Temperature Sensors
ECE 371 JB
Prof. Bernhard
A Simple Thermal System
Sensor
Work Load
Temperature
Controlling
Device
Heat Source
Sensor Input
Output
Types
Thermocouples
Resistance temperature devices (RTD)
Thermistors
Infrared sensors
Thermocouples
Mostly widely used in
industry
Range: sub-zero to
4000oF(2000oC)
Formed by joining two
different metal alloy wires
(A,B) at point called
junction
Junction called the
measuring or “hot” junction
Lead ends attached to
temp indicator or controller
Connection point called
reference or “cold” junction
Measuring Junction
A
+
B
-
Reference
Junction
Display
Device
How does it work?
Measuring junction is heated, small DC voltage
(millivolts) generated in thermocouple wires
Thermocouple converts thermal energy into
electrical energy
Note: thermocouple only generates a
millivoltage signal when there is temperature
difference between “hot” and “cold” junctions
“cold” junction usually set to 32oF(0oC)
Thermocouple Types
Made up of two different metal
alloy wires.
Different alloys result in
different temperature ranges
Ex:
Standard
Type
Metal
Content
(Pos. Leg)
Metal
Content
(Neg. Leg)
Temp.
Range
B
70.4% (Pt) 93.9% (Pt)
29.6% (Rh) 6.1% (Rh)
1600-3100 oF
870-1700 oC
E
90% (Ni)
10% (Cr)
32-1650 oF
870-1700 oC
55% (Cu)
45% (Ni)
Pros/Cons
Each thermocouple type has advantages & disadvantages
– Cost:
Rare metals (i.e. noble metals)  $$$
– Types B, R, S
Common metals (i.e. base metals) $
– Types E, J, K, N, T
Rarer metals = high temperature range & better accuracy
– Temperature Range
– Accuracy a.k.a. tolerance
– Life Expectancy
Operating Temp.
Wire size
Thermocouple protection
Environment
Accuracy required
Type
Max.
Temp.
Tolerances
B
3100oF
1700oC
(+/-) 0.5%
E
1650oF
900oC
(+/-) 1.7oC (+/-) 3.06oF
or (+/-) 0.5%
Whichever greater
Life Expectancy
Failed = inaccuracy
- When wires are heated/cooled changes take place on
molecular level
Physically: molecular structure changes
Chemically: wires react with oxygen or other substances,
changing chemical composition
- Result: millivolt signal “drifts”
EMF
(mV)
Tolerance Band
Time
- Recalibration: adjust controller to compensate for errors
Thermocouple Constructions
3 General constructions
– Insulated Wire
– Ceramic-beaded
– Metal-sheathed
Insulated Wire Thermocouples
Bare wires wrapped with insulation
– Insulations
Fibrous, woven material made of fiber-glass, mica,
or ceramic fiber
Plastics (Teflon)
Polyimides (Kapton)
– Purpose
Electrically isolate wires
Protects wires from contamination
Easier wire installation
Metal - Sheathed Thermocouples
Junction and wires are assembled in small
diameter metal tubes
– Insulation
Fiberglass
MgO
– Purpose
Protects against contamination
Defends against chemical attack
Provides mechanical stability
Metal - Sheathed Thermocouples
Orientation of thermocouple junction during assembly
– Grounded
Weld junction directly to inside tip of sheath
Ensures rapid heat transfer from sheath to junction
