rhodium

Green -50 to 1768 0.011

S Platinum/platinum-10% rhodium

Green -50 to 1768 0.012

E Tungsten, 5%rhenium/tungsten,26% rhenium

White,Red trace

0 to 2320 0.020

Characteristics of Standard Thermocouples

Common Sheathed Thermocouple Probe

Place the wires and junction and ceramic insulation inside a stainless steel or inconel sheath to protect the thermocouple wires from damage or chemical contamination.

Thermopile

When n thermocouples are connected in series in a device called a thermopile

Which increases the sensitivity of the system, however, it provides a method to average several thermocouples which are distributed in a spatial region.

T1 T2

C 0 1 2 3 4

300 2.401 2.410 2.420 2.430 2.440

Example:

A type R thermocouple with an ice reference gives a voltage reading of 2.415 mV. What is its temperature?

Thermoelectric voltage in mV

2.415C 5.301

1 010.0

005.0C 301

CT

C 0 1 2 3 4

30 0.171 0.177 0.183 0.189 0.195

Example:

A type R thermocouple with an ice reference gives a voltage reading of 2.415 mV. What would it read

with a 30 °C reference?

Thermoelectric voltage in mV

V = 2.415 mV - 0.171 mV = 2.244 mV

C 0 1 2 3 4

280 2.207 2.217 2.226 2.236 2.246

Example:

A type R thermocouple with 30 °C reference gives a voltage reading of 2.244 mV. What is its temperature?

Thermoelectric voltage in mV

0.010 mV

2.244

0.008 mVC 8.310

30 1 010.0

008.0C 283

CCT

NO!

C 0 1 2 3 4

300 2.401 2.410 2.420 2.430 2.440

Thermoelectric voltage in mV

2.244 + 0.171 = 2.415

C 5.301

1 010.0

005.0C 301

CT

Example:

A type R thermocouple with 30 °C reference gives a voltage reading of 2.244 mV. What is its temperature?

Correct the voltageFIRST!

(30o voltage)

Thermistors

- work by changing resistivity,

A

LR

change L and A(strain gauge)

change (thermistor)

Thermistors

Characteristics Resistive Device, active, non-linear Less accurate than TCs and RTDs -100 to 300oC

Applications Automobile engine water temperature VCR overheating controller Electronic circuits temperature dependency

compensator

Thermistor Types

Rods Chips Flakes

Beads a) a bare bead b) a glass-encapsulated bead c) an assembly with extension wires

Thermistors

Non-linear Small Range: -100 to +300oC Sensitivity

High coefficient of resistivity:

4 to 6% per oC

or greater!

0

25

50

0 20 40 60 80 100

T (deg C)

R (k

Ohm

s)

Thermistor Example: Our Lab#6

Resistance = 34.17 k at 22oC Change Temperature by 1 to 23oC Resistance is now 32.71K a difference of

1.46 k or 1460 Change of 4.2% from the original reading

Does this sound readable?

Thermistors

Conversion Equations:

To is typically 25oC

Ro ranges from 50 to 2 M

R R e T T 0

1 10

3lnln1

RCRBAT

The Steinhart-Hart equation:

Measuring the Resistance

Voltage Divider CircuitSimple to buildFind the output voltage given the temperature:

Solve for the voltage divider for RT

Look up the temperature for a given resistance

Vs

R1

RT Vout

VR

R RVou t

T

Ts

( )1

Basic Thermistors

Voltage Divider Circuit The range:

VR

R RVou t

T

Ts

( )1

0

25

50

75

100

125

0 20 40 60 80 100

T (deg C)

R (

kOh

ms)

0

2

4

6

8

10

0 20 40 60 80 100

T (deg C)

V (

Vo

lts)

011

0TTeRRT

Vs

R1

RT Vout

Thermistor’s Linearized

Linearizing with more hardware:

T2

R2

Vout

+

-Vs

R1

T1

You will do this this week in lab!