Thermocouples (cont’d) and Variable Resistors (Thermistors) 23 Feb 2000 Introduction to...
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Transcript of Thermocouples (cont’d) and Variable Resistors (Thermistors) 23 Feb 2000 Introduction to...
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!