Protects junction while minimizing heat transfer delays.
– Ungrounded
Similar to grounded except junction isolated from metal sheath
Electrically isolates junction from sheath
Prevents stray voltages from inducing measuring error
More shock resistant & better under rapid temperature changes
DISADVANTAGE: Slows down heat transfer to junction (2x-3x
slower)
– Exposed
Junction protrudes from end of sheath, but insulated from it
Due to direct exposure with heated material, very quick response to
temp. changes
No sheath to slow down heat transfer
DISADVANTAGE: Not protected from mechanical damage &
chemical attack
Resistance Temperature Devices
(RTD)
Precision Temperature Sensors
– More accurate than thermocouple elements
– Maintain accuracy over longer period of time
– Range up to 1200oF (650oC)
Styles
– Wire-Wound
– Thin film
– Kapton Insulated
How do RTDs work?
RTD’s resistance
as temp.
– Controller measures resistance value and converts to
temp. reading, fairly linear relationship.
– Unlike thermocouple, no electrical signal generated
– Controller measures resistance by passing current
through RTD
– Use a base resistance value (ex: for Platinum, value
of 100 ohms at 0oC (32oF)
RTD Resistance Vs. Temp. (TCR) Curve
Resistance
(Ohms)
TCR = Temperature coefficient of resistance
Temperature (oC)
RTD Vs. Thermocouples
Advantages of RTDs
– Stability
– Repeatability
– Accuracy
Disadvantages of RTDs
– Cost: Platinum = $$$, 2x more expensive
– Temp. Range limited
– Response Time slower, 2x-4x times slower
Heat must transfer through epoxy or glass coating
Entire RTD element must reach uniform temp. before
accurate measurement taken.
Lead Wire Effect
Alters reading due to lead wire resistance
Two approaches
– Determine lead wire resistance and have controller
compensate
– Attach additional lead wire to one end of RTD
– Connect a transmitter, converts resistance to low amp
signal and sent to temperature controller
2
1
3
1
2
3-wire RTD
RTD
3
4
4-wire RTD
RTD
Effect of Lead Resistance: Platinum
Wire RTD
Most Common: DIN 43760
– Standard temp. coefficient (alpha=0.00385)
For 100 ohm wire  +0.385 ohms/OC @ 0oC
alpha = average slope from 0oC – 100oC
– A 10 ohm lead impedance implies 10/3.85 =
26oC error in measurement
R=5 
Lead
100  RTD
Lead
R=5 
How to correct this problem?
Wheatstone: 3-Wire Bridge
– Wires A & B are perfectly matched in length, respective
impedances effects will cancel out due to being on opposite legs
– Wire C acts as sense lead & carries no current
A
DVM
C
RTD
B
– Non-linear relationship between resistance change and bridge
output voltage change
– Additional equation required to convert bridge output voltage to
equivalent RTD impedance
3-Wire Bridge Calculations
If Vs & Vo known, Rg can be found.
Unbalanced Vo of bridge with R1=R2
 R3 
  Vs (1 / 2)
Vo  Vs 
R R 
g 
 3
If Rg=R3  Vo=0 & bridge is balanced
To determine Rg assuming lead resistance is
zero


Vs  2Vo

Rg  R3 
 Vs  2Vo 
3-Wire Bridge Calculations
If Rg located some distance from 3-wire configuration 
RL appears in series with Rg & R3
 Vs  2Vo 
 4Vo 
  RL 

Rg  R3 
 Vs  2Vo 
 Vs  2Vo 
V3/2
+
-
-
+
RL
Vo
RL
Rg
Another Approach
4-Wire Ohms
– DVM is directly proportional to RTD resistance  1 conversion
equation required
– Insensitive to length of lead wires
– Accuracy better than 3-wire
– Disadvantage: One more extension wire required.
+
Current
Source
i=0
100 W RTD
DVM
-
i=0
RTD=Rg
Vs
+
-
-
+
Vo
Resistance to Temperature
Conversion
RTD more linear than thermocouple, curvefitting still required
Callendar-Van Dusen Equation
3



 T
 T
 T
 T  
   
RT  Ro T    
 1
 1
 
 100  100) 
 100  100  

RT = Resistance at Temperature T
Ro = Resistance at T=0oC

= Temperature coefficient at T=0oC

= 1.49 (typical value for 0.00392 platinum)

= 0 T>0, 0.11 (typical) T<0
Identification
2-wire RTD uses same color lead wire for
both leads
3-wire has 2 red leads & 1 white lead
4-wire has 2 red leads & 2 white leads
1
2
3
4
4-wire RTD
RTD
Lead-to-lead
Measurement
Distance at Room
Temperature
1 to 2; 3 to 4
Less than 1ohm to a
few ohms max.
1 to 3; 1 to 4
2 to 3; 2 to 4
107 to 110 ohms
RTD Assembly
Wire Wound
– For 500oF (260oC), element welded to copper or
nickel lead wires
– Sub-assembly placed in closed-end tube
– Powder, cement or thermal grease fills tube
– Epoxy seal seals out moisture & locks RTD/leads to
tube
Thin Film
– For 1200oF (650oC), element fitted into cavity of MgO
metal-sheathed cable
– Wires in cable welded to RTD element
– Cap filled with MgO and placed on element end &
mounted
What are Thermistors?
Semiconductor used as temperature sensor
Made from mixture of metal oxides pressed to bead or wafer form
Bead heated under pressure at high temp & encapsulated with
glass/epoxy
RESULT: Distinct non-linear resistance vs. temp. relationship
Non-linear decrease in resistance
as temperature increases.
Resistance
(Ohms)
Temperature (oC)
So Sensitive…
Very large resistance change = small
temp. change
3 – 5% per oC (vs. 0.4% per oC for RTDs)
Temp. changes as small as 0.1oC
Significantly smaller in size
Temp range: -100oC – 300oC (-120oF –
570oF)
Thermistor Standards
No Industrial Standards
Base resistance range: 103 – 106 ohms
– Typically measured at 25oC vs. 0oC for RTDs
TCRs vary widely
Thermistor’s accuracy limited to small
temp. range
Thermistor Lead Wire Effects
Lead wire does add overall resistance
NOTE: base resistance of thermistor very
large (>103 ohms), added lead wire
resistance insignificant.
RESULT: No resistance compensation
required!
Infrared Sensors
Intercepts portion of infrared energy radiated by
object (  = 8 - 14 microns).
Waves focused through lens on infrared
detector, converting to an electric output signal
Heat Source
Non-Contact Temp. Sensor
Optics
Infrared Detector
Temp. Indicator
Emissivity
Def: The ability of a material to radiate or absorb
electromagnetic waves. Higher = Better!
– Ex: Given values below & emissivity varies by 0.05, what is
measuring error?
Ans: IR Sensor A 5.5% (0.05/0.9)
IR Sensor B 10% (0.05/0.5)
IR Sensor A
e = 0.9
IR Sensor B
e = 0.5
Field of View
All infrared radiation in this filed of view will be
detected by the sensor
0.60 in
(15 mm)
0.75 in
(19 mm)
1.0 in
(25 mm)
Infrared
Sensor
25 mm
76 mm
152 mm
1.4 in
(36 mm)
2.5 in
(64 mm)
4.5 in
(114 mm)
Good vs. Bad Radiation
Position 1, IR sensor sees both target object & background objects
Position 2, IR sensor only sees target object. True target
temperature can now be measured.
RULE: target size should be at least 1.5 to 2 times the “spot size.”
Infrared
Sensor
2
Correct
Target
Placement
1
Incorrect
Target
Placement
Background
“Noise”
Scenarios to Avoid
Figure 1: Thin film materials & background radiation
enter sensor
Figure 2: Polished metals will not function well with
infrared sensing due to the reflecting radiation.
Infrared
Sensor
Figure 1
Infrared
Sensor
Figure 2
Sensor to Target Distance
To reduce reflected radiant energy, set IR sensor at right
angle with respect to target
If space limitation, mount IR up to a maximum of 45O
Sensor
<45o
Product
Operating Environment
Smoke, dust vapors absorb or reflect infrared
radiation before getting to sensor lens.
Causes controller to maintain target at wrong
temperature
Target
Infrared
Sensor
Smoke or Vapors
So which one is better? Advantages
Thermocouple
RTD
Thermistor
Infrared
Simple, rugged
High temp. operation
Low Cost
No resistance lead wire problems
Point temp. sensing
Fastest response to temperature changes
Most stable over time
Most accurate
Most repeatable temp. measurement
Very resistant to contamination/corrosion of the
RTD element
High sensitivity to small temperature changes
Temperature measurements become more
stable with use
Copper or nickel extension wires can be used
No contact with the product required
Response times as fast or faster than
thermocouples
No corrosion or oxidation to affect sensor
accuracy
High repeatability
So which one is better? Disadvantages
Thermocouple
RTD
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
of not shielded
Lowest accuracy
High Cost
Slowest response time
Low sensitivity to small temperature
changes
Sensitive to vibration
Decalibration if used beyond sensor’s
temperature ratings
Somewhat fragile
So which one is better? Disadvantages
Thermistor
Infrared
Limited temperature range
Fragile
Some initial accuracy “drift”
Decalibration if used beyond the sensor’s
temperature rating
Lack of standards for replacement
High initial cost
More complex – support electronics
required
Emissivity variations affect temperature
measurement accuracy
Field of view and spot size may restrict
sensor application
Measuring accuracy affected by dust,
smoke, background radiation etc.